# P2

## Velocity and Distance-Time Graphs

Speed is just how fast you are going, whereas velocity is the speed you are going in a given direction.

-flat sections are where it is stationary                                                                                         -straight uphill or downhill sections mean it is a steady speed                                                       -the steeper the graph, the faster it is going                                                                                 -downhill sections mean it is going back towards its starting point

y-axis= distance  x-axis=time

speed=distance/time

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## Acceleration and Velocity-Time Graphs

Acceleration is how quickly the velocity is changing.

Acceleration=change in velocity/time taken   < it is measured in m/s^2

Velocity-time graphs

-flat sections represent steady speed                                                                                           -the steeper the graph, the greater the acceleration or deceleration                                               -uphill=acceleration                                                                                                                   -downhill=deceleration                                                                                                               -the area under a graph is equal to the distance travelled in that time interval

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## Weight, Mass and Gravity

Gravitational force is the force of attraction between all masses.

-it gives everything a weight                                                                                                       -on the surface of a planet, it makes all things accelerate towards the ground, all with the same acceleration. On earth it is about 10m/s^2.

Mass is how much matter is in an object. It is measured in kg with a mass balance.

Weight is the force of gravity acting on a mass. It is measured in newtons(N) using a spring balance or a newton meter.

weight=mass x gravitational field strength

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## Resultant Forces

Resultant force is the overall force on a point or object.

A resultant force means a change in velocity. If there is a resultant force acting on an object, then the object will change its state of rest or motion.

If the forces all act along the same line then the overall effect is just found by adding or subtracting them.

<-------------driving force------------ CAR -----air resistance---->                                                                             1000N                                    600N

R.F=> 1000N-600N = 400N TO THE LEFT

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## Forces and Acceleration

An object needs a force to start moving.

If the resultant force on a stationary object is zero, then the object will remain stationary.

No resultant force means no change in velocity.

If there is no resultant force on a moving object it'll just carry on moving at the same velocity. To keep going at a steady speed, there must be zero resultant force.

A resultant force means acceleration.

If there is a non-zero resultant force then the object will accelerate in the direction of the force.

This acceleration can taake five different forms: starting, stopping, speeding up, slowing down and changing direction. On a force diagram, the arrows will be unequal.

F=ma (F= resultant force in newtons)

Reaction forces are equal and opposite.

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

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## Frictional Force and Terminal Velocity

When two surfaces slide past each other, the interaction between them produces a force of friction. To travel at a steady speed, the driving force needs to balance the frictional forces.

Resistance or drag from fluids (air or liquid).

-Reducing drag- keep the object streamlined                                                                               -drag increases as the speed increases

Objects falling through fluids reach a terminal veocity.

when falling objects first set off, gravity is much more than friction, so they accelerate. As the speed increases, the friction builds up. This gradually reduces the acceleration until the frictional force is equal to the accelerating force and it won't accelerate anymore. It has reached its terminal velocity.

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## Stopping Distances

Stopping distance is the distance covered in the time between the driver first spotting a hazard and the vehicle coming to a complete stop.

Stopping distance= thinking distance + braking distance

The faster you are going then the greater your stopping distance.

1) THINKING DISTANCE- affected by tiredness, drugs, alcohol, how fast you're going

2) BRAKING DISTANCE- affected by how fast you're going, condition of brakes, condition of tyres, weather conditions affecting grip or road surfaces.

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## Work and Potential Energy

When a force moves an object through a distance, ENERGY IS TRANSFERRED and WORK IS DONE.

work done=energy tansferred

Work done= force x distance (force=N, distance=m)

Gravitational potential energy is energy due to height.

GPE= mass x g x height

GPE is measured in joules.

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## Kinetic Energy

Kinetic energy= 1/2 x mass x speed^2

kinetic energy is movement energy.

To stop a car, the kinetic energy has to be converted to heat energy as friction because energy cannot be created or destroyed- only converted into different forms.

Kinetic energy transferred= work done by brakes                                                                                                 1/2mv^2= F x d

When something falls, its potential enrgy is converted into kinetic energy. So the further it falls, the faster it goes.

