7.1.1Energy – Forces Doing Work
Energy Stores and Transfers — Edexcel GCSE Combined Science 1SC0 - Physics Revision Notes & Topical Questions
5 min read 8 parts Lesson 2 of 8
Section 1 of 8Energy as a quantity
Energy is a measure of how much change a system can bring about.
A useful habit is to think of energy as being stored in objects and then transferred from one store to another. The energy is the same stuff throughout; only the way it is held changes.
Section 2 of 8The main energy stores
You should be able to name and recognise these stores:
Light and sound are not storesthey are ways of moving energy, which brings us to transfers.
Section 3 of 8Ways energy is transferred
Energy moves between stores by four main pathways.
When describing a change, name the store that loses energy, the store that gains it, and the pathway. For a kettle: electrical energy is transferred to the thermal store of the water.
Section 4 of 8Describing real systems
A bouncing ball dropped from your hand starts with GPE.
A person running uses energy from the chemical store of food. Most of it ends up as thermal energy (you get hot), the rest as kinetic energy of the body and the work done against air resistance.
Energy changes in a circuit
- a battery's chemical store transfers energy electrically to a motor;
- the motor transfers some of it mechanically to the kinetic store of a lifted load (and raising its GPE) and some by heating to the surroundings.
Section 5 of 8Useful and wasted energy
In most devices only part of the energy supplied ends up in the store we wanted. The rest, often thermal, is called wasted energy. It spreads out into the surroundings and becomes hard to reuse. This idea leads to conservation of energy and efficiency, covered in the next two sub-topics.
Section 6 of 8Worked examples
A 0.50 kg ball is thrown straight up and, at the very top, it momentarily stops. Describe the energy transfers from the moment it leaves the hand to the top, ignoring air resistance.
At the hand, the ball has kinetic energy (it is moving fast).
Worked examplesAs it rises, the gravitational force does negative work on it, so energy is transferred mechanically from the kinetic store to the gravitational potential store.At the top, speed is zero, so the kinetic store is zero and the GPE store has gained all of the energy.The ball has not lost energy, it has changed store.An electric toy car runs on a battery. Name the stores and pathways for the first few seconds of motion.
The chemical store of the battery decreases. Energy is transferred electrically through the wires to the motor. The motor transfers energy mechanically to the kinetic store of the car. Some energy is transferred by heating to the thermal store of the wires, motor and surroundings, and some as sound waves. Stores losing energy: chemical. Stores gaining energy: kinetic and thermal.
A hot cup of tea is left on a table and cools. State the store that decreases, the store that increases, and the pathways.
The thermal store of the tea decreases. The thermal stores of the air and table increase. Energy leaves by heating through conduction (to the table), convection (to the air above) and radiation (infrared). Evaporation also removes thermal energy from the tea.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Energy store | A place where energy is held, such as kinetic, chemical or thermal. |
| Energy transfer | The movement of energy from one store to another. |
| Joule (J) | The SI unit of energy. |
| Gravitational potential energy | Energy stored because of an object's height above the ground. |
| Elastic energy | Energy stored in a stretched or compressed object. |
| Thermal energy | Total random kinetic and potential energy of particles in a substance. |
| Wasted energy | Energy transferred to stores that are not useful for the purpose. |
Section 8 of 8Exam technique
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