physics - all

Energy stores and systems

A system is an object or group of objects.
There are changes in the way energy is stored when a system changes.

Students should be able to describe all the changes involved in the way energy is stored when a system changes, for common situations. For example:

  • an object projected upwards
  • a moving object hitting an obstacle
  • an object accelerated by a constant force
  • a vehicle slowing down
  • bringing water to a boil in an electric kettle. 
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Changes in energy - kinetic

The kinetic energy of a moving object can be calculated using the equation:

kinetic energy = 0.5 × mass × page16image4952speed 2

E k = 12 m v 2
kinetic energy, Ek, in joules, J

mass, m, in kilograms, kg
speed, v, in metres per second, m/s 

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elastic potential energy

The amount of elastic potential energy stored in a stretched spring can be calculated using the equation:

elastic potential energy = 0.5 × spring constant × extension 2

1 2 Ee = 2 k e

(assuming the limit of proportionality has not been exceeded) elastic potential energy, Ee, in joules, J
spring constant, k, in newtons per metre, N/m
extension, e, in metres, m 

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gravitational potential energy

The amount of gravitational potential energy gained by an object raised above ground level can be calculated using the equation: 

g.p.e. = mass × gravitational field strength × height

Ep = m g h

gravitational potential energy, Ep, in joules, J

mass, m, in kilograms, kg

gravitational field strength, g, in newtons per kilogram, N/kg (In any

calculation the value of the gravitational field strength (g) will be given.) height, h, in metres, m 

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Energy changes in systems

The amount of energy stored in or released from a system as its temperature changes can be calculated using the equation:

change in thermal energy = mass × specific heat capacity × temperature change

E=mcθ

change in thermal energy, ∆E, in joules, J

mass, m, in kilograms, kg

specific heat capacity, c, in joules per kilogram per degree Celsius, J/kg °C

temperature change, ∆θ, in degrees Celsius, °C

The specific heat capacity of a substance is the amount of energy required to raise the temperature of one kilogram of the substance by one degree Celsius. 

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power

Power is defined as the rate at which energy is transferred or the rate at which work is done.

power = energy transferred/time 

P = Et
power =  work done/time 

P = Wt
power, P, in watts, W

energy transferred, E, in joules, J
time, t, in seconds, s
work done, W, in joules, J
An energy transfer of 1 joule per second is equal to a power of 1 watt. 

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Energy transfers in a system

Energy can be transferred usefully, stored or dissipated, but cannot be created or destroyed. 

closed systems are systems where neither matter nor energy can enter or leave. the net change in the total energy of a closed system is always 0 

in all system changes energy is dissipated, so that it is stored in less useful ways. This energy is often described as being ‘wasted’.

can reduce unwanted energy transfers eg by lubrication to reduce frictional forces and insulation which reduces the rate of energy transfer by heating

The higher the thermal conductivity of a material the higher the rate of energy transfer by conduction across the material. 

the thicker the walls and the lower their thermal conductivity, the slower the rate of energy transfer will be (will cool more slowly)

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Efficiency

he energy efficiency for any energy transfer can be calculated using the

equation:

efficiency = useful output energy transfer / total input energy transfer 

Efficiency may also be calculated using the equation:

efficiency = useful power output / total power input 

the less energy wasted the more efficient the device is said to be 

you can improve the efficiency of energy transfers by insulating objects, lubricating them or making them more streamlined 

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National and global energy resources

The main energy resources available for use on Earth include: fossil fuels (coal, oil and gas), nuclear fuel, biofuel, wind, hydro-electricity, geothermal, the tides, the Sun and water waves.

A renewable energy resource is one that is being (or can be) replenished as it is used.

The uses of energy resources include: transport, electricity generation and heating. 

transport - non renewable: petrol and diesel powered vehicles (including most cars) use fuel created from oil. coal is used in some old fashioned steam trains-boil water to produce steam renewable: vehicles that run on pure bio-fuels or a mix of a bio-fuel and petrol/diesel 

heating - non renewable: natural gas is most widely used for heating homes in UK. coal is commonly burnt in fireplaces. electric heaters.                                                                     renewable: geothermal heat pump to heat buildings. solar water heaters - used for radiators. burning bio-fuel or using electricity generated from renewable resources can be used for heating 

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wind power

advantages:

  • no pollution
  • no fuel costs and minimal running costs
  • no permanent damage to landscape                                                                                                                            

disadvantages:                                                                                                                               

  • spoil view
  • lots needed (1500 = 1 coal-fired power station)
  • very noisy
  • turbines stop if no wind or wind is too strong
  • impossible to increase supply if extra demand
  • initial costs are quite high
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solar cells

advantages: 

  • no pollution 
  • energy is free
  • low running costs 
  • very reliable source in sunny countries (only in day)
  • cost effective even in cloudy places eg Britian

disadvantages:

  • generate electricity on small scale 
  • cant increase power output when theres extra demand 
  • initial costs are high 
  • use a lot of energy to munufacture 
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geothermal power

energy in underground thermal energy stores. possible in volcanic areas or where hot rocks are near to surface 

can be used to generate electricity or heat buildings directly 

advantages:

  • free energy
  • reliable 
  • little damage to environment 

disadvantages:

  • arent many suitable locations for power plants 
  • cost of building power plant is high compared to amount of energy produced 
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hydro-electric power

uses falling water. usually requires flooding of valley by building a big dam

advantages:

  • no pollution (as such)
  • can provide an immediate response to increased demand for electricity 
  • reliable (except in times of drought)
  • no fuel costs and minimal running costs 
  • useful for generating electricity on small scale and in remote areas
  • putting power stations in remote valleys tends to reduce impact on humans

disadvantages

  • initial costs are high
  • big environmental impact due to flooding of valley (releases methane and CO2)
  • possible loss of habitat for some species
  • reservoirs can look unsightly when dry up  
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wave power

wave power turbines located around the coast 

advantages:

  • no pollution 
  • no fuel costs and minimal running costs 
  • can be very useful on small scale eg on small islands 

disadvantages:

  • disturb seabed and habitats of marine animals 
  • spoil view 
  • hazard to boats 
  • fairly unreliable (waves tend to die out when wind drops)
  • initial costs are high 
  • not likely to provide energy on high scale 
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tidal barrages

tides made by sun and moons gravity. big dams built acorss river estuaries with turbines in them

advantages:

  • no pollution 
  • reliable (happen twice a day without fail)
  • no fuel costs and minimal running costs 
  • has potential to generate significant amount of energy 

disadvantages:

  • prevent free access by boats
  • spoil view 
  • alter habitat of wildlife
  • height of tide is variable (lower tides produce less energy)
  • dont work when water same height on either side of barrage 
  • initial costs moderately high 
  • only used in suitable estuaries 
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non-renewable energy

it is reliable but can cause many problems such as:

  • coal oil and gas produce CO2 when burnt. CO2 adds to greenhouse effect and global warming 
  • burning coal and oil releases sulfur dioxide which causes acid rain - harmful (acid rain can be reduced by taking sulfur out before fuel burnt or by cleaning up emissions)
  • coal mining makes mess of landscape. view spoilt by power plants 
  • oil splillages cause environmental problems - affects mammals and birds in/around sea
  • nuclear power is clean but nuclear waste is very dangerous and difficult to dispose of 
  • nuclear fuel relatively cheap but overall cost is high 
  • nuclear power always carries risk of major catasrophe eg Fukushima disaster in Japan
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Electricity

Electric charge is a fundamental property of matter everywhere. Understanding the difference in the microstructure of conductors, semiconductors and insulators makes it possible to design components and build electric circuits. Many circuits are powered with mains electricity, but portable electrical devices must use batteries of some kind.

Electrical power fills the modern world with artificial light and sound, information and entertainment, remote sensing and control.

The fundamentals of electromagnetism were worked out by scientists of the 19th century. However, power stations, like all machines, have a limited lifetime. If we all continue to demand more electricity this means building new power stations in every generation – but what mix of power stations can promise a sustainable future? 

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standard circuit diagrams and symbols

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Electrical charge and current

For electrical charge to flow through a closed circuit the circuit must include a source of potential difference.

Electric current is a flow of electrical charge. The size of the electric current is the rate of flow of electrical charge. Charge flow, current and time are linked by the equation:

charge flow = current × time

Q =It
charge flow, Q, in coulombs, C
current,
I, in amperes, A (amp is acceptable for ampere)
time,
t, in seconds, s
A current has the same value at any point in a single closed loop. 

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Current, resistance and potential difference

The current (I) through a component depends on both the resistance (R) of the component and the potential difference (V) across the component. The greater the resistance of the component the smaller the current for a given potential difference (pd) across the component.

