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Heat energy can be transferred by radiation (transfer of heat by infrared radiation), conduction and convection (transfer of energy by particles). Radiation can be emitted by solids, liquids and gases. Conduction is in solids and convection is in liquids and gases. 

All objects continually emit and absorb infrared radiation. The hotter the object is, the more radiation if radiates in a given time.

An object that is hotter than its surroundings emits more than it absorbs as it cools down.

An object that is cooler than its surroundings absorbs more than it emits as it warms up.

Dark, matt surfaces are the best absorbers and the emitters of radiation, but the worst reflectors.

Light, shiny surfaces are the worst absorbers and emitters of radiation, but the best reflectors. 

EXAMPLE: SOLAR PANELS                                                                                          Solar panels contain pipes under a black surface. Radiation is absorbed by the black surface to hear the water in the pipes. 

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Kinetic Theory

SOLIDS                                                                                                                      Strong forces of attraction hold the particles in a fixed, regular arrangement. Particles vibrate in fixed positions. Solids cannot be compressed or squashed as particles have no space to move into. 

LIQUIDS                                                                                                                               Weaker forces of attraction between particles mean particles form irregular arrangements. The particles are close together but can move past one another. The particles have more energy than in solids and move in random directions at low speeds

GASES                                                                                                                         Almost no forces of attraction between particles. Particles have more energy and are free to move in random directions at high speeds. 

When substances are heated, the particles are given more kinetic energy and vibrate/move faster. This causes solids to melt and liquids to boil.


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Conduction is where vibrating particles pass on their extra kinetic energy to neighbouring particles.

This will continue throughout the solid as the heat energy is passed along. This causes a rise in temperature at the other side, and an increase in heat radiating from the solid's surface.

Conduction is usually faster in denser solids, and the particles are closer together. As a result, they collide more often, passing heat energy faster as a result. 

Materials that have larger spaces between particles conduct heat more slowly, and are INSULATORS.

Why are metals good conductors?

  • Contain free electrons 
  • At the hot end, the electrons move faster and collide with the other free electrons, transferring energy
  • This is much faster than simply passing the energy on to neighbouring particles
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Convection occurs where the more energetic particles move from a hotter region to a cooler region, and take their heat energy with them. 

Convection in immersion heaters:                                                                            

  • Heat energy is transferred from the heater coils to the water by conduction.
  • Particles near the coils gain kinetic energy and move faster.
  • Therefore there is more distance between particles and the water expands, becoming less dense.
  • This means that the hotter water rises above the cooler, denser water and displaces it, making it sink. 
  • The cold water is then heated by the coils in the same way and so on.
  • Convection currents circulate this heat energy.

Convection is most efficient in round/square containers as the convection currents work the best.

In radiators, the hot air rises and becomes less dense as the particles move further apart. The hot air displaces the denser, cooler air. The cooler air then falls and so on. 

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Evaporation = particles escaping from a liquid. LIQUID TO GAS.

Particles can escape and become gas particles if they are travelling in the right direction and have enough kinetic energy to overcome the forces of attraction of the other particles.

The fastest particles are most likely to evaporate. When they do, the average speed of those remaining decreases and the kinetic energy they have decreases. As a result, the TEMPERATURE FALLS.

The rate of evaporation is quicker if:

  • The temperature is higher (average energy is higher so more particles have enough energy to escape).
  • The density is lower (forces between particles are weaker so more will have enough energy to overcome these forces).
  • The surface area is larger (more particles are near enough to the surface to escape.)
  • The airflow over the liquid is greater (if the concentration of an evaporating substance in the air is lower) - a greater airflow means that the air above the liquid can be replaced quicker. 
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Condensation involves a gas changing into a liquid.

As a gas cools, the partices lose kinetic energy and slow down. As a result, attractive forces between particles pull them closer together. Energy is also released when this happens, which is why steam touching the skin can cause scalds. 

Condensation takes place when the temperature is cool enough and the gas particles are close enough together. Water vapour in the air condenses when it touches cold surfaces.         EXAMPLE: Steam from a kettle is water vapour condensing as it spreads into cooler air. 

The rate of condensation is quicker if...

