Energy transfer diagrams and energy stores

What does 'changes in energy' mean?

Scientists describe energy changes by identifying energy stores before and after an event. A moving object has a kinetic energy store; a raised object has a gravitational potential energy store; a stretched spring has an elastic energy store; fuels, foods and batteries have chemical energy stores; and warm objects have thermal energy stores.

The pathway explains how energy moves between those stores. A force can transfer energy mechanically, an electric current can transfer it electrically, and a temperature difference can transfer it by heating. Light, infrared and other waves transfer energy by radiation.

Examples of energy changes

When a cyclist speeds up, energy is transferred from a chemical store in the rider's body to the kinetic stores of the bicycle and rider. When a ball falls, its gravitational potential store decreases while its kinetic store increases. When bicycle brakes slow a wheel, energy is transferred mechanically from the wheel's kinetic store and by heating to thermal stores in the brakes and surroundings.

Real processes are not perfectly efficient. Friction, air resistance and electrical resistance transfer some energy to thermal stores in the surroundings. That energy still exists, but because it is spread out it is usually harder to use again.

Three energy-store and pathway questions

• An electric kettle is used to heat water. Name the pathways and the store that increases. Answer: Energy is transferred electrically to the kettle's heating element and then by heating to the water. The energy in the water's thermal store increases.
• A wind-up toy car is released and speeds across the floor. Describe the transfer using stores and pathways. Answer: The spring's elastic store decreases. Energy is transferred mechanically to the car's kinetic store, with some transferred by heating to the thermal stores of the car, floor and surroundings.
• Why is it wrong to say a lamp ‘turns electricity into light energy’? Answer: It is wrong because light is not an energy store. The lamp is supplied electrically. Some energy is transferred away by visible electromagnetic radiation and some by heating. Ultimately the transferred energy increases thermal stores in the surroundings.