Physics

• The distance-time graph for any object that is:
  • stationary is horizontal
  • moving at a constant seed is a straight line that slopes upwards

• The gradient of a distance-time graph for an object represents the object's speed

• Speed in metres per second, m/s = 
  • distance travelled in metres, m
  • time taken in seconds, s

• Velocity is speed in a given direction

• Acceleration is change of velocity per second. The unit of acceleration is the metre per second squared (m/s^2 (where ^2 is squared)).

• Acceleration = change of velocity / time taken

• Deceleration is the change of velocity per secong when an object slows down

• If a velocity-time graph is a horizontal line, the acceleration is zero.

• The gradient of the line on a velocity-time graph represents acceleration.

• The area under the line on a velocity-time graph represents distance travelled.

• A force can change the shape of an object or change its motion or its state of rest.

• The unit of force is the newton (N).

• When two objects interact, they always exert equal and opposite forces on each other.

• The resultant force is a single force that has the same effect as all the forces acting on an object.

• If the resultant force on an object is zero, the object stays at rest or at constant velocity. If the resultant forve on an object is not zero, the velocity of the object will change.

• If two forces act on an object along the same line, the resultant force is:
  •  
    •  1. their sum if the forces act in the same direction
    •  2. their difference if the forces act in opposite directions

• The bigger the resultant force on an object is, the greater its acceleration.

• The greater the mass of an object is, the smaller its acceleration is for a given force.

• Resultant force (newtons, N) = mass (kg) x acceleration (m/s^2 (where ^2 is squared)).

• Friction and air resistance oppose the driving force of a car.

• The stopping distance of a car depends on the thinking distance and the braking distance.

• High speed, poor weather conditions, and poor maintenance all increase the braking distance.

• Poor reaction time and high speed both increase the thinking distance.

• The weight of an object is the force of gravity on it. Its mass is the quantity of matter in it.

• An object acted on only by gravity accelerates at about 10m/s^2 (where ^2 is squared).

• The terminal velocity of a falling object is the velocity it reaches when it is falling in a fluid. The weight is then equal to the drag force on the object.

• The extension is the difference between the length of the spring and its original length.

• The extension of a spring is directly proportional to the force applied to it, provided the limit of proportionality is not exceeded.

• The spring constant of a spring is the force per unit extension needed to stretch it.

• Fuel economy of road vehicles can be improved by reducing the speed or fitting a wind deflector.

• Average speed cameras are linked in pairs and they measure the average speed of a vehicle.

• Anti-skid surfaces increase the friction between car tyres and the road surface. This reduces skids, or even prevents them altogether.

• The electromagnetic spectrum (in order of decreasing wavelength, increasing frequency and energy) is:
  • radiowaves, microwaves, infrared radiation, visible light, ultraviolet radiation, gamma radiation, X-rays
• The wave speed equation is used to calculate the frequency or wavelength of electromagnetic waves.
• The wave speed equation is: v = f x 
  • where v = wavespeed (m/s)
  •    f = frequency (Hz)
  •     = wavelength (m)

• White light contains all the colours of the visible spectrum.

• Infrared radiation is used for carrying signals from remote handsets and inside optical fibres.

• We use microwaves to carry satellite TV programmes and mobile phone calls.

• Radio-waves are used for radio and TV broadcasting, radio communication, and mobile phone calls.

• Different types of EM radiation are hazardous in different ways. Micro- and radio-waves can cause internal heating. Infrared radiation can cause burns.

• Radio waves of different frequencies are used for different purposes because the wavelength (and therefore frequency) of waves affects:
  • how far they can go
  • how much they spread
  • how much information they can carry

• Microwaves are used for satellite TV signals.

• Further research is needed to evaluate whether or not mobile phones are safe.

• Optical fibres are very thin transparent fibres that are used to transmit signals by light and infrared radiation.

• The red-shift of a distant galaxy is the shift to longer wavelengths of the light from it because the galaxy is moving away from us.

• The faster a distant galaxy is moving away from us, the greater the red-shift.

• All the distant galaxies show a red-shift. The further away a distant galaxy is from us, the greater its red-shift is.

• Distant galaxies are all moving away from us because the universe is expanding.

• Infrared radiation is energy transfer by electromagnetic waves.

