Energy stores and energy transfers

Energy stores are the ways an object or system can hold energy. GCSE Physics uses eight: kinetic, gravitational potential, elastic potential, thermal, chemical, magnetic, electrostatic and nuclear. Energy is never used up. It moves from one store to another through energy transfers: a force doing work, a current doing work, heating, or radiation.

This is the first topic in the Energy unit of AQA GCSE Physics (section 4.1) and in Topics 3 and 8 of Edexcel GCSE Physics. Get the language right here and every later energy question, from kettles to roller coasters, follows the same pattern.

What are energy stores?

An energy store is a way of describing where energy is at one moment. A moving car has energy in its kinetic store. A book on a high shelf has energy in a gravitational potential store, which really belongs to the book and the Earth together. A hot cup of tea has energy in its thermal store.

Physicists talk about a system: AQA defines it as an object or group of objects. When a system changes, the way its energy is stored changes too. Drop the book and the gravitational potential store empties while the kinetic store fills. The total stays the same.

The 8 energy stores

These are all eight energy stores, the full list most GCSE courses and the Institute of Physics use. Four of them have an equation you need at GCSE.

In the equations, m is mass in kg, v is speed in m/s, g is gravitational field strength in N/kg, h is height in m, k is the spring constant in N/m, e is extension in m, c is specific heat capacity in J/kg °C and Δθ is the temperature change in °C. Every answer comes out in joules (J).

Kinetic store

Every moving object has energy in its kinetic store. Speed matters far more than mass, because speed is squared: double the speed and the energy goes up four times.

Here x is the speed in m/s and y is the energy in the kinetic store in joules, from Ek = ½ × 80 × v² = 40v². Going from 3 m/s to 6 m/s takes the energy from 360 J to 1,440 J: twice the speed, four times the energy. That is why stopping distances grow so quickly with speed.

Gravitational potential store

Lift an object and its gravitational potential store fills. Ep = mgh gives the change when it moves through a height h. On Earth g is about 9.8 N/kg, and your exam paper will tell you the value to use.

Elastic potential store

Stretch, squash or bend something that springs back and you fill its elastic potential store. Ee = ½ke² works for a spring as long as you don't stretch it past its limit of proportionality.

Thermal store

Heat an object and its particles move faster, so its thermal store fills and its temperature rises. ΔE = mcΔθ gives the energy needed. Water has a high specific heat capacity, about 4,200 J/kg °C, so it takes a lot of energy to warm up.

Chemical, magnetic, electrostatic and nuclear stores

You describe these four but don't calculate them at GCSE. A chemical store empties when a reaction happens: fuel burning, food being respired, a battery driving a current. Strictly, the store belongs to the fuel and the oxygen together, since neither releases energy alone. Nuclear stores empty in fission, fusion and radioactive decay. Magnetic and electrostatic stores change when magnets or charges move closer together or further apart.

Energy transfers: the 4 pathways

Energy moves between stores in four ways, called energy transfers or pathways. AQA and Edexcel list three ways the energy of a system can change: work done by forces, work done when a current flows, and heating. Radiation is the fourth pathway.

Work done is energy transferred, so both are measured in joules. A force of 1 N moving an object 1 m in the direction of the force transfers 1 J.

Energy stores and transfers in five exam situations

AQA and Edexcel both name the same five situations in their specifications, so these come up again and again. For each, say which store empties, how the energy is transferred and which store fills.

Example: a ball thrown straight up

A 0.2 kg ball is thrown straight up at 12 m/s. At the start its kinetic store holds:

As it rises, gravity does work against the motion and the energy moves to the gravitational potential store. At the top the ball stops for an instant, so the kinetic store is empty. Ignoring air resistance, all 14.4 J is now in the gravitational potential store, which gives the height:

Here x is time in seconds and y is the energy in the kinetic store in joules. The kinetic store empties on the way up, is empty at the top after 1.22 s, and fills again on the way down. The gravitational potential store does the opposite, so the two always add up to 14.4 J. With air resistance, a little energy goes to the thermal store of the air each moment, so the ball comes back slightly slower.

Calculating the energy in a store: worked examples

Write the equation, put in the numbers in standard units, then give the answer in joules. These rise in difficulty.

Example 1: kinetic store

A cyclist on a bike, 80 kg in total, rides at 6 m/s. How much energy is in the kinetic store?

The kinetic store holds 1,440 J. Check it in the kinetic energy calculator.

Example 2: gravitational potential store

A 2.5 kg box is lifted from the floor onto a shelf 1.8 m high. Take g = 9.8 N/kg. How much energy is transferred to the gravitational potential store?

The store gains 44.1 J, the same as the work done lifting the box.

