IGCSE Physics / Energy transfers
Energy stores and how energy is transferred
Edexcel International GCSE Physics (4PH1), chapter 14: Energy transfers, specification point 4.2, 4.1. Free lesson.
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Energy is held in stores and can be transferred from one store to another in four ways. Naming the store, the object and the transfer helps you describe energy changes clearly.
The five stores linked to motion, position or shape
Energy is measured in joules (J) and is always held in a store. Five stores belong to objects because of how they move, where they are or their shape. Kinetic: anything moving, such as a rolling ball. Gravitational: anything raised up, such as a book on a shelf. Elastic: anything stretched or squashed, such as a stretched spring. Magnetic: magnets attracting or repelling, such as two fridge magnets. Electrostatic: electric charges attracting or repelling, such as a charged balloon.
The three stores inside substances
These stores depend on what a substance is made of and how hot it is. They are held inside the material itself. Chemical: energy released in chemical reactions. Fuels, food and batteries have chemical stores. Thermal: an object has more energy in its thermal store when it is hotter, such as a cup of tea. Nuclear: energy stored in an atom's nucleus and released in nuclear reactions, such as in uranium in a reactor.
The four ways energy is transferred
Energy moves from one store to another through a transfer. Name how it moves, rather than treating the transfer as another store. Mechanically: a force moves something, such as when you push a trolley. Electrically: a current flows, such as when charging a phone. By heating: energy moves from a hotter object to a colder one, such as a hob warming a pan. By radiation: waves carry energy, such as sunlight reaching Earth.
Describe where energy starts and ends
An energy description names the starting store, the transfer and the ending store. Include the object that holds each store. This is clearer than saying “light energy” or “heat energy”. In a torch, energy goes from the chemical store of the battery, electrically to the lamp, then by light to the thermal store of the surroundings. In a kettle, energy is supplied electrically from the mains and transferred by heating to the thermal store of the water. As a ball falls, energy moves from the gravitational store of the ball to its kinetic store. When a car brakes, energy moves…
Say where the energy goes
Energy is not lost. Saying it is “lost as heat and sound” does not name the transfer or the store where the energy ends up. Heat is not a store. Instead, say that energy is transferred by heating and by sound to the thermal store of the surroundings. A complete description gives both the way energy is transferred and the store it ends up in.
Apply the store-to-store description
When a stretched rubber band is released, its store changes as it starts moving. State the transfer and name the object holding each store.