Kinetic energy gained= Potential energy lost

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## Forces and Elasticity

Work done to an elastic object is stored as elastic potential energy. Any object that can go back to its orignal shape after the force has been removed is an elastic object.

Extension(e) of an elastic object is directly proportional to force(F). Extension is measured in metres and force is measured in newtons.

F= k x e       < k= the spring constant(N/m)

There is a limit to the amount of force you can apply for the extension to keep on increasing proportionally. This is known as the limit of proportionality.

If you increase the force past the limit of proportionality, the material will be permanently stretched. When the force is removed, the material will be longer than at the start.

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## Power

Power is the rate of doing work- i.e. how much per second.

Power= work done/ time taken        P=E/T

Power is measured in watts or J/s

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## Momentum and Collisions

Momentum= mass x velocity

The greater the mass of an object and the greater its velocity, the more momentum the object has.

Momentum before= momentum after (closed system)

A closed system means that there are no external forces acting on it.

Forces cause changes in momentum.

-a larger force means a faster change in momentum                                                                     -if someone's momentum changes very quickly(car crash), the forces on the body will be very large, and more likely to cause injury.                                                                                                   -the longer it takes for a change in momentum then the smaller the force.

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## Car Design and Safety

The brakes reduce the kinetic energy of a car by transferring it into heat.

Regenerative brakes(electric/hybrid cars) put the vehicle's motor into reverse, slowing the wheels. At the same time, the motor acts as an electric generator, converting the kinetic energy into electrical energy that is stored as chemical energy in the vehicle's battery. This is the advantage of regenerative brakes.

CRUMPLE ZONES: Increase the impact time, decresing the force produced from the change in momentum. Also, the kinetic energy is converted into other forms of energy by the car body as it changes shape.

SIDE IMPACT BARS: Strong, metal tubes fitted into car door panels to help to direct the kinetic energy of the crash away from the passengers to areas such as the crumple zone.

SEAT BELTS: Stretch slightly to increase the time taken for the wearer to stop. some of the kinetic energy is absorbed by the seat belt stretching.

AIR BAGS: Also slow you down more gradually and prevent you from hitting hard surfaces inside the car.

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## Static Elecricity

Build-up of static is caused by friction.

When certain insulating materials are rubbed together, negatively charged electrons will be scraped off one and dumped on the other.                                                                                               This'll leave a positive static charge on one and a negative static charge on the other.

ONLY ELECTRONS MOVE. Never the positive charges.

A positive static charge is always caused by electrons moving elsewhere.

Two opposite charges attract each other. Electrical charges can also move easily through conductors.

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## Current and Potential Difference

Current is the rate of flow of electric charge round the circuit(amps,A)

Potential difference is the driving force that pushes current round- it is the work done per unit charge(volts,V)

Resistance is anything in the circuit which slows the flow down(ohms)

The greater the resistance across a component, the smaller the current that flows (for a given potential difference across a component).

Current= charge/time   <charge= coulombs

P.D.= work done/charge

Voltage is the same as potential difference.

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## Ammeter and Voltmeter

Ammeter:

Measures the current in amps flowing through the component.

Must be placed in series.

Can be put anywhere in series in the main circuit, but NEVER PARALLEL.

Voltmeter:

Measuers the potential difference in volts across the component.

Must be placed in parallel around the component under test.

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## Resistance and V= I x R

Different resistors- the current through a resistor at a constant temperature is directly proportional to P.D.

Filament lamp- as the temperature of the filament increases, the resistance increases.

Diode- current will only flow through a diode in one direction. The diode has a very high resistance in the opposite direction.

Resistance increases with temperature.

-When an electrical charge flows through a resistor, some of the electrical energy is transferred to heat energy and the resistor gets hot.                                                                                         -This causes the ions in the conductor to vibrate more, making it more difficult for charge-carrying electrons to get through the resistor. The resistance increases, the current decreases.                     -More current means an increase in temperature, which means an increase in resistance, which means that the current decreases again.

Potential difference= current x resistance         V= I x R

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## Circuit Devices

DIODE: Used to regulate the P.D. in circuits. It lets current flow through it one direction, but not the other. It is made from a semiconductor material such as silicon.

