Efficiency, Power and Energy Resources
Efficiency is the fraction of the energy supplied to a device that is transferred usefully, calculated as useful output energy transfer divided by total input energy transfer, and no device is 100 percent efficient because some energy is always dissipated to the surroundings, mainly by heating. Power is the rate of energy transfer, in watts, where one watt is one joule per second, and energy resources are compared on reliability, cost, environmental impact and scale of supply.
Before you start
Make sure you're comfortable with these topics first:
Method
- Choose the right form of the efficiency equation for the data given: energies for a total transfer, powers for a rate. Both give the same answer for the same device.
- Find the useful output by subtraction when it is not given directly: total input minus the energy dissipated (usually stated as the energy transferred by heating) equals the useful output.
- Express efficiency as a decimal or a percentage, and never above 1 or 100 percent; an answer above 100 percent means useful and total have been swapped.
- Improve efficiency by naming a mechanism, not a wish: lubricating to reduce friction, insulating to reduce energy transferred by heating, using thicker or lower-resistance wires to reduce heating in a circuit, streamlining to reduce air resistance.
- Use power = energy transferred / time, converting time to seconds, and be able to rearrange it to find the energy transferred over a stated period, which is how domestic electricity bills are calculated.
- Compare energy resources on four axes every time: whether it is renewable, how reliable the supply is, the environmental impact including carbon dioxide and other pollutants, and the cost including the initial build cost against the running cost. Finish with a judgement that names the criterion you weighted most heavily.
Worked example
An electric motor is supplied with 800 J of energy. It transfers 240 J usefully as kinetic energy and the rest is dissipated by heating. Calculate the efficiency as a percentage, and state how much energy is wasted.
- Identify the useful output: 240 J transferred as kinetic energy.
- Identify the total input: 800 J supplied electrically.
- Write the equation: efficiency = useful output energy transfer / total input energy transfer.
- Substitute: 240 / 800 = 0.30.
- Convert to a percentage: 0.30 x 100 = 30 percent.
- Find the wasted energy: 800 - 240 = 560 J, dissipated to the surroundings by heating, mainly through friction in the bearings and resistance in the coils.
Practice questions
Try each question, then tap to reveal the answer.
Q1Give the equation for efficiency in terms of energy.Show answer
Answer: Efficiency = useful output energy transfer / total input energy transfer.
Q2A lamp is supplied with 60 J and transfers 6 J as light. Calculate its efficiency as a percentage.Show answer
Answer: 6 / 60 = 0.1, so 10 percent.
Q3Define power and give its unit.Show answer
Answer: Power is the rate of energy transfer (or the rate of doing work), measured in watts, where one watt is one joule per second.
Q4Why can no device be 100 percent efficient?Show answer
Answer: Some energy is always dissipated to the surroundings, mainly by heating due to friction or electrical resistance.
Q5Name three renewable energy resources.Show answer
Answer: Any three of: wind, solar, hydroelectric, tidal, wave, geothermal, biofuel.
Q6Give one advantage and one disadvantage of wind power.Show answer
Answer: Advantage: no fuel cost and no carbon dioxide released during generation. Disadvantage: the supply is unreliable because it depends on the wind, and some people object to the visual and noise impact.
Q7State one way to increase the efficiency of a motor.Show answer
Answer: Lubricate the moving parts to reduce friction, so less energy is dissipated by heating.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
A 2000 W heater is used for 5 minutes. Calculate the energy transferred, and calculate the efficiency if 540 kJ of that energy usefully heats the room.
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A country is deciding between building a new gas-fired power station and a large offshore wind farm. Compare the two options and give a reasoned recommendation.
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See real GCSE Science past-paper questions, with official mark schemes →
Free printable worksheet
Want more practice on paper? Download the efficiency, power and energy resources worksheet pack - 13 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 17 of GCSE Physics Workbook, the whole course as one free printable PDF.
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