  • The temperature of the gas is lower (the energy of the particles is lower, so the particles move more slowly and clump together to form droplets).
  • The temperature of the surface is lower
  • The density is lower (stronger forces between particles, fewer can overcome these forces, more clump together).
  • The airflow is lower (concentration of substance in the air is higher). 
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Factors affecting Heat Loss

The rate of heat loss can be increased by increasing the surface area and the material an object is on.

By increasing the surface area, there is more surface for heat to be radiated off of. By changing the material, the rate of heat loss changes if the material is an insulator or conductor. More heat will be lost from a metal surface than a wooden surface.

Cooling fins are used in fridges, engines, microchips and motorbike engines, to get rid of heat or to cool down. The fins create a larger surface area.

Animals have to adapt to their climates. For example:

  • Elephants have large, flat ears to maximise their surface area.
  • Whales have a large volume to surface area ratio minimising heat loss.
  • Desert foxes live in hot climates and have larger ears than arctic foxes living in cold climates.
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A U-Value is the rate of heat transfer through a material.

HIGH U VALUE: A lot of heat energy is transferred

LOW U VALUE: A little heat energy is transferred

Low U-Values are good for insualting buildings. 

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Specific Heat Capacity

An substance's specific heat capacity is the amount of energy required to raise the temperature of 1kg of a substance by 1°C. It tells us how much energy a substance can store. 

Materials gain energy when warming up and release energy when cooling down - therefore, they can store heat energy. Specific heat capacity of water - 4200J/kg°C. 

  • Materials in heaters have high specific heat capacities to store lots of energy.
  • Water has a high specific heat capacity and can be easily pumped around.
  • Electric storage heaters store heat energy at night and release in during the day - they store heat using concrete or bricks, which have a high specific heat capacity.
  • Oil heating systems are often not as good as water heating systems. Oil has a lower SHC. However, oil has a higher boiling point so can safely reach higher temperatures. 
  • Formula: E = m × c × θ
  • E = energy transferred (in J)
  • M = mass of substance (in kg)
  • C = specific heat capacity (J/kg°C)
  • θ = temperature change (°C)
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Energy Efficiency/Energy Transfer

Energy can be transferred from one form to another, stored or dissipated, but can never be created or destroyed

  • Electrical energy: whenever a current flows
  • Light energy: from the sun, light bulbs etc
  • Sound energy: from anything that makes a noise
  • Kinetic energy: anything that moves
  • Nuclear energy: released from nuclear reactions
  • Thermal energy: flows from hot to cold objects
  • Gravitational potential energy: anything which can fall 
  • Elastic potential energy: anything that is stretched, strings, elastic bands etc
  • Chemical energy: possessed by food, fuel, batteries etc 

Gravitational potential, elastic potential and chemical energy are all forms of stored energy. 


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Payback Times

Measures of minimising heat loss in the home:

  • Loft insulation in roof: prevents heat loss through roof
  • Cavity wall insulation in walls: double thickness of wall with an insulating material between the two layers to prevent heat loss
  • Double glazing on windows: 2 layers of glass with vacuum in the middle. No heat loss through convection.
  • Draft excluders on doors: stops cool air entering or warm air leaving
  • LED lighting (uses less lighting), energy efficient machines, heat exchangers (removes heat from warm air that leaves the house).

Payback time = initial cost divided by the annual saving.

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Electrical Energy/Energy costs

Houses may have higher electricity bills if there are more people or appliances in a house, or if the appliances have been used for longer.

Energy transferred (kilowatt hours) = power (kilowatts) x time (hours)


The cost of running electrical appliances could be different each time depending on the temperature, the heaviness of the load and the amount of time run for. 

One unit = one kilowatt hour. 

Cost = no. of kilowatt hours x cost each unit

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Generating Electricity

Water can be heated to produce steam. This steam can turn a turbine, which is connected to a generator - the generator converts this kinetic energy into electricity.

Currently, we mainly burn coal, oil, gas and sometimes biofuels to heat the water. The steam produced from heating the water connects to a turbine and then a generator. It is then cooled and condensed, to keep this cycle going.

Gas has the fastest start-up time, whereas coal has the slowest start-up time.

