Key Stage 4 science · Physics – Energy

GCSE energy: stores, transfers, power and conservation

At GCSE, energy stops being a vague idea and becomes something students calculate. They describe a system as energy being moved between stores, work out the energy in kinetic, gravitational and elastic stores and the energy needed to heat a material, and define power as the rate at which energy is transferred. Throughout, they use the principle that energy is conserved, with some of it dissipated to the surroundings.

Energy stores, transfers and power, explained

Teachers describe energy as being held in stores and moved between them by transfers. The stores include kinetic (moving objects), gravitational potential (anything lifted up), elastic potential (stretched or squashed springs and bands), thermal, chemical (food, fuel, batteries), nuclear, magnetic and electrostatic. The transfers are by forces doing work, by an electric current doing work, by heating, and by waves such as light and sound. A student explaining a falling ball would say that energy is transferred mechanically from the gravitational store to the kinetic store.

The calculations all follow from this picture. Kinetic energy depends on mass and the square of the speed. Gravitational potential energy depends on mass, the gravitational field strength and the height raised. Elastic energy depends on the stiffness of the spring and the square of the extension. Energy needed to warm a material depends on its mass, its specific heat capacity and the temperature change. Work done by a force is the force multiplied by the distance moved in the direction of the force, and it equals the energy transferred. In every case energy is measured in joules, mass must be in kilograms, and distances in metres. Which of these equations students must recall, rather than being given, depends on the course, so it is worth asking the school.

Power is how quickly energy is transferred, or how quickly work is done. It is measured in watts, where one watt is one joule per second, and is found by dividing the energy transferred by the time taken. The same job done more quickly needs more power, even though the energy is unchanged. A 2 kilowatt kettle and a 100 watt lamp are therefore very different in how fast they transfer energy, but which one costs more to run depends on how long each is switched on. The same idea gives the energy transferred by an electric current: power multiplied by time. A 2,000 W kettle switched on for 3 minutes (180 seconds) transfers 2,000 × 180 = 360,000 J.

The principle that ties everything together is conservation of energy: energy cannot be created or destroyed, only transferred, so the total in a closed system never changes. Students sometimes find this odd because everyday objects slow down and stop. The answer is dissipation. Some energy always spreads out to the thermal store of the surroundings, through friction, air resistance or electrical heating, and becomes too spread out to be useful. It is not lost from the universe, but it can no longer do the job we wanted. Lubricating moving parts and insulating buildings are practical ways of reducing unwanted transfers.

Words your child will hear

Energy store: A way of describing where energy is held, such as kinetic, gravitational potential, elastic potential, thermal or chemical.
Work done: Energy transferred when a force moves an object. Force multiplied by distance moved in the direction of the force.
Joule (J): The unit of energy and of work done.
Power: The rate of transferring energy, or doing work. Measured in watts (W).
Closed system: A system where no energy enters or leaves, so the total energy stays the same.
Dissipated: Spread out into the surroundings, usually as thermal energy, and so not useful.
Specific heat capacity: The energy needed to raise the temperature of 1 kg of a substance by 1 °C.

What the National Curriculum says

The programme of study for Key Stage 4 says:

Students should be taught about energy changes in a system involving heating, doing work using forces, or doing work using an electric current: calculating the stored energies and energy changes involved
Students should be taught about power as the rate of transfer of energy
Students should be taught about conservation of energy in a closed system, dissipation

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

A worked example

A student of mass 60 kg climbs a flight of stairs with a vertical height of 3.0 m in 5.0 s. Use a gravitational field strength of 10 N/kg. How much energy is transferred to the gravitational store, what is the student’s useful power, and where does the rest of the energy go?

Choose the equation. Gravitational potential energy gained = mass × gravitational field strength × height. Check that the units are already kilograms, N/kg and metres.

Substitute and calculate.

E = m × g × h
  = 60 × 10 × 3.0
  = 1,800 J

Power is energy transferred divided by time taken.
```
P = E ÷ t
  = 1,800 ÷ 5.0
  = 360 W
```
Sense check. 360 W is less than a fifth of the power of a 2 kW kettle, which is a plausible effort for a short, fairly quick climb. If the student walked up in 10 s instead, the same 1,800 J would give a power of only 180 W.
Where does the rest go? The student’s body transfers more energy from its chemical store than the 1,800 J that ends up in the gravitational store. The extra heats the muscles and the surroundings, so it is dissipated to the thermal store. The total energy is still conserved.

1,800 J is transferred to the gravitational store, at a power of 360 W. The rest of the energy taken from the chemical store is dissipated as thermal energy. Using g = 9.8 N/kg would give 1,764 J and about 353 W.

Common mistakes and how to help

Saying energy is “used up” or “lost”

In daily life we say a battery is flat or a car has used its petrol. The language suggests the energy has gone out of existence.

What helps: Always ask where it went. The petrol’s chemical store has been transferred to kinetic and, mostly, thermal stores of the engine, exhaust and air. Drawing the stores before and after makes the total easy to compare.

Treating power and energy as the same thing

Both appear on appliances and bills, and people say “this uses lots of power” to mean either.

What helps: Power is a rate: joules per second. Energy is an amount: joules. Use a car analogy of speed and distance. A powerful heater transfers a lot of energy each second, but if it is on only briefly the total energy may be small.

Thinking that doubling the speed doubles the kinetic energy

Students assume that most quantities scale in step. The speed is squared in the kinetic energy equation, which is easy to overlook.

What helps: Work it through with numbers: at 5 m/s a 2 kg ball has 25 J, and at 10 m/s it has 100 J, four times as much. This is also why braking distances grow so quickly with speed.

Using grams or centimetres in the equations

Everyday measurements are in grams and centimetres, and the units are not always converted before substituting.

What helps: Make unit checks part of every question: write kg, m, s and N beside each number. A 500 g mass is 0.5 kg and a 20 cm extension is 0.20 m. Estimating the size of the answer catches most mistakes.

How to help at home

Measure your own stair power. Measure the height of one stair with a ruler and count the stairs. Time a calm, steady climb, with no running. Use the mass shown on bathroom scales, or a sensible estimate such as 60 kg if anyone would rather not be weighed. Your child can then work out energy and power, and compare answers within the family.
Drop and bounce a ball. Drop a ball from a measured height and see how high it bounces. It never returns to the starting height, so ask where the missing energy went. Some becomes sound, and some warms the ball, the floor and the air.
Follow the stores in a routine. Pick an everyday device, such as a toaster, torch or hairdryer, and ask your child to describe the stores and transfers, from the start to the final useful and wasted energy. It works in a car, on a bike or when making a cup of tea.
Compare power ratings. Read the rating plates on a few appliances and sort them by power. Then guess which uses the most energy in a week, and discuss why the ranking changes once the time on is taken into account.
Feel dissipation. Rub your hands together briskly for ten seconds and notice the warmth, or slide a book across a table and feel the surface. Kinetic energy is being dissipated as thermal energy by friction. Ask your child to explain why the book stops.

Builds on

Key Stage 3: Fuel uses and energy costs at home
Key Stage 3: Energy changes and transfers
Key Stage 3: Energy in systems and conservation