• All objects emit infrared radiation.

• The hotter an object is, the more infrared radiation it emits in a given time.

• Dark, matt surfaces emit more infrared radiation than light, shiny surfaces.

• Dark, matt surfaces absorb more infrared radiation that light, shiny, surfaces.

• Light, shiny surfaces reflect more infrared radiation than dark, matt surfaces.

• Metals are the best conductors of electricity.

• Materials such as fibreglass and wool are the best insulators.

• Conduction of energy in a metal is due mainly to free electrons transferring energy inside the metal.

• Non-metals are poor conductors because they do not contain free electrons.

• Convection is the circulation of a fluid (liquid or gas) caused by heating it.

• Convection takes place only in liquids and gases.

• Heating a liquid or gas makes it less dense so it rises and causes circulation.

• Evaporation is when a liquid turns into a gas.

• Condensation is when a gas turns into a liquid.

• Cooling by evaporation of a liquid is due to the faster moving molecules escaping from the liquid.

• Evaporation can be increased by increasing the surface area of the liquid, by increasing the liquid's temperature, or by creating a draught of air across the liquid's surface.

• Condensation on a surface can be increased by increasing the surface area of the surface or reducing the temperature of the surface.

• The rate of energy transferred to or from an object depends on:
  • the shape, size and type of material of the object
  • the materials the object is in contact with
  • the temperature difference between the object and its surroundings

• The greater the mass of an object, the more slowly its temperature increases when it is heated.

• The rate of temperature change of a substance when it is heated depends on:
  •  
    • the energy supplied to it
    • its mass
    • its specific heat capacity

• Storage heaters use off-peak electricity to store energy in special bricks.

• Energy transfer from our homes can be reduced by fitting:
  •  
    • loft insulation
    • cavity wall insulation
    • double glazing
    • draught proofing
    • aluminium foil behind radiators

• U-valves tell us how much energy per second passes through different materials.

• Solar heating panels do not use fuel to heat water but they are expensive to buy and install.

• Energy exists in different forms.

• Energy can change from one form into another form.

• When an object falls and gains speed, its gravitational potential energy (GPE) decreases and its kinetic energy (KE, or Ek) increases.

• Energy cannot be created or destroyed.

• Conservation of energy applies to all energy changes.

• Useful energy is energy in the place we want it and in the form we need it.

• Wasted energy is energy that is not useful energy.

• Useful and wasted energy both end up being transferred to the surroundings, which become warmer.

• As energy spreads, it gets more and more difficult to use for further energy transfers.

• The efficiency of a device = useful energy transferred by device
  •  
    •  
      •  
        •         total energy supplied to device x 100%

• No machine can be more than 100% efficient.

• Measures to make machines more efficient include reducing friction, air resistance, electrical resistance, and noise due to vibrations.

• Electrical appliances can transfer electrical energy into useful energy at the flick of a switch.

• Uses of everyday electrical appliances include heating, lighting, making objects move (using an electric motor) and creating sound and visual images.

• An electrical appliance is designed for a particular purpose and should waster as little energy as possible.

• Power is rate of transfer of energy.

• P = E/t

• Efficiency = useful power out
•        total power in  x 100%

• The kilowatt-hour is the energy supplied to a 1kW appliance in 1 hour.

• E = P x t

• Total cost = number of kWh used x cost per kWh

• Cost effectiveness means getting the best value for money.

• To compare the cost of effectiveness of different appliances, we need to take account of costs to buy it, running costs and other costs such as environmental costs.

• Electricity generators in power stations are driven by turbines.

• Coal, oil and natural gas are burned in fossil fuel power stations.

• Uranium and plutonium are used as the fuel in nuclear power stations. Much more energy is released per kg from uranium or plutonium than from fossil fuels.

• Biofuels are renewable sources of energy. Biofuels such as methane and ethanol can be used to generate electricity.

• A wind turbine is an electricity generator on top of a tall tower.

• Waves generate electricity by turning a floating generator.

• Hydroelectricity generators are turned by water running downhill.

• A tidal power station traps each high tide and uses it to turn generators.

• Solar cells are flat solid cells that convert solar energy directly into electricity.

• Solar heating panels use the Sun's energy to heat water directly.

• Geothermal energy comes from the energy released by radioactive substances deep inside the Earth.