LIGHT-EMITTING DIODE: Emits light when a current flows through it in the forward direction. LEDs are being used more as lighting because they use a much smaller current than filaments. Used in traffic lights, digital clocks and remote controls.

LIGHT DEPENDENT RESISTOR: In bright light, the resistance falls. In darkness, the resistance is highest. Used in automatic night lights, outdoor lighting and burgular detectors.

THERMISTOR: A thermistor is a temperature dependent resistor. In hot conditions, the resistance drops. In cold conditions, the resistance goes up. Used in temperature detectors, e.g. car engine temperature sensors and electronic thermostats.

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## Series Circuits

1) Potential differnec is shared, so the voltages always add up to equal the source voltage.

V = V1 + V2 + ...

2) Current is the same eveywhere. The size of the current is determined by the total P.D. of the cells and the total resistance of the circuit i.e. I= V/R

A1 = A2

3) Resistance adds up, the total resistance is just the sum of all the resistances. The bigger the resistance of a component, the bigger the share of thentotal P.D.

There is a bigger P.D. when more cells are in series, provided the cells are all connected in the same way.

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## Parallel Circuits

In parallel circuits, each component is connected to the +ve and -ve of the supply. If you remove or disconnect one of them, it will hardly affect the others at all.

1) P.D. is the same across all components. This means that identical bulbs connected in parallel will all be at the same brightness.

V1 = V2 =V3

2) Current is shared between branches. If two identical components are connected in parallel then the same current will flow through each component. The total current going into a junction has to equal the total current leaving that junction. The total current flowing around the circuit is equal to the total of all the currens through the separate components.

A = A1 + A2 + ...

-Voltmeters and ammeters are exceptions to the rule and always placed in series(ammeter) and parallel(voltmeter).

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## Mains Electricity

Mains supply= AC        Battery supply= DC

Alternating current means that the current is constantly changing direction. The UK mains supply is approx 230voltsThe supply has a frequency of 50 hertz.

Direct current means that the current always keeps flowing in the same direction.

Electricity supplies can be shown on an oscilloscope screen. A cathode ray oscilloscope (CRO) is a snazzy voltmeter.

A DC source is always at the same voltage, so you get a straight line.

An AC source gives a regularly repeating wave. From that, you can work out the period and the frequency of the supply.

Frequency= 1/time period(s)

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## Electricity in the home

Likely hazards in the home: long cables, frayed cables, cables touching something wet or hot, water near sockets, shoving things into sockets, damaged plugs, too many plugs in one socket, lighting sockets without bulbs, appliances without covers on.

Most cables have three separate wires-                                                                                       -the brown live wire in a mains supply alternates between a high +ve and -ve voltage.                     -the blue neutral wire is always at 0V. Electricity usually flows in and out through the live and neutral wires only.                                                                                                                     -the green and yellow earth wire is for protecting the wiring, and for safety- it works together with a fuse to prevent fire and shocks. It is attached to the metal casing and carries the electricity to eath should something go wrong and the live or neutral wires touch the metal case.

PLUGS:1) the right coloured wire is connected to each pin and firmly screwed in.                                       2) no bare wires showing inside the pug                                                                                     3) cable grip tightly fastened over the cable outer layer                                                                 4) thicker cables have less resistance, therefore they carry more current

The metal parts are made of copper or brass because they are good conductors. The case, cable grip and cable insulation are made of rubber or plastic because they are good insulators.

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## Fuses and Earthing

Earthing and fuses prevent electrical overloads.

• if a fault develops and the live wire touches the metal case because the case is earthed too great a current flows through the live wire, into the case and out of the earth wire. This surge in current melts the fuse if the current is greater than the fuse rating, This cuts off the live supply and breaks the circuit.
• Earthing isolates the appliance and stops you getting an electric shock from the case. It also stops the risk of there being a fire.

If the appliance has a plastic casing and no metal parts showing thenit's said to be double insulated.