Questions will be set using the term potential difference. Students will gain credit for the correct use of either potential difference or voltage.

Current, potential difference or resistance can be calculated using the equation:

potential difference = current × resistance

V=IR
potential difference, V, in volts, V
current,
I, in amperes, A (amp is acceptable for ampere) resistance, R, in ohms, Ω 

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Resistors

for some resistors, the value of R remains constant but that in others it can change as the current changes.

The resistance of a thermistor decreases as the temperature increases.

The applications of thermistors in circuits eg a thermostat is required.

The resistance of an LDR decreases as light intensity increases.

The application of LDRs in circuits eg switching lights on when it gets dark is required. 

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filament lamp

The resistance of components such as lamps, diodes, thermistors and LDRs is not constant; it changes with the current through the component.

The resistance of a filament lamp increases as the temperature of the filament increases. 

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ohmic conductor

The current through an ohmic conductor (at a constant temperature) is directly proportional to the potential difference across the resistor. This means that the resistance remains constant as the current changes.

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diode

The current through a diode flows in one direction only. The diode has a very high resistance in the reverse direction.

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Series and parallel circuits

There are two ways of joining electrical components, in series and in parallel. Some circuits include both series and parallel parts.

For components connected in series:

  • there is the same current through each component
  • the total potential difference of the power supply is shared between the components
  • the total resistance of two components is the sum of the resistance of each component.
  • Rtotal = R1 + R2         resistance, R, in ohms, Ω

For components connected in parallel:

  • the potential difference across each component is the same
  • the total current through the whole circuit is the sum of the currents through the separate components
  • the total resistance of two resistors is less than the resistance of the smallest individual resistor. 
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Direct and alternating potential difference

Mains electricity is an ac supply. In the United Kingdom the domestic electricity supply has a frequency of 50 Hz and is about 230 V.

direct current (dc) - cells and batteries supply direct current. direct current is a current that always flows in the same direction. its created by direct voltage

alternating current (ac) - current constantly changing direction. produced by alternating voltages in which the positive and negative end keeps alternating small.pngsmall.png

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Mains electricity

Most electrical appliances are connected to the mains using three-core cable.

The insulation covering each wire is colour coded for easy identification:                                   - live wire – brown                                                                                                                       - neutral wire – blue                                                                                                                     - earth wire – green and yellow stripes

The live wire carries the alternating potential difference from the supply. The neutral wire completes the circuit. The earth wire is a safety wire to stop the appliance becoming live.

The potential difference between the live wire and earth (0 V) is about 230 V. The neutral wire is at, or close to, earth potential (0 V). The earth wire is at 0 V, it only carries a current if there is a fault.

 

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the live wire

your body ( just like the earth) is at 0 V. if you touch a live wire, a large potential difference is produced across your body and a current flows through you 

a live wire may be dangerous even when a switch in the mains circuit is open as there is still a pd in the wire. if you made contact with the wire your body would provide a link between the supply and earth so a current would flow through you. 

any connnection between live and earth can be dangerous. if the link creates a low resistance path to earth, a huge current will flow, which could result in fire 

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energy transfers - Power

  • when an electrical charge goes through a change in potential difference, then energy is transferred 
  • energy is supplied to the charge at the power source to 'raise' it through a potential
  • the charge gives up the energy when it 'falls' through any potential drop in components elsewhere in the circuit 

power = potential difference × current

P=VI
power = current 2 × resistance

P = 2 R

power, P, in watts, W
potential difference, V, in volts, V
current, I, in amperes, A (amp is acceptable for ampere) resistance, R, in ohms, Ω 

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Energy transfers in everyday appliances

Everyday electrical appliances are designed to bring about energy transfers.

The amount of energy an appliance transfers depends on how long the appliance is switched on for and the power of the appliance. 

Kettles transfer energy electrically from the mains ac supply to the thermal energy store of the heating element inside the kettle 

energy is transferred electrically from the battery of a handheld fan to the kinetic energy store of the fans motor 

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work done

Work is done when charge flows in a circuit.

The amount of energy transferred by electrical work can be calculated using the equation:

  • energy transferred = power × time     (E =Pt)
  • energy transferred = charge flow × potential difference  (E =QV)

energy transferred, E, in joules, J

power, P, in watts, W

time, t, in seconds, s

charge flow, Q, in coulombs, C

potential difference, V, in volts, V

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energy transferred depends on power

the total energy transferred by an appliance depends on how long the appliance is on for and its power 

the power of an appliance is the energy that it transfers per second so the more energy it transfers in a given time the higher its power 

appliances are given a power rating which is the maximum safe power they can operate at

the power rating tells you the maximum amount of energy transferred between stores per second when the appliance is in use 

a higher power doesnt mean that it transfers more energy usefully. an appliance may be more powerful but less efficient 

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The National Grid

The National Grid is a system of cables and transformers linking power stations to consumers.

Electrical power is transferred from power stations to consumers using the National Grid.

Step-up transformers at power stations produce the very high voltages needed to transmit electricity through the National Grid power lines. This is because high voltages improve efficiency by reducing heat loss in the power lines.

But high voltages are too dangerous for use in the home, so step-down transformers are used locally to reduce the voltage to safe levels.

Power lines and substations are potentially dangerous as an electric shock can kill someone who gets too close to such a high voltage supply.

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static electricity

When certain insulating materials are rubbed against each other they become electrically charged. Negatively charged electrons are rubbed off one material and on to the other. The material that gains electrons becomes negatively charged. The material that loses electrons is left with an equal positive charge.

When two electrically charged objects are brought close together they exert a force on each other. Two objects that carry the same type of charge repel. Two objects that carry different types of charge attract. Attraction and repulsion between two charged objects are examples of non-contact force. 

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electric fields

A charged object creates an electric field around itself. The electric field is strongest close to the charged object. The further away from the charged object, the weaker the field.

A second charged object placed in the field experiences a force. The force gets stronger as the distance between the objects decreases. 

if the field is strong enough, charges can be forced though insulators such as air and a spark will occur. This is what happens during a lightning strike. It may also happen if a charged person touches a conductor. 

A Van de Graaff generator removes electrons to produce a positive charge. A person does not have to touch the Van de Graaff generator to start feeling the effects, as static electricity is a non-contact force. This force will act on any charged particle in the electric field around the generator.

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Particle model of matter

The particle model is widely used to predict the behaviour of solids, liquids and gases and this has many applications in everyday life. It helps us to explain a wide range of observations and engineers use these principles when designing vessels to withstand high pressures and temperatures, such as submarines and spacecraft. It also explains why it is difficult to make a good cup of tea high up a mountain! 

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Density of materials

The density of a material is defined by the equation:

density = mass / volume 

ρ = Vm

density, ρ, in kilograms per metre cubed, kg/m3 mass, m, in kilograms, kg
volume,
V, in metres cubed, m3

The particle model can be used to explain

  • the different states of matter
  • differences in density. 
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Changes of state

a change of state conserves mass

when you heat a liquid it boils (or evaporates) and becomes a gas. when you heat a solid it melts and becomes a liquid. these are changes of state. the state can also change due to cooling. the particles lose energy and form bonds

Changes of state are physical changes which differ from chemical changes because the material recovers its original properties if the change is reversed. 

the number of particles doesnt change, they are just arranged differently so mass is conserved 

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Internal energy

Energy is stored inside a system by the particles (atoms and molecules) that make up the system. This is called internal energy.

Internal energy is the total kinetic energy and potential energy of all the particles (atoms and molecules) that make up a system.

Heating changes the energy stored within the system by increasing the energy of the particles that make up the system. This either raises the temperature of the system or produces a change of state. 

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Temperature changes in a system and specific heat

If the temperature of the system increases, the increase in temperature depends on the mass of the substance heated, the type of material and the energy input to the system.

The following equation applies:

change in thermal energy = mass × specific heat capacity x temperature change

E=mcθ

change in thermal energy, ∆E, in joules, J
mass, m, in kilograms, kg

specific heat capacity, c, in joules per kilogram per degree Celsius, J/kg °C temperature change, ∆θ, in degrees Celsius, °C.

The specific heat capacity of a substance is the amount of energy required to raise the temperature of one kilogram of the substance by one degree Celsius. 

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Changes of heat and specific latent heat

If a change of state happens:

The energy needed for a substance to change state is called latent heat. When a change of state occurs, the energy supplied changes the energy stored (internal energy) but not the temperature.