• Water pumped into hot rocks underground produces steam to drive turbines that generate electricity.

• Fossil fuels produce increased levels of greenhouse gases which could cause global warming.

• Nuclear fuels produce radioactive waste.

• Renewable energy resources can affect plant and animal life.

• The National Grid is a network of cables and transformers that distributes electricity to our homes from distant power stations and renewable energy generators.

• Step-up transformers are used to step up power station voltages to the grid voltage.

• Step-down transformers are used to step the grid voltage down for use in our homes.

• A high grid voltage reduces energy loss and makes the system more efficient.

• Gas-fired power stations and pumped-storage stations can meet variations in demand.

• Nuclear, coal and oil power stations can meet base-load demand.

• Nuclear power stations, fossil-fuel power stations using carbon capture, and renewable energy are all likely to contribute to future energy supplies.

• Work is done on an object when a force makes the object move.

• Energy transferred = work done

• Work done (joules) = force (newtons) x distance moved in direction of force (metres)

• Work done to overcome friction is transferred as energy that heats the objects that rub together and the surroundings.

• The GPE of an object depends on its weight and how far it moves vertically.

• The GPE of an object increases when the object goes up and decreases when the object goes down.

• The change of GPE of an object is equal to its mass x the gravitational field strength x its change of height

• The KE of a moving object depends on its mass and its speed.

• Kinetic energy (J) = 1/2 x mass(kg) x speed^2 (m/s^2)

• Elastic potential energy is the energy stored in an elastic object when work is done on the object.

• Momentum = mass x velocity

• The unit of momentum is kgm/s

• Momentum is conserved whenever objects interact, provided the objects are in a closed system so that no external forces can act on them

• Momentum is mass x velocity and velocity is speed in a certain direction.

• When two objects push each other, they move apart:
  • with different speeds if they have unequal masses
  • with equal and opposite momentum so their total momentum is zero

• When vehicles collide, the force of the impact depends on mass, change of velocity, and the duration of impact.

• The longer the impact time is, the more the impact force is reduced.

• When two vehicles collide:
  • the exert equal and opposite forces on each other
  • their total momentum is unchanged

• Seat belts and air bags spread the force across the chest and they also increase the impact time.

• Side impact bars and crumple zones 'give way' in an impact so increasing the impact time.

• We can use the conservation of momentum to find the speed of a car before impact.

• Certain insulating materials become charged when rubbed together.

• Electrons are transferred when objects become charged:
  • insulating materials that become positively charged when rubbed lose electrons
  • insulating materials that become negatively charged when rubbed gain electrons

• Like charges repel; unlike charges attract.

• Every component has its own agreed symbol. A circuit diagram shows how components are connected together.

• A battery consists of two or more cells connected together.

• The size of an electric current is the rate of flow of charge.

• Electric charge = charge flow / time taken

• Potential difference across a component (in volts) = 
  •  
    • work done or energy transferred (J)
    •      charge (coulombs)

• Resistance (in ohms) = potential difference (volts) / current (amperes)

• Ohm's law states that the current through a resistor at constant temperature is directly proportional to the potential difference across the resistor.

• Reversing the current through a component reverses the potential difference across it.

• Filament: resistance increases with increase of the filament temperature

• Diode: 'forward' resistance is low; 'reverse' resistance high

• Thermistor: resistance decreases if its temperature increases

• LDR: resistance decreases if the light intensity on it increases

• For components in series:
  •  
    • the current is the same in each component
    • adding the potential difference gives the total potential difference

• Adding the resistances gives the total resistance of resistors in series.

• For cells in series, acting in the same direction, the total potential difference is the sum of their individual potential differences.

• For components in parallel:
  • the total current is the sum of the currents through the separate components
  • the bigger the resistance of a component, the smaller its current is
• In a parallel circuit the potential difference is the same across each component.
• To calculate the current through a resistor in a parallel circuit, use this equation: currents (amperes) = potential difference (volts)
• resistance (ohms)

• Direct current is in one direction. Alternating current repeatedly reverse its direction.

• The peak voltage of alternating potential difference is the maximum voltage measured from zero volts.

• A mains circuit has a live wire that is alternately positive and negative every cycle and a neutral wire at zero volts.

