How is energy transferred and stored, and how do we calculate efficiency?
Energy stores and transfers, the conservation of energy, wasted energy, and calculating efficiency.
A focused answer to the WJEC GCSE Science Double Award Unit 3 topic on energy, covering energy stores and transfers, the conservation of energy, energy wasted as heat, and calculating efficiency with the efficiency equation.
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What this dot point is asking
WJEC Double Award Unit 3 wants you to describe energy stores and transfers, state the conservation of energy, identify wasted energy, and calculate efficiency.
Energy stores and transfers
For example, a falling ball transfers energy from its gravitational store to its kinetic store; a battery-powered torch transfers energy from a chemical store electrically and then as light.
Conservation of energy
This is why you can never get more energy out of a device than you put in, and why "wasted" energy is not really lost but transferred to a less useful store (usually heating the surroundings).
Wasted energy
In every device, some of the input energy is transferred to useful stores and some is wasted. The wasted energy is usually transferred to the thermal store of the surroundings by heating, often because of friction or electrical resistance. For example, a light bulb usefully gives out light but also wastes energy as heat. Wasted energy spreads out and becomes very hard to use again.
Efficiency
No device is 100% efficient, because some energy is always wasted, usually as heat.
Sankey diagrams
Energy transfers are often shown on a Sankey diagram, where the width of each arrow is proportional to the amount of energy. The total energy in enters on the left, the useful energy continues as one or more arrows, and the wasted energy branches off (usually downwards) as heat. Because energy is conserved, the widths of the useful and wasted arrows together must equal the input arrow. You can read the useful and total energy off a Sankey diagram and use them to calculate efficiency, so being able to interpret one is a key skill in this topic.
Reducing wasted energy
Devices can be made more efficient by reducing the energy wasted, usually the heat lost to the surroundings. Lubricating moving parts reduces friction, so less energy is wasted as heat; insulating a hot device keeps more of the energy in the useful store; and using lower-resistance wiring reduces heat lost electrically. For example, modern LED bulbs are far more efficient than old filament bulbs because they waste much less energy as heat. Linking a method to the type of wasted energy it cuts is a common application question.
Try this
Q1. State the law of conservation of energy. [1 mark]
- Cue. Energy cannot be created or destroyed, only transferred between stores or stored.
Q2. A device takes in 500 J and usefully transfers 400 J. Calculate its efficiency. [2 marks]
- Cue. , or 80 percent.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style3 marksA motor is supplied with 200 J of energy and transfers 150 J usefully. Calculate its efficiency as a percentage.Show worked answer →
A Unit 3 calculation. Efficiency = useful energy out / total energy in (1 mark) = = 0.75 (1 mark); as a percentage, (1 mark). Markers reward the equation, the substitution and the percentage. A common error is to divide the wrong way round or to forget to multiply by 100.
WJEC style4 marksDescribe the energy transfers when an electric kettle heats water, and explain why it is not 100 percent efficient.Show worked answer →
A Unit 3 explain question worth 4 marks. Reward: the kettle transfers electrical energy to the thermal (heat) energy store of the water (1); useful energy heats the water (1); but some energy is wasted, heating the kettle itself and the surroundings (1); so not all the input energy ends up usefully heating the water, making it less than 100 percent efficient (1). Markers credit the useful transfer, the wasted energy and the conclusion. A common error is to say energy is "lost" (destroyed) rather than transferred to less useful stores.
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