The specific latent heat of a substance is the amount of energy required to change the state of one kilogram of the substance with no change in temperature.

energy for a change of state = mass × specific latent heat

E =mL
energy, E, in joules, J
mass, m, in kilograms, kg
specific latent heat, L, in joules per kilogram, J/kg
Specific latent heat of fusion – change of state from solid to liquid Specific latent heat of vaporisation – change of state from liquid to vapour 

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Particle motion in gases

The molecules of a gas are in constant random motion. The temperature of the gas is related to the average kinetic energy of the molecules.

  • as gas particles move at high speeds they collide and exert a force on the thing they collide with 
  • faster particles and more frequent collisions lead to increase in net force and so gas pressure. increasing temp will increase the speed and so pressure (if volume kept constant)
  • if temp is constant increasing volume of gas means particles get more spread out and hit walls less often. the gas pressure decreases 
  • PRESSURE AND VOLUME ARE INVERSELY PROPORTIONAL

Changing the temperature of a gas, held at constant volume, changes the pressure exerted by the gas. 

  • the pressure of a gas causes a net outwards force at right angles to the surface of its container 
  • there is also a force on the outside of the container due to pressure of the gas around it 
  • if a container can easily change its size then any change in these pressures will cause the container to compress or expand due to the overall force 
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Pressure in gases

A gas can be compressed or expanded by pressure changes. The pressure produces a net force at right angles to the wall of the gas container (or any surface). 

For a fixed mass of gas held at a constant temperature: pressure × volume = constant

p V = constant
pressure, p, in pascals, Pa volume, V, in metres cubed, m

Increasing the pressure of a gas 

Work is the transfer of energy by a force.

Doing work on a gas increases the internal energy of the gas and can cause an increase in the temperature of the gas.  transfers energy 

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Atomic structure

Ionising radiation is hazardous but can be very useful. Although radioactivity was discovered over a century ago, it took many nuclear physicists several decades to understand the structure of atoms, nuclear forces and stability. Early researchers suffered from their exposure to ionising radiation. Rules for radiological protection were first introduced in the 1930s and subsequently improved. Today radioactive materials are widely used in medicine, industry, agriculture and electrical power generation. 

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The structure of an atom

Atoms are very small, having a radius of about 1 × 10-10 metres.

The basic structure of an atom is a positively charged nucleus composed of both protons and neutrons surrounded by negatively charged electrons.

The radius of a nucleus is less than 1/10 000 of the radius of an atom. Most of the mass of an atom is concentrated in the nucleus.

The electrons are arranged at different distances from the nucleus (different energy levels). The electron arrangements may change with the absorption of electromagnetic radiation (move further from the nucleus; a higher energy level) or by the emission of electromagnetic radiation (move closer to the nucleus; a lower energy level). 

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Mass number, atomic number and isotopes

In an atom the number of electrons is equal to the number of protons in the nucleus. Atoms have no overall electrical charge.

All atoms of a particular element have the same number of protons. The number of protons in an atom of an element is called its atomic number.

The total number of protons and neutrons in an atom is called its mass number.

Atoms can be represented as shown in this example:

  (mass number)   23 ^Na    (atomic number) 11

Atoms of the same element can have different numbers of neutrons; these atoms are called isotopes of that element.

Atoms turn into positive ions if they lose one or more outer electron(s). 

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The development of the model of the atom

  • New experimental evidence may lead to a scientific model being changed or replaced.
  • Before the discovery of the electron, atoms were thought to be tiny spheres that could not be divided.
  • The discovery of the electron led to the plum pudding model of the atom. The plum pudding model suggested that the atom is a ball of positive charge with negative electrons embedded in it.
  • The results from the alpha particle scattering experiment led to the conclusion that the mass of an atom was concentrated at the centre (nucleus) and that the nucleus was charged. This nuclear model replaced the plum pudding model.
  • Niels Bohr adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances. The theoretical calculations of Bohr agreed with experimental observations.
  • Later experiments led to the idea that the positive charge of any nucleus could be subdivided into a whole number of smaller particles, each particle having the same amount of positive charge. The name proton was given to these particles.
  • The experimental work of James Chadwick provided the evidence to show the existence of neutrons within the nucleus. This was about 20 years after the nucleus became an accepted scientific idea. 
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Radioactive decay and nuclear radiation

Some atomic nuclei are unstable. The nucleus gives out radiation as it changes to become more stable. This is a random process called radioactive decay.

Activity is the rate at which a source of unstable nuclei decays. Activity is measured in becquerel (Bq)

Count-rate is the number of decays recorded each second by a detector (eg Geiger-Muller tube).

The nuclear radiation emitted may be:

  • an alpha particle (α) – this consists of two neutrons and two protons, it is the same as a helium nucleus
  • a beta particle (β) – a high speed electron ejected from the nucleus as a neutron turns into a proton
  • a gamma ray (γ) – electromagnetic radiation from the nucleus
  • a neutron (n). 
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alpha, beta, gamma

alpha particles

  • dont penetrate very far into materials and are stopped quickly
  • they can only travel a few cm  in the air and are
  • absorbed by a sheet of paper 
  • because of their size they are strongly ionising 

beta 

  • moderately ionising. penetrate moderately far into materials before colliding 
  • can travel a few metres in air 
  • absorbed by sheet of aluminium 
  • for every beta particle emitted, a neutron in the nucleus has turned into a proton 

gamma

  • penetrate far into materials without being stopped. travel long distance through air 
  • weakly ionising - pass through rather than collide with atoms. eventually hit something and do damage 
  • can be absorbed by thick sheets of lead or metres of concrete 
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Nuclear equations

Nuclear equations are used to represent radioactive decay.

In a nuclear equation an alpha particle may be represented by the symbol:   4^ He                                                                                                                                                                     .                                                                                                                        2 

and a beta particle by the symbol:   0^e                                                                                                    .                                                       -1

The emission of the different types of nuclear radiation may cause a change in the mass and /or the charge of the nucleus. 

So alpha decay causes both the mass and charge of the nucleus to decrease.

So beta decay does not cause the mass of the nucleus to change but does cause the charge of the nucleus to increase. 

The emission of a gamma ray does not cause the mass or the charge of the nucleus to change 

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Half-lives and the random nature of radioactive de

Radioactive decay is random.

The half-life of a radioactive isotope is the time it takes for the number of nuclei of the isotope in a sample to halve, or the time it takes for the count rate (or activity) from a sample containing the isotope to fall to half its initial level. 

Calculating the isotope remaining  It should also be possible to state how much of a sample remains or what the activity or count should become after a given length of time. This could be stated as a fraction, decimal or ratio. For example the amount of a sample remaining after four half-lives could be expressed as:
a fraction - a ½ of a ½ of a ½ of a ½ remains, which is ½ × ½ × ½ × ½ = 1/16 of the original sample This could then be incorporated into other data. So if the half-life is two days, four half-lives is 8 days. So suppose a sample has a count rate of 3,200 Becquerel (Bq) at the start, what its count rate would be after 8 days would be 1/16th of 3,200 Bq = 200 Bq.

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Radioactive contamination

Radioactive contamination is the unwanted presence of materials containing radioactive atoms on other materials. The hazard from contamination is due to the decay of the contaminating atoms. The type of radiation emitted affects the level of hazard.

Irradiation is the process of exposing an object to nuclear radiation. The irradiated object does not become radioactive.

Suitable precautions must be taken to protect against any hazard that the radioactive source used in the process of irradiation may present

Suitable precautions must be taken to protect against any hazard that the radioactive source used in the process of irradiation may present. 

it is important for the findings of studies into the effects of radiation on humans to be published and shared with other scientists so that the findings can be checked by peer review. 

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contamination

Advantages of contamination

Radioactive isotopes can be used as medical and industrial tracers

Disadvantages of contamination

Radioactive isotopes may not go where they are wanted

Use of isotopes with a short half-life means exposure can be limited

It can be difficult to ensure that the contamination is fully removed so small amounts of radioisotope may still be left behind Imaging processes can replace some invasive surgical procedures

Exposure to radioactive materials can potentially damage healthy cells

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irradiation.

Contamination Occurs when an object is exposed to a source of radiation outside the object Occurs if the radioactive source is on or in the object Doesn’t cause the object to become radioactive A contaminated object will be radioactive for as long as the source is on or in it Can be blocked with suitable shielding Once an object is contaminated, the radiation cannot be blocked from it Stops as soon as the source is removed It can be very difficult to remove all of the contamination

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Background radiation

Background radiation is around us all of the time. It comes from:

  • natural sources such as rocks and cosmic rays from space
  • man-made sources such as the fallout from nuclear weapons testing and nuclear accidents.