• To measure the frequency of an a.c. supply, we measure the time period of the waves then use the formula: frequency =                    1                
• time taken form 1 cycle

• Sockets and plug cases are made of stiff plastic materials that enclose the electrical connections. Plastic is used because it is a good electrical insulator.

• Mains cable consists of two or three insulated copper wires surrounded by an outer layer of flexible plastic material.

• In a three-pin plug or a three-core cable, the live wire is brown, the neutral wire is blue, and the earth wire is green and yellow.

• The earth wire is connected to the longest pin and is used to earth the metal case of a mains appliance.

• A fuse contains a thin wire that heats up, melts, and cuts off the current if the current is too large.

• A fuse is always fitted in series with the live wire. This cuts the appliance off from the live wire if the fuse blows.

• A circuit breaker is an electromagnetic switch that open (i.e. 'trips') and cuts off the current if too much current passes through the circuit breaker.

• A mains appliance with a plastic case does not need to be earthed because plastic is an insulator and cannot become live.

• The power supplied to a device is the energy transferred to it each second.

• Electrical power supplied (watts) = current (amperes) x potential difference (volts)

• Correct rating (in amperes) for a fuse: electrical power (watts)
•  potential difference (volts)

• An electric current is the rate of flow of charge.

• Charge (coulombs) = current (amperes) x time (seconds)

• When an electrical charge flows through a resistor, energy transferred to the resistor makes it hot.

• Energy transferred (joules) = potential difference (volts) x charge flow (coulombs)

• When charge flows round a circuit for a certain time, the electrical energy supplied by the battery is equal to the electrical energy transferred to all the components in the circuit.

• Electrical faults are dangerous because they can cause electric shocks and fires.

• Never touch a mains appliance (or plug or socket) with wet hands. Never touch a bare wire or a terminal at a potential of more than 30V.

• Check cables, plugs and sockets for damage regularly. Check smoke alarms and infrared sensors regularly.

• When choosing an electrical appliance, the power and efficiency rating need to be considered.

• Filament bulbs and halogen bulbs are much less efficient than low energy bulbs.

• A radioactive substance contains unstable nuclei that become stable by emitting radiation.

• There are three main types of radiation from radioactive substances: alpha beta and gamma.

• Radioactive decay is a random event - we cannot predict or influence when it will happen.

• Background radiation is from radioactive substances in the environment or from space or from devices such as X-ray machines.

• Rutherford used the measurements from alpha-scattering experiments to prove that an atom has a small positively charged central nucleus where most of the mass of the atom is located.

• The plum pudding model could not explain why some alpha particles were scattered through large angles.

• The nuclear model of the atom correctly explained why the alpha particles are scattered and why some are scattered through large angles.

• Isotopes of an element are atoms with the same number of protons but different numbers of neutrons. Therefore they have the same atomic number but different mass numbers.

change in the nucleus:  alpha particle - nucleus loses 2 protons and two    neutrons beta - neutron in the nucleus changes into a   proton

particle emitted: alpha particle - 2 protons and 2 neutrons emitted as an alpha       particle      beta - electron is created in nucleus and instantly emitted

• Alpha radiation is stopped by paper, has a range of a few centimetres in air, and consists of particles, each composed of two protons and two neutrons.

• Beta radiation is stopped by thin metal, has a range of about a metre in air, and consists of fast-moving electrons emitted from the nucleus.

• Gamma radiation is stopped by really thick lead, has an unlimited range in air and consists of electromagnetic radiation.

• A magnetic or an electric field can be used to separate a beam of alpha, beta and gamma radiation.

• Alpha, beta and gamma radiation ionise substances they pass through. Ionisation in a living cell can damage or kill the cell

• The half-life of a radioactive isotope is the average time it takes for the number of nuclei of the isotope in a sample to halve.

• The activity of a radioactive source is the number of nuclei that decay per second.

• The number of atoms of a radioactive isotope and the activity both decrease by half every half-life.

•  
  •  
    • its half life
    • the type of radiation it gives out

• For monitoring, the isotope should have a long half-life.

• Radioactive tracers should be beta or gamma emitters that last long enough to monitor but not too long.

• For radioactive dating of a sample, we need a radioactive isotope that is present in the sample which has a half-life about the same as the age of the sample.