What is work done, how is power calculated, and what is efficiency?
Work done by a force, the equation for power, the calculation and meaning of efficiency, and the difference between renewable and non-renewable energy resources.
A focused answer to Edexcel GCSE Combined Science Topics 3 and 8 (CP3 and CP8), covering work done by a force, the equation for power, the calculation and meaning of efficiency, and the difference between renewable and non-renewable energy resources.
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What this dot point is asking
Edexcel wants you to calculate the work done by a force, use the equation for power, calculate and interpret efficiency, and compare renewable and non-renewable energy resources.
Work done
Power
A more powerful device transfers the same energy in less time, or more energy in the same time.
Efficiency
Efficiency is always less than 1 (or 100%) because some energy is always dissipated, usually as heat. Multiply by 100 to give a percentage.
Energy resources
A non-renewable resource will run out because it is used faster than it forms: the fossil fuels (coal, oil, gas) and nuclear fuel. A renewable resource is replenished and will not run out: wind, solar, hydroelectric, tidal, wave, geothermal and biomass.
Renewables release little or no carbon dioxide when generating electricity, helping to reduce climate change, but many are unreliable (wind and solar depend on the weather) or have high set-up costs. Fossil fuels are reliable and currently cheap but release carbon dioxide and other pollutants and will run out.
There is often a trade-off to weigh up. Nuclear fuel produces a lot of energy with no carbon dioxide and is reliable, but it produces radioactive waste and has a high decommissioning cost. Wind and solar are clean and free to run once built, but they only generate when the wind blows or the Sun shines. Hydroelectric and tidal are reliable and clean but can damage habitats and are limited to suitable locations. Exam questions often ask you to choose a resource for a particular situation (for example a remote sunny location, or a country wanting to cut carbon emissions) and to justify the choice by balancing reliability, cost and environmental impact.
The reason no device is ever perfectly efficient links back to the conservation of energy: the input energy is conserved, but some of it is always transferred to less useful stores (usually the thermal store of the surroundings). Reducing this wasted energy, for example by lubricating moving parts or insulating a building, raises the efficiency.
Try this
Q1. State the equation for work done. [1 mark]
- Cue. Work done = force times distance.
Q2. A motor supplies and does of useful work. Calculate its efficiency. [2 marks]
- Cue. Efficiency .
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20204 marksA motor does of useful work in while taking in of energy. Calculate the power output of the motor and its efficiency.Show worked answer →
A 4-mark calculation using power and efficiency.
Power = work done / time (2 marks). Efficiency = useful energy output / total energy input , which is (2 marks).
Markers reward power with units (watts), and efficiency as a fraction or percentage. Efficiency has no unit and is always less than 1 (or 100%).
Edexcel 20224 marksCompare a renewable and a non-renewable energy resource, giving one example of each and one advantage and one disadvantage of using renewables.Show worked answer →
A 4-mark compare question on energy resources.
A non-renewable resource will run out and is not replaced on a human timescale, for example a fossil fuel such as coal, oil or gas (1 mark). A renewable resource is replenished and will not run out, for example wind, solar or hydroelectric (1 mark). An advantage of renewables is that they do not release carbon dioxide when generating electricity, so they reduce climate change (1 mark). A disadvantage is that many are unreliable (for example wind and solar depend on the weather) or have a high set-up cost (1 mark).
Markers reward a correct example of each type and a balanced advantage and disadvantage of renewables.
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Sources & how we know this
- Edexcel GCSE (9-1) Combined Science (1SC0) specification — Pearson (2016)