The level of background radiation and radiation dose may be affected by occupation and/or location.
Radiation dose is measured in sieverts (Sv) 1000 millisieverts (mSv) = 1 sievert (Sv)

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Different half-lives of radioactive isotopes

Radioactive isotopes have a very wide range of half-life values.  However, they do not stay radioactive forever. Each radioactive material has a half-life, and after this time it will have only half the activity it had before. Another half-life, and it's down to a quarter. Eventually, the activity will be similar to background radiation, and the material will be safe. For some sources, this could be thousands of years.

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Uses of nuclear radiation

Nuclear radiations are used in medicine for the:

  • exploration of internal organs
  • control or destruction of unwanted tissue

IonisationNuclear radiation ionises materials. This changes atoms or molecules into charged particles.

Uses of alpha radiationIonisation is useful for smoke detectors. Radioactive americium releases alpha radiation, which ionises the air inside the detector. Smoke from a fire absorbs alpha radiation, altering the ionisation and triggering the alarm. 

Uses of gamma radiationGamma radiation is used in the treatment of cancer, testing equipment and sterilising medical instruments.

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uses of beta radiation

Beta radiation is used for tracers and monitoring the thickness of materials.

Doctors may use radioactive chemicals called tracers for medical imaging. Certain chemicals concentrate in different damaged or diseased parts of the body, and the radiation concentrates with it. Radiation detectors placed outside the body detect the radiation emitted and, with the aid of computers, build up an image of the inside of the body.

Radiation is used in industry in detectors that monitor and control the thickness of materials such as paper, plastic and aluminium. The thicker the material, the more radiation is absorbed and the less radiation reaches the detector. It then sends signals to the equipment that adjusts the thickness of the material. 

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Nuclear fission & nuclear fusion

Nuclear fission is the splitting of a large and unstable nucleus (eg uranium or plutonium).

Spontaneous fission is rare. Usually, for fission to occur the unstable nucleus must first absorb a neutron.

The nucleus undergoing fission splits into two smaller nuclei, roughly equal in size, and emits two or three neutrons plus gamma rays. Energy is released by the fission reaction.

All of the fission products have kinetic energy. The neutrons may go on to start a chain reaction.

The chain reaction is controlled in a nuclear reactor to control the energy released. The explosion caused by a nuclear weapon is caused by an uncontrolled chain reaction. 

Nuclear fusion is the joining of two light nuclei to form a heavier nucleus. In this process some of the mass may be converted into the energy of radiation. 

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Forces

Engineers analyse forces when designing a great variety of machines and instruments, from road bridges and fairground rides to atomic force microscopes. Anything mechanical can be analysed in this way. Recent developments in artificial limbs use the analysis of forces to make movement possible 

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Scalar and vector quantities

Scalar quantities have magnitude only.
Vector quantities have magnitude and an associated direction.

A vector quantity may be represented by an arrow. The length of the arrow represents the magnitude, and the direction of the arrow the direction of the vector quantity. 

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Contact and non-contact forces

A force is a push or pull that acts on an object due to the interaction with another object. All forces between objects are either:

  • contact forces – the objects are physically touching
  • non-contact forces – the objects are physically separated.

Examples of contact forces include friction, air resistance, tension and normal contact force.

Examples of non-contact forces are gravitational force, electrostatic force and magnetic force.

Force is a vector quantity.

when two objects interact there is a force produced on both objects.

an ineraction pair is a pair of forces that are equal and opposite and act on two interacting objects (basically Newton's third law)

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gravity

Weight is the force acting on an object due to gravity. The force of gravity close to the Earth is due to the gravitational field around the Earth.

The weight of an object depends on the gravitational field strength at the point where the object is.

The weight of an object can be calculated using the equation:
weight = mass × gravitational field strength W =mg
weight, W, in newtons, N mass, m, in kilograms, kg gravitational field strength, g, in newtons per kilogram, N/kg (In any calculation the value of the gravitational field strength (g) will be given.) The weight of an object may be considered to act at a single point referred to as the object’s ‘centre of mass’. The weight of an object and the mass of an object are directly proportional. Weight is measured using a calibrated spring-balance (a newtonmeter). 

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Resultant forces

A number of forces acting on an object may be replaced by a single force that has the same effect as all the original forces acting together. This single force is called the resultant force. 

A stationary object remains stationary if the sum of the forces acting upon it - resultant force - is zero. A moving object with a zero resultant force keeps moving at the same speed and in the same direction.
If the resultant force acting on an object is not zero, a stationary object begins to accelerate in the same direction as the force. A moving object speeds up, slows down or changes direction.
Acceleration depends on the force applied to an object and the object's mass.

A single force can be resolved into two components acting at right angles to each other. The two component forces together have the same effect as the single force. 

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Work done and energy transfer

When a force causes an object to move through a distance work is done on the object. So a force does work on an object when the force causes a displacement of the object.

The work done by a force on an object can be calculated using the equation:

work done = force × distance moved along the line of action of the force W =Fs

work done, W, in joules, J force, F, in newtons, N

distance, s, in metres, m
One joule of work is done when a force of one newton causes a 
displacement of one metre.

1 joule = 1 newton-metre 

Work done against the frictional forces acting on an object causes a rise in the temperature of the object. 

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Forces and elasticity

to stretch bend or compress an object you need more than one force acting on it otherwise it would move in direction of the applied force 

an object has been elastically deformed if it can go back to its original shape and length after the force has been removed eg a spring 

an object has been inelastically deformed if it doesnt return to its original shape and length after the force has been removed 

The extension of an elastic object, such as a spring, is directly proportional to the force applied, provided that the limit of proportionality is not exceeded. 

work is done when a force stretches or compresses an object and causes energy to be transferred to the elastic potential energy store of the object. if it is elastically deformed ALL this energy is transferred to the objects elastic potential energy store 

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forces and elasticity 2

force = spring constant × extension

F=ke

force, F, in newtons, N

spring constant, k, in newtons per metre, N/m

extension, e, in metres, m
This relationship also applies to the compression of an elastic object, where

e’ would be the compression of the object. 

elastic potential energy = 0.5 × spring constant × extension

Ee   = 1/2 ke ^2 

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Moments, levers and gears

A force or a system of forces may cause an object to rotate.

The turning effect of a force is called the moment of the force. The size of the moment is defined by the equation:  moment of a force = force × distance   (M=Fd)

moment of a force, M, in newton-metres, Nm. force, F, in newtons, N. distance, d, is the perpendicular distance from the pivot to the line of action of the force, in metres, m.

If an object is balanced, the total clockwise moment about a pivot equals the total anticlockwise moment about that pivot. 

A simple lever and a simple gear system both used to transmit the rotational effects of forces. 

leavers increase distance from pivot at which force is applied - less force = same moment. leavers make it easier to do work

different sized gears used to change moment of force. force is transmitted to a larger gear will cause a bigger moment as distance to pivot is bigger. 

larger gears turn slower than smaller gears 

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Pressure in a fluid

A fluid can be either a liquid or a gas.

The pressure in fluids causes a force normal (at right angles) to any surface.

The pressure at the surface of a fluid can be calculated using the equation:

pressure =  force normal to a surface / area of that surface 

p= f/a

pressure, p, in pascals, Pa force, F, in newtons, N
area, A, in metres squared, m

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Pressure in a fluid 2

The pressure due to a column of liquid can be calculated using the equation:

 pressure = height of the column × density of the liquid × gravitational field strength [p=hρg]

pressure, p, in pascals, Pa.  height of the column, h, in metres, m. density, ρ, in kilograms per metre cubed, kg/m3 .  gravitational field strength, g, in newtons per kilogram, N/kg (In any calculation the value of the gravitational field strength (g) will be given.)

the more dense a given liquid is the more particles it has in a certain space. this means that there are more particles that are able to collide so the pressure is higher. as the depth of a liquid increases the number of particles above that point increases. the weight of these particles adds to the pressure felt at that point, so liquid pressure increases with depth 

A partially (or totally) submerged object experiences a greater pressure on the bottom surface than on the top surface. This creates a resultant force upwards. This force is called the upthrust.

upthrust is equal to the weight of the fluid that has been displaced by the object. 

an object floats if its weight = upthrust. sinks if weight is larger than upthrust 

 

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Atmospheric pressure

The atmosphere is a thin layer (relative to the size of the Earth) of air round the Earth. The atmosphere gets less dense with increasing altitude.

Air molecules colliding with a surface create atmospheric pressure.
The number of air molecules (and so the weight of air) above a surface decreases as the height of the surface above ground level increases. So as height increases there is always less air above a surface than there is at a lower height. So atmospheric pressure decreases with an increase in height. 