Circuit breakers are an electrical safety device that protects the circuit from damage if too much current flows. When a circuit breaker detects a current, it breaks the circuit by opening a switch. They are more convenient than fuses because they can be reset by just flicking a switch, whereas fuses have to be completey replaced. Circuit breakers are a lot more expensive than fuses. An example of a circuit breaker is a residual current circuit breaker (RCCB).

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## Energy and Power in Circuits

Anything which supplies electricity is also supplying energy.

If an appliance is efficient then it wastes less energy. These appliances transfer more of their total electrical energy output to useful energy.

The power of an appliance is the energy that it uses per second.

Energy transferred= power rating x time

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## Power and Energy Change

Power= current x P.D.

Most electrical goods show their power rating and voltage rating. To work out the size of the fuse needed, you nees to work out the current that the item will normally use.

When an electrical charge goes through a change in P.D., then energy is transferred.

Energy transformed= charge x P.D.

The bigger the change in P.D. the more energy is transferred for a given amount of charge passing through the circuit.

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## Atomic Structure

J J Thomson discovered that electrons could be removed from atoms. He came to a conclusion  that the atom was a sphere of positive charge with tiny negative electrons stuck in them like plums in a plum pudding - this was called the plum pudding theory.

However later on Rutherford and Marsden carried out an experiment where they fired alpha particles at thin gold foil. their prediction was that the alpha particles would be slightly deflected.However most of the alpha particles went straight through with the odd one coming back at them. They came to the conclusion that most of the mass was located in the centre in a nucleus and the rest of the atom was mainly empty space. The nucleus had an overall positive charge as it repelled the positive protons  that were fired at it. This is called the nuclear model.

particle         mass               charge

proton              1                     +1

neutron            1                      0

electron         1/2000                -1

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Isotopes are different forms of the same element. They have the same number of protons but a different number of neutrons., so they have the same atomic number, but different mass numbers.

Radioactive substances give out radiation from the nuclei of their atoms- it is a completely random process. Radioactive substances spit out one or more of the three types of radiation: alpha, beta or gamma.

Background radiation is present around us at all times:

1) radioactivity of naturally occuring unstable isotopes which are all around us- in the air, in food, in building materials and in the rocks under our feet.

2) radiation from space, which is known as cosmic rays. These come mostly from the sun.

3) radiation due to man-made sources, e.g.fallout from nuclear weapons tests, nuclear accidents(Chernobyl) or dumped nuclear waste.

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## Alpha, Beta and Gamma

Alpha particles are helium nuclei:                                                                                             -2 neutons, 2 protons                                                                                                               -relatively big, heavy and slow moving                                                                                         -don't penetrate very far into materials and are stopped quickly, even when travelling through air       -strongly ionising(due to size), which just means that they bash into a lot of atoms and knock            electrons off them before they slow down, which creates a lot of ions- ionising.

Beta particles are electrons:                                                                                                       - move quite fast and are quite small.                                                                                         -penetrate moderately into materials before colliding, have a long range in air, and are moderately ionising too.                                                                                                                             -for every b-particle emitted, a neutron turns to a proton in the nucleus.                                         -a b-particle is simply an electron, with virtually no mass and a charge of -1

Gamma rays are very short wavelength EM waves:                                                                     -the opposite of alpha particles.                                                                                               -they penetrate far into materials without being stopped and pass straight though air.                     -weakly ionising because they tend to pass through rather than collide with atoms. Eventually, they hit something and do damage. No mass and no charge.

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## Half-Life

The radioactivity of a sample always decreases over time.

HALF LIFE is the AVERAGE TIME it takes for the NUMBER OF NUCLEI in a RADIOACTIVE ISOTOPE SAMPLE to HALVE

• Each time a decay happens and an alpha, beta or gamma is given out, it means one more radioactive nucleus has disappeared.
• Obviously, as the unstable nuclei all steadily disappear, the activity will decrease. So the older a sample becomes, the less radiation it will emit.
• The problem with trying to meaure this is that the activity never reaches zero, which is why we use the idea of half-life to measure how quickly the activity drops off.
• A short half-life means the activity falls quickly, because lots of nuclei decay quickly.
• A long half-life means the activity gass more slowly, because most of the nucleu don't decay for a long time.
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Smoke detectors- a weak source of alpha radiation is placed in the detector, close to two electrodes. The source causes ionisation, and a current flows between the electrodes. If there is a fire then smoke will absorb the radiation- so the current stops and the alarm sounds.

