Units Used In This Topic

Heating an object increases its thermal energy. Cooling an object decreases its thermal energy.

Some materials need more energy to heat them by 1°C than others. It depends on the specific heat capacity of the substance.

The specific heat capacity of a substance is the amount of energy required to raise the temperature of 1kg of the substance by 1°C.

(unit: J/kg°C)

Learn this definition

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

Question: Geothermal power stations pump water through heated rocks.
The temperature of the water increases from 20 °C to its boiling point of 100 °C
Calculate the change in thermal energy when the mass of water heated is 150 kg
Specific heat capacity = 4 200 J/kg °C

Question: The air in a balloon has a mass of 0.00320 kg
The temperature of the air in the balloon decreased by 215 °C
The change in thermal energy of the air in the balloon was 860 J
Calculate the specific heat capacity of the air in the balloon.

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

Insulating Houses

Thermal conductivity is how ‘good’ a material is at allowing thermal energy to flow across it. The higher the thermal conductivity of a material the higher the rate of energy transfer by conduction across the material (that means energy flows through it quickly).

An insulator is a material which prevents energy transfer. An insulator has a low thermal conductivity.

A conductor is a material which allows energy transfer. When particles gain thermal energy they vibrate more. This makes them bump into the particles next to them, causing those particles to vibrate more too. This process of vibrations being passed between particles is called thermal conduction.

Insulating Houses can reduce the amount of thermal energy which is transferred through the walls/windows/roofs to the outside. Air is a good insulator; most house insulation types use air to prevent thermal energy transfer.

Loft insulation is full of air pockets which prevent thermal energy transfer

You could use these methods to insulate your house effectively:

  1. Cavity wall insulation: the gap between the layers of the wall is filled with foam. Foam contains air which is a good insulator, which reduces thermal energy transfer from inside to outside.
  2. Double glazing: the gap between glass panes is filled with a gas called Argon. Argon has a low thermal conductivity which reduces thermal energy transfer from inside to outside.
  3. Loft insulation: the insulation is full of air pockets. Air is a good insulator, which reduces thermal energy transfer from inside to outside.


Efficiency is how good a device is at transforming the energy/power you give it (input energy/power) into useful output energy/power.

efficiency is a number between 0-1 or a percentage between 0% – 100%. Efficiency is never over 1 or 100%.

The energy efficiency for any energy transfer can be calculated using the equation:  

efficiency = useful power output / total input power  or efficiency = useful energy output / total input energy

Question: The total power input to a pumped storage power station is 600 MW.
The useful power output is 540 MW.
Calculate the efficiency of this pumped storage power station.

Question:  The efficiency of a motor was 0.15.
The student calculated that the useful output energy transfer was 1.20 J
Calculate the total input energy transfer.

You can reduce ‘wasted’ energy and improve efficiency by Reducing unwanted energy transfers. Examples include:

  1. Lubrication; Reduce energy transfer by heating by oiling squeaky joints or parts which rub together
  2. The use of thermal insulation; prevents energy transfer by conduction


Infrared is one of the regions of the electromagnetic spectrum. Infrared transfers thermal energy from place to place

The hotter a material the more infrared it emits (gives out to the surroundings)

A good emitter of infrared is also a good absorber of infrared. 

Colour of a material changes how well it emits and absorbs Infrared.

Dark Matt surfaces absorb and emit infrared better than light shiny surfaces. This is why you feel hotter when wearing a black shirt on a sunny day then a white one.

Question: Three mugs (white, grey and black) are filled with hot water, each at the same temperature.
They are left to cool for 5 minutes.
Which mug will be the coolest after 5 minutes? Why?