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Distance and displacement

Distance is how far an object moves. Distance does not involve direction. Distance is a scalar quantity.

Displacement includes both the distance an object moves, measured in a straight line from the start point to the finish point and the direction of that straight line. Displacement is a vector quantity ( both magnitude and direction)

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speed

Speed does not involve direction. Speed is a scalar quantity.

The speed of a moving object is rarely constant. When people walk, run or travel in a car their speed is constantly changing.

The speed at which a person can walk, run or cycle depends on many factors including: age, terrain, fitness and distance travelled. 

Typical values may be taken as:

  • walking  - 1.5 m/s
  • running - 3 m/s
  • cycling - 6 m/s 
  • a car - 25 m/s
  • a train - 55 m/s
  • a plane 250 m/s

 

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speed 2

It is not only moving objects that have varying speed. The speed of sound and the speed of the wind also vary.

A typical value for the speed of sound in air is 330 m/s. 

For an object moving at constant speed the distance travelled in a specific time can be calculated using the equation:

distance travelled = speed × time

s =vt

distance, s, in metres, m

speed, v, in metres per second, m/s

time, t, in seconds, s

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Velocity

The velocity of an object is its speed in a given direction. Velocity is a vector quantity. 

you can have objects travelling at a constant speed with a changing velocity. this happens when the object is changing direction whilst staying at the same speed. 

an object moving in a circle at a constant speed has a constantly changing velocity, as the direction is always changing 

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The distance–time relationship

If an object moves along a straight line, the distance travelled can be represented by a distance–time graph.

The speed of an object can be calculated from the gradient of its distance–time graph.

If an object is accelerating, its speed at any particular time can be determined by drawing a tangent and measuring the gradient of the distance–time graph at that time. 

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Acceleration

The average acceleration of an object can be calculated using the equation:

  • acceleration = change in velocity / time taken  [a = ∆v / t]
  • acceleration, a, in metres per second squared, m/s2. change in velocity, ∆v, in metres per second, m/s. time, t, in seconds, s 

An object that slows down is decelerating. the acceleration of an object can be calculated from the gradient of a velocity–time graph. The distance travelled by an object (or displacement of an object) can be calculated from the area under a velocity–time graph. 

The following equation applies to uniform acceleration: 

  • (final velocity)²− (initial velocity)²= 2 × acceleration × distance  [v² u²= 2as]
  • final velocity, v, in metres per second, m/s. initial velocity, u, in metres per second, m/s acceleration, a, in metres per second squared, m/s2. distance, s, in metres, m

Near the Earth’s surface any object falling freely under gravity has an acceleration of about 9.8 m/s2An object falling through a fluid initially accelerates due to the force of gravity. Eventually the resultant force will be zero and the object will move at its terminal velocity. 

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Newton’s First Law

Newton’s First Law:

If the resultant force acting on an object is zero and:

  • the object is stationary, the object remains stationary
  • the object is moving, the object continues to move at the same speed and in the same direction. So the object continues to move at the same velocity.

So, when a vehicle travels at a steady speed the resistive forces balance the driving force. So, the velocity (speed and/or direction) of an object will only change if a resultant force is acting on the object. The tendency of objects to continue in their state of rest or of uniform motion is called inertia. 

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Newton’s Second Law

The acceleration of an object is proportional to the resultant force acting on the object, and inversely proportional to the mass of the object.

As an equation:
resultant force = mass × acceleration  [F = ma]

force, F, in newtons, N. mass, m, in kilograms, kg. acceleration, a, in metres per second squared, m/s

inertia is the tendency for motion to remain unchanged

  • until acted upon by a resultant force objects at rest stay at rest and objects moving at a steady speed will stay moving at that speed (newtons first law). this tendency to continue in the same state of motion is called inertia
  • an objects inertial mass measures how difficult it is to change the velocity of an object 
  • inertial mass can be found using Newtons second law of F = ma. rearranging this gives         m = F/a so inertial mass is just the ratio of the force of acceleration 
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Newton’s Third Law

Whenever two objects interact, the forces they exert on each other are equal and opposite. 

it can be easy to get confused with Newtons Third Law when an object is in equilibrium. a book resting on the ground is in equilibrium. the weight of the book is equal to the normal contact force

but this is NOT Newton's Third Law because the two forces are different types and both acting on the book 

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Stopping distance

The stopping distance of a vehicle is the sum of the distance the vehicle travels during the driver’s reaction time (thinking distance) and the distance it travels under the braking force (braking distance). For a given braking force the greater the speed of the vehicle, the greater the stopping distance. - combination of thinking distance and braking distance 

in an emergency a driver may perform an emergency stop. this is where maximum force is applied by the brakes in order to stop the car in the shortest possible distance. the longer it takes to perform an emergency stop the higher the risk of crashing into whatevers in front

stopping distance = thinking distance / braking distance 

speed affects braking distance more than thinking distances 

  • as a car speeds up the thinking distance increases at the same rate as speed. this is because thinking time stays pretty constant - but the higher the speed the more distnace you cover in that time. 
  • braking distance increases faster the more you speed up. the work done to stop the car is equal to the energy in the cars kinetic energy store (1/2mv²)
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reaction time

Reaction times vary from person to person. Typical values range from 0.2 s to 0.9 s.

A driver’s reaction time can be affected by tiredness, drugs and alcohol. Distractions may also affect a driver’s ability to react. 

you can measure reaction times with the ruler drop test 

another way of measuring reaction times is to use a computer-based test 

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Factors affecting braking distance 1

The braking distance of a vehicle can be affected by adverse road and weather conditions and poor condition of the vehicle. 

Adverse road conditions include wet or icy conditions. Poor condition of the vehicle is limited to the vehicle’s brakes or tyres. 

  • Speed - the faster a vehicle travels, the longer it takes to stop 
  • weather or road surface - if its wet or icy there is less grip between vehicles tyres and the road which causes tyres to skid
  • condition of tyres - if tyres are bald (dont have any thread left) they cant get rid of water in wet conditions. this leads to them skidding on top of water 
  • brakes - if brakes are worn or faulty they wont be able to apply as much force as well-maintained brakes, which could be dangerous when you need to brake hard 
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Factors affecting braking distance 2

When a force is applied to the brakes of a vehicle, work done by the friction force between the brakes and the wheel reduces the kinetic energy of the vehicle and the temperature of the brakes increases.

The greater the speed of a vehicle the greater the braking force needed to stop the vehicle in a certain distance.

The greater the braking force the greater the deceleration of the vehicle. Large decelerations may lead to brakes overheating and/or loss of control.

a larger braking distance means larger deceleration. very large decelerations can be dangerous because they may cause breaks to overheat (dont work as well) or cause vehicle to skid 

 

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thinking distance

how far car travels during drivers reaction time 

affected by:

  • speed - faster vehicle travels, longer it takes to stop
  • reaction time - longer your reaction time, the longer your thinking distance 
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Momentum is a property of moving objects

Momentum is defined by the equation:

momentum = mass × velocity

p = mv

momentum, p, in kilograms metre per second, kg m/s

mass, m, in kilograms, kg

velocity, v, in metres per second, m/s 

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Conservation of momentum

In a closed system, the total momentum before an event is equal to the total momentum after the event.

This is called conservation of momentum. 

momentum in a collision:

before - the momentum was equal to mass of moving object x its velocity                                   after - the mass of the moving object has increased but its momentum is equal to the momentum         before the collision. so an increase in mass causes a decrease in velocity 

if the momentum before an event is 0, the momentum after will also be 0 e.g in an explosion the momentum before is 0. after the explosion pieces fly off in diffrent directions so the total momentum cancels out to 0. 

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Changes in momentum

When a force acts on an object that is moving, or able to move, a change in momentum occurs.

The equations F = m × a and a= v-u / t  combine to give the equation F = mv / ∆t 

where mv = change in momentum - ie force equals the rate of change of momentum

cars have many safety features such as:

  • crumple zones crumple on impact, increasing time taken for the car to stop 
  • seat belts stretch slightly increasing time taken for wearer to stop 
  • air bags inflate before you hit the dashboard of a car. the compressing air inside it slows you down more gradually than if you had just hit the dashboard 

bike helmets contain a crushable layer of foam which helps lengthen the time taken for your head to stop in a crash. this reduces the impact on your brain 

crash mats and cushioned playground flooring increase the time taken for you to stop if you fall on them. this is because they are made from soft, compressible materials 

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waves

Wave behaviour is common in both natural and man-made systems. Waves carry energy from one place to another and can also carry information. Designing comfortable and safe structures such as bridges, houses and music performance halls requires an understanding of mechanical waves. Modern technologies such as imaging and communication systems show how we can make the most of electromagnetic waves. 