The heat to produce the steam can also be generated through nuclear fuel. Plutonium or uranium are used. Nuclear fuel is generated through nuclear fission, whereby nuclei are split, producing lots of energy.

PROS of Nuclear Fuel: Produce lots of energy from little amounts of fuel. Do not produce pollutant gases.

CONS of Nuclear Fuel: Produces radioactive waste and have high decommissioning costs

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Renewable Energy

1. HYDROELECTRICITY - A dam separates high and low level water. A pipeline allows water to flow past a turbine, which generates a generator. Only produces enough for local area, visual pollution and habitat destruction. No pollutant gases, quite reliable.                                        

2. TIDAL ENERGY - A tidal barrage uses the water's kinetic energy to turn attached turbines. Only enough for local area, habitat destruction, visual pollution, unreliable (only useful when tides move). No pollutant gases. 

3. WAVES - Waves moving up and down drive air to turn turbines. Bell shaped object draws air in when waves go down and vice versa, turning turbines. Dependent on weather. 

4. GEOTHERMAL - used on volcanic areas. Pump cold water to where hot rocks are. The hot rocks heat the water, converting it to steam etc. Can't be build everywhere, only for local area, but produces lots of energy, is reliable and no pollutant gases. 

5. SOLAR - Converts light into electricity. Not the same as solar panels, which use heat. Not a lot of electricity, many panels required, can't be used at night, weather dependent. 

6. WIND - Wind turns the fans. Weather dependent, not lots of electricity, noise pollution

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Generating Energy Pros and Cons

Fossil fuel power stations: 

  • Fossil fuels produce CO2 and SO2 when burned. CO2 contributes to the greenhouse effect, which contributes to global warming. SO2 can cause acid rain.Scientists are finding new ways to lower CO2 and SO2, e.g Carbon Capture, a method by which CO2 is piped underground to disused oil and gases fields, so cannot be released into atmosphere.
  • Very reliable and produces lots of electricity.

Nuclear power stations:

  • Produces radioactive waste, which is dangerous (the stations are usually built deep underground). Expensive to build and take down.
  • No pollutant gases and lots of electricity produced. 

Renewable energy:

  • Can be variable in reliability and often only enough for local areas.
  • No pollutant gases whatsoever.
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Pumped Storage

A base load is the average amount of electricity required over 24 hours. There are often peaks and drops in demand for this electricity.

Pumped storage works in a similar way to hydroelectric energy.

Water from a high level flows downwards, turns the turbine and produces electricity - it has a very quick start up time, so can be used when there are peaks in demand.

When there is less demand, the low level water can be pumped back up through the turbine and is stored at the high level, ready for when it is needed. 

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The National Grid


The National Grid is a system of delivering electricity nationally.

  • Electricity leaves the power station at around 11,000V.
  • The step up transformer increases the voltage to around 132,000V.
  • It does this in order to decrease the current. The current has a heating effect, and this heat energy would be given off by the cable, as wasted energy.
  • The step down transformers decrease the current to around 11,000V again and then to 240V, the voltage of household sockets. 
  • Overhead cabling spoil landscapes, and some believe they may cause health issues, due to the electromagnetic field that surrounds them.
  • Underground cables mean that the cables are hidden (less visual pollution), but are expensive to lay and to repair, and must be waterproofed with special casing. 
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Transverse and Longitudinal Waves

Transverse Waves

  • The wavelength is the distance between 2 successive peaks or troughs.
  • The amplitude is the distance the peak/trough and the middle line. 
  • They are made by oscillations that move up and down. The oscillation is perpendicular to the direction of the wave/of the energy transfer.

Longitudinal Waves

  • The wavelength is the distance between two compressions or two rarefactions.
  • The compressions are the squashed together parts, and the rarefactions are the spread out parts.
  • They are made by oscillations that are parallel to the direction of the wave/energy transfer.

The frequency of a wave is the number of waves per second, and is measured in Hertz (Hz). 

Velocity of wave (m/s) = frequency (Hz) x wavelength (m). 

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Diffraction is the spreading of waves under certain conditions.

The amount of diffraction depends on how large the gap is in comparison the wavelength. Maximum diffraction occurs when the gap is the same length as the wavelength. 