Medical tracers use beta or gamma radiation...                                                                           -A source that emits beta or gamma radiation is injected or swallowed.                                       -The radiation penetrates the body tissues and can be detected externally                                   -As the source moves around the body, a radiographer uses a detector to monitor it's progress       -A computer converts the reading to an on-screen display, doctors use this method to check      whether organs of the body are working properly                                                                         -The radioactive source has to have a short half-life                                                                   -An alpha source wouldn't work because it is stopped by skin also because it's highly ionising it will damage lots of cells.

Radiotherapy- High doses of gamma rays will kill living cells, they can be used to treat cancers. The gamma rays have to be carefully directed and at the right dosage so as to kill the cancer cells without damaging too many normal cells. However, a fair bit of damage is inevitable, which makes the patient feel very ill.

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Alpha, beta and gamma radiation will carefully enter living cells and colide with molecules. These collisions cause ionisation, which damages or destroys the molecules. Lower doses tend to cause minor damage without killing the cell. Higher doses tend to kill cells completely, which causes radiation sickness if a lot of body cells get blasted at once.

Safety Precautions                                                                                                                                                             -When conducting experiments, use radioactive sources for a short amount of time as possible so your exposure is kept to a minimum.                                                                                                                                                           -Never allow skin contact with a source. Always hold tongues.                                                                                       -Hold the source at arm's length to keep it as far from tthe body as possible, this decreases the amount of radiation that hits you.                                                                                                                                                        -Keep the source pointing away from the body and avoid looking directly at it.                                                             -Lead absorbs all thre types of radiation. Always keep the radioactive sources in a lead box.                                   -When someone needs an x-ray or radiotheraphy, only one area of the body needs to be treated us exposed to the radiation, the rest of the body is protected by lead.

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## Nuclear Fission and Fusion

FISSION: Nuclear power stations generate electricity using nuclear reactors. In a nuclear reactor, a controlled chain reaction takes place in which atomic nuclei split up and release energy in the form of heat. This heat is then simply used to heat water to make steam, which is used to drive a steam turbine connected an electricty generator. The "fuel" that's split is usually uranium-235 or plutonium-239.

FUSION: Two light nuclei can join to create a larger nucleus - this is called nuclear fusion. Fusion releases a lot of energy. Fusion doesn't leave a lot of radiactive waste like fission, and there's plenty of hyrdrogen that can be used as fuel. The big problem is that fusion can only happen at really high temperatures(about 10,000,000 degrees celcius). You can't hold the hydrogen at the high temperatures and pressures required for fusion in an ordinary container. There are a few experimental reactors around, but none of them are generating electricty yet. At the moment it takes more power to get up to temperature than the reactor can produce.

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## The Life Cycle of Stars

1) Initially form from clouds of GAS and DUST                                                                                                                2) Gravity makes the clouds come spiralling in, gravitational energy turns into heat energy and the temp rises     3) When the temp is high enough, hydrogen nuclei do nuclear fusion to form helium nuclei, giving out heat and light (a star is born). Enters a long stable period where the heat created by nuclear fusion provides an outward pressure to balance the inward pressure from gravity - MAIN SEQUENCE STAR. Can last millions of years            4) Eventually hydrogen runs out and the star swells and becomes a RED GIANT - red because the surface cools    5) Small stars, like the Sun, will cool and contract into a WHITE DWARF then as light fades completely into a BLACK DWARF                                                                                                                                                            6) Big stars glow brightly as they undergo more fusion and expand and contract several times, forming heavier elements, they'll eventually explode and become aSUPERNOVA                                                                                7) This throws outer layers of dust and gas into space, leaving a dense core called aNEUTRON STAR or if the star is big enough, a BLACK HOLE                                                                                                                                       8) The dust and gas thrown out will turn into SECOND GENERATION STARS (like our Sun). Heavier elements are only made in the final stages of a big star (sometimes during the supernova) - these elements found in the Sun is evidence to support the big bang theory

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