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Transverse and longitudinal waves

Waves may be either transverse or longitudinal.
The ripples on a water surface are an example of a transverse wave.

Longitudinal waves show areas of compression and rarefaction. Sound waves travelling through air are longitudinal. 

in transverse waves, the oscillations (vibrations) are perpendicular to the direction of energy transfer. examples - electromagnetic waves (eg light), ripples and waves in water, a wave on a string

in longitudal waves, the oscillations are parallel to the direction of energy transfer examples are sound waves in air, ultrasound, shock waves eg some seismic waves 

wave speed = frequency x wavelength 

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Properties of waves

The amplitude of a wave is the maximum displacement of a point on a wave away from its undisturbed position.

The wavelength of a wave is the distance from a point on one wave to the equivalent point on the adjacent wave.

The frequency of a wave is the number of waves passing a point each second.

period = 1 / frequency

T = 1 / f 

period, T, in seconds, s

frequency, f, in hertz, Hz

The wave speed is the speed at which the energy is transferred (or the wave moves) through the medium. 

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Reflection of waves

Waves can be reflected at the boundary between two different materials.

Waves can be absorbed or transmitted at the boundary between two different materials.

when waves arrive at a boundary three things can happen:

  • the waves are absorbed by the material the wave is trying to cross into - this transfers energy to the materials energy stores 
  • the waves are transmitted - the waves carry on travelling through the new material - this often leads to refraction 
  • the waves are reflected 

specular reflection happens when a wave is reflected in a single direction by a smooth surface 

diffuse reflection is when a wave is reflected by a rough surface and the reflected rays are scattered into lots of different directions

this happens because the normal is diffrent for each incoming ray so the angle of incidence is different for each ray                                                                                                                when light is reflected by a rough surface, the surface appears matte and you dont get a clear reflection of objects

 

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ray diagrams

In a ray diagram, the mirror is drawn a straight line with thick hatchings to show which side has the reflective coating. The light rays are drawn as solid straight lines, each with an arrowhead to show the direction of travel. Light rays that appear to come from behind the mirror are shown as dashed straight lines.

Make sure that the incident rays (the solid lines) obey the law of reflection: the angle of incidence equals the angle of reflection. Extend two lines behind the mirror. They cross where the image appears to come from.

The image in a plane mirror is:

  • virtual (it cannot be touched or projected onto a screen)
  • upright (if you stand in front of a mirror, you look the right way up)
  • laterally inverted (if you stand in front of a mirror, your left side seems to be on the right in the reflection)

17c2e50490d35c218924bdd2cc06069072232253.jpg

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sound waves

Sound waves can travel through solids causing vibrations in the solid. 

  • eg - paper diaphragm in a speaker vibrates back and forth which causes surrounding air to vibrate, creating compressions and rarefactions. a sound wave is created -
  • when the sound wave hits a solid object the air particles hitting the object causes the particles in the solid to move back and forth 
  • these particles hit the next particles in line and so on passing the sound wave through the object as a series of vibrationsWithin the ear, sound waves cause the ear drum and other parts to vibrate which causes the sensation of sound.

The conversion of sound waves to vibrations of solids works over a limited frequency range. This restricts the limits of human hearing.

the range of normal human hearing is from 20 Hz to 20 kHz. human hearing is limited by the size and shape of our ear drum as well as the structure of all parts within the ear that vibrate to transfer energy from the sound wave different materials can convert different frequencies of sound waves into vibrations sound cant travel in space because its mostly a vacuum (no particles to move or vibrate) 

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Waves for detection and exploration

Ultrasound waves have a frequency higher than the upper limit of hearing for humans. Ultrasound waves are partially reflected when they meet a boundary between two different media. The time taken for the reflections to reach a detector can be used to determine how far away such a boundary is. This allows ultrasound waves to be used for both medical and industrial imaging.

Seismic waves are produced by earthquakes. P-waves are longitudinal, seismic waves. P-waves travel at different speeds through solids and liquids. S-waves are transverse, seismic waves. S-waves cannot travel through a liquid. P-waves and S-waves provide evidence for the structure and size of the Earth’s core.

Echo sounding, using high frequency sound waves is used to detect objects in deep water and measure water depth. 

 the study of seismic waves provided new evidence that led to discoveries about parts of the Earth which are not directly observable. 

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Types of electromagnetic waves

Electromagnetic waves are transverse waves that transfer energy from the source of the waves to an absorber.

Electromagnetic waves form a continuous spectrum and all types of electromagnetic wave travel at the same velocity through a vacuum (space) or air.

The waves that form the electromagnetic spectrum are grouped in terms of their wavelength and their frequency. Going from long to short wavelength (or from low to high frequency) the groups are: radio, microwave, infrared, visible light (red to violet), ultraviolet, X-rays and gamma rays.

Our eyes only detect visible light and so detect a limited range of electromagnetic waves. 

long wavelength                                                                                               short wavelength 

radio waves, microwaves, infared, visible light, ultraviolet, x-rays, gamma rays 

low frequency                                                                                                     high frequency 

Our eyes only detect visible light and so detect a limited range of electromagnetic waves. 

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Properties of electromagnetic waves 1

different substances may absorb, transmit, refract or reflect electromagnetic waves in ways that vary with wavelength.

Some effects, for example refraction, are due to the difference in velocity of the waves in different substances. 

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properties of electromagnetic waves 2

Radio waves can be produced by oscillations in electrical circuits.

When radio waves are absorbed they may create an alternating current with the same frequency as the radio wave itself, so radio waves can themselves induce oscillations in an electrical circuit.

Changes in atoms and the nuclei of atoms can result in electromagnetic waves being generated or absorbed over a wide frequency range. Gamma rays originate from changes in the nucleus of an atom.

Ultraviolet waves, X-rays and gamma rays can have hazardous effects on human body tissue. The effects depend on the type of radiation and the size of the dose. Radiation dose (in sieverts) is a measure of the risk of harm resulting from an exposure of the body to the radiation.

1000 millisieverts (mSv) = 1 sievert (Sv)

Ultraviolet waves can cause skin to age prematurely and increase the risk of skin cancer. X-rays and gamma rays are ionising radiation that can cause the mutation of genes and cancer

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Uses and applications of electromagnetic waves

Electromagnetic waves have many practical applications. For example:

  • radio waves – television and radio ( transmissions have very short wavelengths - reflected from the ionoshphere an electrically charged layer in earths atmosphere so recieved by transmitter)
  • microwaves – satellite communications (microwaves can pass easily through earths atmosphere), cooking food penetrate up to a few cm into food before being absorbed and tramsferring energy they are carrying to water molecules in food causing it to heat up)
  • infrared – electrical heaters, cooking food (absorbing IR radiation causes objects to get hotter), infrared cameras
  • visible light – fibre optic communications (light isnt easily absorbed or scattered as it travels along a fibre)
  • ultraviolet – energy efficient lamps, sun tanning 
  • X-rays and gamma rays – medical imaging and treatments (x-rays pass easily through flesh but not through denser materials eg bones) (gamma rays used as tracer as it can pass through the body to be detected) 
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Lenses

A lens forms an image by refracting light. In a convex lens, parallel rays of light are brought to a focus at the principal focus. The distance from the lens to the principal focus is called the focal length. Ray diagrams are used to show the formation of images by convex and concave lenses.

The image produced by a convex lens can be either real or virtual. The image produced by a concave lens is always virtual.

The magnification produced by a lens can be calculated using the equation: magnification = image height / object height 

Magnification is a ratio and so has no units.

Image height and object height should both be measured in either mm or cm.

In ray diagrams a convex lens will be represented by: ff139f242ec01222147e875d52a90cac446ac5a3.gif

A concave lens will be represented by:ed6d98e09e8c6ba14f6f3361e1c6582551ad3d9b.gif

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Visible light

Each colour within the visible light spectrum has its own narrow band of wavelength and frequency.

Reflection from a smooth surface in a single direction is called specular reflection. Reflection from a rough surface causes scattering: this is called diffuse reflection.

Colour filters work by absorbing certain wavelengths (and colour) and transmitting other wavelengths (and colour). 

The colour of an opaque object is determined by which wavelengths of light are more strongly reflected. Wavelengths that are not reflected are absorbed. If all wavelengths are reflected equally the object appears white. If all wavelengths are absorbed the objects appears black. 

for opaque objects that arent a primary colour they may be reflecting either the wavelengths of light corresponding to that colour or the wavelengths of the primary colours that can mix together to make it that colour 

Objects that transmit light are either transparent or translucent. 