Little diffraction occurs when the gap is much wider than the wavelength, and only a little diffraction is seen when the gap is a little wider than the wavelength. 

Light has a very small wavelength so can be diffracted but only by a tiny gap.

In hilly area, waves with longer wavelengths in comparison to the gap can reach houses on the other side, as the waves can diffract more. 


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Refraction is when waves change direction. The normal line is at a 90° to the surface. When going from one medium to another, the light ray will bend away from or towards the normal. 

Example 1: Glass block: when going from the air to the glass, the light ray bends TOWARDS the normal. When going from the glass to the air, the light ray bends AWAY from the normal.

Example 2: Water waves: the same occurs with water waves, when going from deep to shallow water. However, if the waves are moving perpendicular to or in line with the normal, they don't change direction.

Example 3: Person seeing something in water. When going from air to water, the ray bends AWAY from the normal, so the person will see the fish in a different position - the ray has been refracted. 


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The Electromagnetic Spectrum

The electromagnetic spectrum are the waves of vibrating electrical or magnetic fields.

On one end are the waves with the high frequencies and the low wavelengths, and on the other side are the waves with the low frequencies and the high wavelengths.

The spectrum, from the highest frequency to the lowest frequency goes:

Gamma, X Rays, Ultraviolet, Visible Light, Infrared, Microwaves, Radiowaves. 

All electromagnetic waves can transfer energy and can travel through a vaccuum at the same speed (the speed of light).

Radiowaves are used in TV signal, for the radio, for bluetooth and for phones.

Microwaves are used for mobile phones and satellite TV.

Infrared waves are used for remote controls and fibre optic cables.

Light waves are used for digital cameras and fibre optic cables. Infrared and light waves are used for fibre optic cables because there will be no loss of signal due to the speed they travel at. 

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Light bounces off objects into our eyes to allow us to see them. 

When light travelling in the same direction reflects from an uneven surface such as paper, the light reflects off at different angles. When light travelling in the same direction reflects from an even surface (e.g a mirror, smooth and shiny), the light reflects off at the same angle and you get a clear reflection. 

Differences between an object and a virtual image:

  • Image is virtual and upright
  • Image is as far behind the mirror as the object is in front. 
  • Image is laterally inverted - the left and right sides are swapped. 


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Sound Waves

Sound is caused by vibrations. Sound waves are longitudinal and require a medium to travel through. Sound cannot travel through a vaccuum as a result. Oscilloscopes convert longitudinal waves into transverse waves.

The amplitude determines the volume and the frequency determines the pitch. 

Echoes are reflections of sound, and sound waves are reflected by hard, flat surfaces. As a result, a room filled with furniture will not cause as much echoes and empty rooms, as the furtniture will absorb the sound quickly and stop it echoing. You hear a delay between the original sound and the echo because the echoed sound has to travel further.

Ships use echoes at the bottom of the ocean to see how deep they are and bats use echoes to navigate. 

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The Doppler Effect

When a car is moving towards you, you will hear a higher pitched sound. This is because as the car moves towards you, the wavelength of the sound wave gets shorter, and the frequency increases, causing a higher pitch.

When the car moves away from you, you will hear a lower pitched sound, as the wavelength gets longer, and the frequency decreases, causing a lower pitched noise. 

In the visible light spectrum, red has the longest wavelength, whereas blue/purple has the shortest wavelength. The colour that something appears depends on its wavelength.

Redshift is shown by objects in the galaxy. Stars often appear yellow or red - this means that they have a large wavelength, meaning they are moving AWAY from us. If objects appear blue, they have shorter wavelengths and are moving towards us.(http://www.gcse.com/eb/doppler.gif)

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The Big Bang Theory

Redshift is used as evidence for The Big Bang Theory. 

Most galaxies appear red, meaning  they have a large wavelength and that they're moving away from us. The ones that appear more red are moving faster.  

This means that at one point in the past, they were all together at one single point - singularity. 

CMBR is cosmic microwave background radiation, that was around at the time of The Big Bang. The Big Bang Theory is the only theory that can explain CMBR. 

It is detected from all parts of the universe, and was emitted after The Big Bang when the universe was still extremely hot. As the universe expanded, it cooled, and this radiation dropped in frequency and is now seen as microwave radiation. 

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