Example 3: elastic store into kinetic store

A toy launcher has a spring with spring constant 40 N/m, compressed by 15 cm. It fires a 0.05 kg dart. Find the energy in the elastic store and the dart's launch speed, assuming every joule reaches the dart.

Change 15 cm to 0.15 m first.

All 0.45 J goes to the dart's kinetic store, so rearrange Ek = ½mv² for v:

The elastic store holds 0.45 J and the dart leaves at 4.24 m/s. In real life some energy is dissipated, so it would be a little slower.

Try it

Potential Energy Calculator

Work out gravitational (mgh) and elastic potential energy

Open the calculatorGo to its page

Example 4: thermal store and the kettle

A 3 kW kettle brings 0.5 kg of water at 10 °C up to boiling point, 100 °C. Take 4,200 J/kg °C for the specific heat capacity of water. How much energy goes into the thermal store of the water, and how long would it take if none were wasted?

A power of 3 kW is 3,000 joules every second, so:

The water gains 189,000 J (189 kJ), which would take 63 s. A real kettle takes longer because some energy heats the kettle itself and the air around it. The specific heat capacity calculator does this calculation step by step.

Example 5: a roller coaster

A 500 kg roller coaster car sits still at the top of a 20 m drop, then rolls down it. Take g = 9.8 N/kg. (a) Find its speed at the bottom if no energy is dissipated. (b) Friction and air resistance actually transfer 20,000 J to thermal stores on the way down. Find its real speed.

(a) The gravitational potential store loses:

All of it goes to the kinetic store, so:

(b) Only 98,000 − 20,000 = 78,000 J reaches the kinetic store:

The car reaches 19.8 m/s with no losses and 17.7 m/s in reality. Notice that in part (a) the mass cancels: any object falling 20 m freely reaches 19.8 m/s.

Conservation of energy

The rule behind all of this is the conservation of energy. In AQA's words, energy can be transferred usefully, stored or dissipated, but cannot be created or destroyed. In a closed system, where nothing gets in or out, there is no net change to the total energy, however much it moves between stores.

Dissipation: where wasted energy goes

In every real change, some energy ends up stored in less useful ways. This is called dissipation, and the energy is often described as wasted. Most of it ends up in the thermal store of the surroundings, spread so thinly that you can't get it back to do anything useful. Friction in a machine, the warm casing of a phone charger and the noise of a car engine are all dissipation.

You can reduce unwanted transfers. Lubrication (oiling moving parts) cuts friction. Thermal insulation slows energy loss by heating: thicker walls, and materials with a lower thermal conductivity, make a building cool more slowly.

Efficiency

Efficiency tells you what fraction of the energy put in comes out as useful energy.

Suppose the kettle in example 4 actually takes 75 s. It transfers 3,000 × 75 = 225,000 J, but only 189,000 J reaches the water, so its efficiency is 189,000 ÷ 225,000 = 0.84, or 84%. You can also work out efficiency from power, as useful power output divided by total power input. The work, energy and power calculator handles both.

Sankey diagrams

A Sankey diagram shows energy transfers as arrows whose widths are drawn to scale. One wide arrow comes in on the left and splits into a useful arrow and a wasted arrow, and the widths always add back up to the input. For the kettle, the input arrow stands for 225,000 J, the useful arrow to the water for 189,000 J and the wasted arrow, bending down, for 36,000 J.

Check your understanding of energy stores and transfers before moving on.

How to describe energy changes in the exam

Questions often ask you to describe the changes in the way energy is stored. A strong answer has three parts:

1. The store that empties, named with its object: "the kinetic store of the car".
2. How the energy is transferred: mechanically, electrically, by heating or by radiation.
3. The store that fills, including where the dissipated energy goes: "the thermal store of the brakes and then the surroundings".

For a battery-powered torch: energy is transferred from the chemical store of the battery, electrically, to the lamp. The lamp transfers energy by radiation (light), and some by heating, so in the end it all ends up in the thermal store of the surroundings.

Common mistakes

• Saying energy is used up or lost. It is transferred or dissipated. The total never changes.
• Calling light, sound or electricity a store. They are transfers. A battery has a chemical store, and the current is how the energy leaves it.
• Leaving out the wasted energy. In every real transfer some energy is dissipated, usually to the thermal store of the surroundings.
• Mixing up heat and temperature. Temperature tells you how hot something is. The energy in a thermal store also depends on how much stuff there is: a bath at 40 °C holds far more energy than a cup of tea at 80 °C.
• Forgetting to square the speed in Ek = ½mv², or squaring ½mv as a whole.
• Wrong units. Change grams to kg, cm to m and kJ to J before you use an equation.

Practise energy questions

Energy appears in every GCSE Physics course, so it comes up often in the daily GCSE Physics Foundation and GCSE Physics Higher papers. Each is a short timed paper of 8 questions worth 25 to 35 marks, with a new one every day.

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