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Emission and absorption of infrared radiation

All bodies (objects), no matter what temperature, emit and absorb infrared radiation. The hotter the body, the more infrared radiation it radiates in a given time.

A perfect black body is an object that absorbs all of the radiation incident on it. A black body does not reflect or transmit any radiation. Since a good absorber is also a good emitter, a perfect black body would be the best possible emitter. 

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Perfect black bodies and radiation

A body at constant temperature is absorbing radiation at the same rate as it is emitting radiation. The temperature of a body increases when the body absorbs radiation faster than it emits radiation.
The temperature of the Earth depends on many factors including: the rates of absorption and emission of radiation, reflection of radiation into space.  Bodies emit a continuous range of electromagnetic radiation at different energy values – this means that the radiation that is emitted is spread out over a range of different frequencies and wavelengths.  A perfect black body is a theoreticalobject. It would have these properties:
- it would absorb all the radiation that falls on it
- it would not reflect or transmit any radiation
An object that is good at absorbing radiation is also a good emitter, so a perfect black body would be the best possible emitter of radiation. White and shiny silvery surfaces are the worst absorbers, as they reflect all visible light wavelengths. Poor absorbers are also poor emitters, and do not emit radiation as quickly as darker colours. Radiators in homes are usually painted white so that the infrared radiation is emitted gradually.

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Magnetism and electromagnetism

Electromagnetic effects are used in a wide variety of devices. Engineers make use of the fact that a magnet moving in a coil can produce electric current and also that when current flows around a magnet it can produce movement. It means that systems that involve control or communications can take full advantage of this. 

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Poles of a magnet

The poles of a magnet are the places where the magnetic forces are strongest. When two magnets are brought close together they exert a force on each other. Two like poles (poles that are the same) repel each other. Two unlike poles attract each other. Attraction and repulsion between two magnetic poles are examples of non-contact force.

A permanent magnet produces its own magnetic field.

An induced magnet is a material that becomes a magnet when it is placed in a magnetic field. Induced magnetism always causes a force of attraction. When removed from the magnetic field an induced magnet loses most/all of its magnetism quickly. 

the force between permanent and indued magnets is always attractive 

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Magnetic fields

The region around a magnet where a force acts on another magnet or on a magnetic material (iron, steel, cobalt and nickel) is called the magnetic field.

The force between a magnet and a magnetic material is always one of attraction.

The strength of the magnetic field depends on the distance from the magnet. The field is strongest at the poles of the magnet.

The direction of the magnetic field at any point is given by the direction of the force that would act on another north pole placed
at that point. The direction of a magnetic field line is from the north (seeking) pole of a magnet to the south(seeking) pole of the magnet.

A magnetic compass contains a small bar magnet. The Earth has a magnetic field. The compass needle points in the direction of the Earth’s magnetic field. 

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Detecting magnetic fields

A magnetic field is invisible, but it can be detected using a magnetic compass. A compass contains a small bar magnet on a pivot so that it can rotate. The compass needle points in the direction of the Earth's magnetic field, or the magnetic field of a magnet.

Magnetic fields can be mapped out using small plotting compasses:

  • place the plotting compass near the magnet on a piece of paper
  • mark the direction the compass needle points
  • move the plotting compass to many different positions in the magnetic field, marking the needle direction each time
  • join the points to show the field lines

The needle of a plotting compass points to the south pole of the magnet.

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Electromagnetism

When a current flows through a conducting wire a magnetic field is produced around the wire. The strength of the magnetic field depends on the current through the wire and the distance from the wire.

Shaping a wire to form a solenoid (coil of wire) increases the strength of the magnetic field created by a current through the wire. The magnetic field inside a solenoid is strong and uniform.

The magnetic field around a solenoid has a similar shape to that
of a bar magnet. Adding an iron core increases the strength of the magnetic field of a solenoid. An electromagnet is a solenoid with an iron core. 

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the motor effect

A wire carrying a current creates a magnetic field. This can interact with another magnetic field, causing a force that pushes the wire at right angles. 

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Fleming’s left-hand rule

you can find the direction of the force using Flemming's left-hand rule:

  • point your First finger in the direction of the Field 
  • point you seCond finger in the direction of the Current 
  • you thuMb will then point in the direction of the force (Motion) 

Flemming's left-hand rule shows that if either current or magnetic field is reversed then the direction of the force will also be reversed. this can be used for things like motors 

For a conductor at right angles to a magnetic field and carrying a current:

force = magnetic flux density × current × length F=BIl

force, F, in newtons, N
magnetic flux density, B, in tesla, T
current, I, in amperes, A (amp is acceptable for ampere)                                                              
length, l, in metres, m

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Electric motors

A coil of wire carrying a current in a magnetic field tends to rotate. This is the basis of an electric motor.  Starting from the position shown in the diagram of the dc motor: - current in the left hand part of the coil causes a downward force, and current in the right hand part of the coil causes an upward force - the coil rotates anticlockwise because of the forces described above
When the coil is vertical, it moves parallel to the magnetic field, producing no force. This would tend to make the motor come to a stop, but two features allow the coil to continue rotating:
- the momentum of the motor carries it on round a little
- a split ring commutator changes the current direction every half turn
Once the conducting brushes reconnect with the commutator after a half turn:
- current flows in the opposite direction through the wire in the coil
- each side of the coil is now near the opposite magnetic pole
This means that the motor effect forces continue to cause anticlockwise rotation of the coil.

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Loudspeakers and headphones

Loudspeakers and headphones use the motor effect to convert variations in current in electrical circuits to the pressure variations in sound waves. 

Alternating current supplied to the loudspeaker creates sound waves in the following way:

  • a current in the coil creates an electromagnetic field
  • the electromagnetic field interacts with the permanent magnet generating a force, which pushes the cone outwards
  • the current is made to flow in the opposite direction
  • the direction of the electromagnetic field reverses
  • the force on the cone now pulls it back in
  • repeatedly alternating the current direction makes the cone vibrate in and out
  • the cone vibrations cause pressure variations in the air, which are sound waves

To make a loudspeaker cone vibrate correctly, the electric current must vary in the same way as the desired sound.

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Induced potential

If an electrical conductor moves relative to a magnetic field or if there is a change in the magnetic field around a conductor, a potential difference is induced across the ends of the conductor. If the conductor is part of a complete circuit, a current is induced in the conductor. This is called the generator effect.

An induced current generates a magnetic field that opposes the original change, either the movement of the conductor or the change in magnetic field. 

you can change the size of induced potential by:

  • increasing the speed of the movement 
  • increasing the strength of the magnetic field 

if you move the magnet (or conductor) in the opposite direction then the potential difference/current will be reversed. likewise if the polarity of the magnet is reversed then the potential difference/current will be reversed too if you keep the magnet (or coil) moving backwards and forwards, you produce a potential difference that keeps swapping direction - an alternating current 

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Uses of the generator effect

The generator effect is used in an alternator to generate ac and in a dynamo to generate dc.

An alternating current (ac) generator is a device that produces a potential difference. A simple acgenerator consists of a coil of wire rotating in a magnetic field.

As one side of the coil moves up through the magnetic field, a potential difference is induced in one direction. As the rotation continues and that side of the coil moves down, the induced potential difference reverses direction. This means that the alternator produces a current that is constantly changing. This is alternating current or ac.

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Alternator output on a graph

The output of an alternator as it rotates can be represented on a potential difference-time graph with potential difference (voltage) on the vertical axis and time on the horizontal axis.               The graph shows an alternating sine curve. The maximum potential difference or current can be increased by:                                                                                                                                       - increasing the rate of rotation                                                                                                              - increasing the strength of the magnetic field                                                                                    - increasing the number of turns on the coil

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A - coil at 0° - moving parallel to direction of magnetic field - no potential difference induced.
B - coil at 90° - moving at 90° to direction of magnetic field - induced potential difference is at its maximum. 
C - coil is 180° - coil is moving parallel to the direction of the magnetic field, so no potential difference is induced.
D - coil at 270° - moving at 90° to direction of magnetic field - induced potential difference at its maximum - induced potential difference travels in the opposite direction to what it did at B. 
A - coil is 360°(back at starting point) - coil moving parallel to the direction of the magnetic field - no potential difference is induced.

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The dc generator

A direct current (dc) generatoris another device that produces a potential difference. A simple dc generator consists of a coil of wire rotating in a magnetic field. However, it uses a split ring commutator rather than the two slip rings found in alternating current (ac) generators.

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In a dynamo, a split ring commutator changes the coil connections every half turn. As the induced potential difference is about to change direction, the connections are reversed. This means that the current to the external circuit always flows in the same direction. 

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Dynamo output on a graph

The output of a rotating dynamo can be shown on a potential difference-time graph. The graph shows a sine curvethat stays in the same direction all the time. The maximum potential difference or current can be increased by:                                                                                              - increasing the rate of rotation                                                                                                                - increasing the strength of the magnetic field                                                                                       - increasing the number of turns on the coil

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A - The coil is at 0°. The coil is moving parallel to the direction of the magnetic field, so no potential difference is induced.
B - The coil is at 90°. The coil is moving at 90° to the direction of the magnetic field, so the induced potential difference is at its maximum. 
C - The coil is at 180°. The coil is moving parallel to the direction of the magnetic field, so no potential difference is induced.
D - The coil is at 270°. The coil is moving at 90° to the direction of the magnetic field, so the induced potential difference is at its maximum. Here, the induced potential difference travels in the same direction as at B. 
A - The coil is at 360°, ie it is back at its starting point, having done a full rotation. The coil is moving parallel to the direction of the magnetic field, so no potential difference is induced.

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Microphones

Microphones use the generator effect to convert the pressure variations in sound waves into variations in current in electrical circuits. 

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In a moving-coil microphone:

  • pressure variations in sound waves cause the flexible diaphragm to vibrate
  • the vibrations of the diaphragm cause vibrations in the coil
  • the coil moves relative to a permanent magnet, so a potential difference is induced in the coil
  • the coil is part of a complete circuit, so the induced potential difference causes a current to flow around the circuit
  • the changing size and direction of the induced current matches the vibrations of the coil
  • the electrical signals generated match the pressure variations in the sound waves
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Transformers

A basic transformer consists of a primary coil and a secondary coil wound on an iron core. Iron is used as it is easily magnetised.

The ratio of the potential differences across the primary and secondary coils of a transformer Vp and Vs depends on the ratio of the number of turns on each coil, np and ns.  Vp/Vs = Np/Ns                                                                                                                 potential difference, Vp and Vs in volts, V                                                                                   In a step-up transformer Vs > Vp                                                                                                                                                                        In a step-down transformer Vs < Vp                                                                                     If transformers were 100 % efficient, the electrical power output would equal the electrical power input.

Vs × Is = Vp × Ip
Where Vs × Is is the power output (secondary coil) and Vp × Ip is the

power input (primary coil).
power input and output, in watts, W 

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Design and use of transformers

A basic transformer is made from two coils of wire, a primary coil from the alternating current (ac) input and a secondary coil leading to the ac output. The coils are not electrically connected. Instead, they are wound around an iron core. This is easily magnetised and can carry magnetic fields from the primary coil to the secondary coil.

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When a transformer is working:

  • a primary voltage drives an alternating currentthrough the primary coil
  • the primary coil current produces a magnetic field, which changes as the current changes
  • the iron core increases the strength of the magnetic field
  • the changing magnetic field induces a changing potential difference in the secondary coil
  • the induced potential difference produces an alternating current in the external circuit
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Our solar system

Within our solar system there is one star, the Sun, plus the eight planets and the dwarf planets that orbit around the Sun. Natural satellites, the moons that orbit planets, are also part of the solar system.

Our solar system is a small part of the Milky Way galaxy.

The Sun was formed from a cloud of dust and gas (nebula) pulled together by gravitational attraction. 

  • stars initially form a cloud of dust and gas. 
  • the force of gravity pulls them together to form a protostar. the temperature rises as the star gets denser and more particles collide. when the temperature gets high enough, hydrogen nuclei undergo nuclear fusion to form helium nuclei. this gives out huge amounts of energy which keeps the core of the star hot. a star is born 
  • the star enters a long stable period where the outward pressure caused by nuclear fusion that tries to expand the star balances the force of gravity pulling everything inwards (equilibrium). in this stable period its called a main sequence star and it typically lasts several billion years. 
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The life cycle of a star

A star goes through a life cycle. The life cycle is determined by the size of the star. 

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fusion

Fusion processes in stars produce all of the naturally occurring elements. Elements heavier than iron are produced in a supernova.

The explosion of a massive star (supernova) distributes the elements throughout the universe. 

how new elements are formed:

all stars 

  • eventually hydrogen begins to run out. the star then swells into a red giant  (if its a small star) and a red super giant (if its a larger star). it becomes red because the surface cools. Fusion of helium (and other elements) occurs. Heavier elements (up to iron) are created in the core of the star 

big stars 

  • big stars start to glow brightly again as they undergo more fusion and expand and contract several times, forming elements as heavy as iron in various nuclear reactions. eventually they explode in a supernova forming elements heavier than iron and ejecting them into the universe to form new planets and stars. stars and their life cycles produce and distribute all naturally occuring elements 
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Orbital motion

Gravity provides the force that allows planets and satellites (both natural and artificial) to maintain their circular orbits. 

planets - large objects that orbit a star. there are 8 in our solar system (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune). They have to be large enough to have "cleared their neighbourhoods" - this means that their gravity is stron enough to have pulled in any nearby objects apart from their natural satellites

dwarf planets - eg Pluto. these are planet like objects that orbit stars but dont meet all the rules for becoming a planet

moons- these orbit planets. they're a type of natural satellite (not man made) 

artificial satellites - satellites that humans have built. they generally orbit earth 

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circular orbits

  • the planets move around the sun in almost circular orbits (same goes for the moon orbiting earth) 
  • if an object is travelling in a circle it is constantly changing direction which means it is constantly accelerating 
  • this also means its constantly changing velocity (but NOT changing speed) 
  • for an object to accelerate there must be a force acting on it. this force is directed towards the centre of the circle 
  • this force would cause the object to just fall towards whatever its orbiting but as the object is already moving, it just causes it to change direction 
  • the object keeps accelerating towards what its orbiting but the instantaneous velocity (which is at a right angle to the acceleration) keeps it travelling in a circle
  • the force that makes this happen is provided by the gravitational force (gravity) between the planet and the Sun (or between the planet and its satellites)
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Red-shift

There is an observed increase in the wavelength of light from most distant galaxies. The further away the galaxies, the faster they are moving and the bigger the observed increase in wavelength. This effect is called red-shift.

The observed red-shift provides evidence that space itself (the universe) is expanding and supports the Big Bang theory.

The Big Bang theory suggests that the universe began from a very small region that was extremely hot and dense.

Since 1998 onwards, observations of supernovae suggest that distant galaxies are receding ever faster. 

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Spectra from distant galaxies

Astronomers can observe light from distant galaxies. When they do this, they see it is different to the light from the Sun. The dark lines in the spectra from distant galaxies show an increase in wavelength. The lines are moved or shifted towards the red end of the spectrum. 

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Astronomers see red-shift in virtually all galaxies. It is a result of the space between the Earth and the galaxies expanding. This expansion stretches out the light waves during their journey to us, shifting them towards the red end of the spectrum. The more red-shifted the light from a galaxy is, the faster the galaxy is moving away from Earth.

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Big Bang Theory

According to the Big Bang theory, about 13.8 billion years ago the whole Universe was a very small, extremely hot and dense region. From this tiny point, the whole Universe expanded outwards to what exists today.Evidence from red-shift                                                                                                                         Astronomers have discovered that, in general, the further away a galaxy is, the more red-shifted its light is. This means that the further away the galaxies are, the faster they are moving. This is similar to an explosion, where the bits moving fastest travel furthest from the explosion. Red-shift data provides evidence that the Universe, including space itself, is expanding.

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The future of the Universe

For many years, scientists have tried to work out the density of the Universe. The answer to this would give them an idea of whether the Universe is going to expand forever, or if the gravitational attraction between all objects will eventually slow to a stop, attracting everything back together in a 'Big Crunch'.

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Dark energy

Since 1998, astronomical observations of supernova have suggested that distant galaxiesare moving away increasingly faster. The expansion of the Universe appears to be accelerating. Scientists do not entirely understand how this could happen, but they have come up with an idea called dark energy  

The nature of dark energy is still a complete mystery, but it is thought to cause the Universe to expand faster all the time. Astronomers have calculated that to make the Universe accelerate as observed, dark energy must account for 68 per cent of the Universe.

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Dark matter

Another recently discovered anomaly is that galaxies seem to rotate too quickly for the mass of their stars. This suggests that there is mass in the Universe that is invisible to the instruments used by scientists. Scientists only know it is there because it has gravity that affects objects nearby. This unknown material has been called dark matter. Astronomers have calculated that 27 per cent of the Universe must be made of this dark matter.

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