Efficiency — AQA GCSE Combined Science
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Efficiency explained
Efficiency measures how much of the energy supplied to a device is transferred usefully rather than wasted.
Read the full explanation
For any energy transfer, identify the total input energy and the useful output energy, then divide useful output by total input. The result is a fraction or percentage, always between 0 and 1, or 0% and 100%. For example, a lamp takes in 100 J of electrical energy and transfers 20 J as useful light, so efficiency = 20 J ÷ 100 J = 0.20 or 20%. The remaining 80 J is dissipated, often by heating the surroundings. The equation applies to mechanical, electrical, thermal and other transfers, provided the same unit is used for both energies. Efficiency cannot exceed 1 because energy is conserved and some is always dissipated.
efficiency = useful output energy transfer / total input energy transfer
This equation defines efficiency as the ratio of useful output energy transfer to total input energy transfer. To use it, identify the energy supplied to the device and the energy transferred usefully, then divide. For example, an electric motor is supplied with 500 J and transfers 350 J usefully as kinetic energy, so efficiency = 350 J ÷ 500 J = 0.70 or 70%. The remaining 150 J is wasted, usually heating the motor and surroundings. Efficiency is a dimensionless number between 0 and 1, or a percentage between 0% and 100%. The same unit must be used for both energies. A higher efficiency means a greater proportion of input energy is transferred usefully, reducing wasted energy and running costs.
Efficiency may also be calculated using the equation:
Efficiency compares the useful energy or power transferred by a device with the total energy or power supplied to it. The word 'also' signals that this is an alternative route: you may already know efficiency = useful output / total input using energy in joules, and the same idea applies using power in watts. Because power is energy transferred each second, dividing useful power output by total power input gives the same fraction as dividing useful energy by total energy over the same time. For example, a motor drawing 500 W and delivering 350 W usefully has efficiency 350 / 500 = 0.70, or 70%. The remaining 150 W is wasted, often as heating of surroundings. Always identify which value is useful and which is total before substituting, and remember efficiency has no unit.
efficiency = useful power output / total power input
This equation gives efficiency as the ratio of useful power output to total power input. Power is the rate of energy transfer, measured in watts, so a 60 W lamp that emits 12 W as useful light has efficiency 12 / 60 = 0.20, or 20%. The other 48 W is transferred to the surroundings, often by heating. Because both quantities are powers in watts, the units cancel and efficiency has no unit; it is a number between 0 and 1, or a percentage between 0% and 100%. No real device reaches 100% efficiency because some energy is always dissipated. When using the equation, identify the useful output first, then the total input, substitute, and evaluate. If a percentage is required, multiply the decimal by 100.
(HT only) Students should be able to describe ways to increase the efficiency of an intended energy transfer.
Efficiency compares the useful energy transferred by a device with the total energy supplied to it. No device is perfectly efficient because some energy is always dissipated to the surroundings, often by heating due to friction or resistance. To increase efficiency, you must reduce these unwanted transfers. Practical methods include lubricating moving parts to reduce friction, tightening loose parts to stop vibration, using thermal insulation to cut heat loss, and streamlining shapes to lower drag. For example, a motor is made more efficient by oiling its bearings, while a hot water tank's efficiency improves by adding thermal insulation to reduce heat loss. Improving efficiency means a greater proportion of the input energy becomes useful, so less energy is wasted for the same useful output.
Your focus
- Select the correct useful output and total input energies for a described transfer.
- Apply the efficiency equation accurately to obtain a decimal or percentage.
- Interpret an efficiency value to compare devices and identify energy losses.
Show all 15 objectives
- Apply the efficiency equation to numerical data from a device or process.
- Convert between decimal and percentage forms of efficiency.
- Explain how efficiency relates to wasted energy and the cost of running a device.
- Select the useful power output and total power input from given data and substitute them into the efficiency equation.
- Calculate efficiency as a decimal or percentage and interpret what the value means for a device.
- Explain why efficiency can be found using either energy or power values and identify where wasted power goes.
- Apply efficiency = useful power output / total power input to numerical data and obtain a correct value.
- Convert between decimal and percentage forms of efficiency and interpret the result.
- Explain why efficiency has no unit and why it is always less than 1 for a real device.
- Identify the useful and wasted energy transfers for a given device.
- Describe at least two practical methods that reduce unwanted energy transfers.
- Explain how reducing wasted transfers increases the efficiency of the intended energy transfer.
Efficiency exam tips
Marking Points
- State the equation as efficiency = useful output energy transfer ÷ total input energy transfer.
- Identify the useful output energy as the energy transferred to the intended store or form, such as light from a lamp or kinetic energy from a motor.
- Identify the total input energy as the energy supplied to the device before any transfer occurs.
- Substitute values with matching units, usually joules, then divide to obtain a decimal or multiply by 100 to obtain a percentage.
- Recognise that efficiency has no unit and must lie between 0 and 1, or between 0% and 100%.
- Explain that wasted energy is not destroyed but transferred to less useful stores, commonly thermal energy in the surroundings.
- Quote the equation in the form efficiency = useful output energy transfer ÷ total input energy transfer.
- Distinguish useful output energy from total input energy in a given device or process.
- Substitute numerical values into the equation, converting to a common unit such as joules where necessary.
- Calculate the efficiency as a decimal and, if required, convert it to a percentage by multiplying by 100.
- Interpret the result, for example stating that 70% of the input energy is transferred usefully and 30% is dissipated.
- Compare efficiencies of two devices by calculating both values and identifying the device that wastes less energy.
- State that efficiency compares useful output with total input and can be found from either energy or power values.
- Identify the useful power output and the total power input from the information given, in watts.
- Substitute correctly into efficiency = useful power output / total power input.
- Evaluate the fraction and express the result either as a decimal or as a percentage.
- Recognise that wasted power is the difference between total power input and useful power output, and that efficiency has no unit.
- Write the equation as efficiency = useful power output / total power input.
- Identify the useful power output and the total power input in watts from the question.
- Substitute the values correctly and evaluate the ratio.
- Convert a decimal answer to a percentage by multiplying by 100 when required.
- Interpret the result, for example 0.25 means 25% of the input power is transferred usefully.
- State that efficiency is the ratio of useful energy transferred to total energy supplied, often expressed as a percentage.
- Identify a named unwanted transfer, such as energy dissipated by friction between moving parts or by heating of wires.
- Describe a specific method that reduces that transfer, for example lubricating bearings, tightening loose components, adding insulation or streamlining a shape.
- Explain that reducing wasted energy raises the useful proportion of the total input, so efficiency increases.
- Apply the idea to a device, such as a motor, vehicle or hot water tank, naming the intended useful transfer and the unwanted transfer being reduced.
Examiner Tips
- 💡Write the equation, then substitute values with units before calculating, so method marks can be awarded even if the arithmetic slips.
- 💡Check whether the question asks for a decimal or a percentage; if a percentage is required, multiply the decimal by 100 and include the % sign.
- 💡Use the answer to judge reasonableness: an efficiency above 100% signals that the values have been reversed or misread.
- 💡Underline the useful output and total input values in the question before substituting them into the equation.
- 💡Show the substitution line clearly, including units, because method marks are often available even when the final value is wrong.
- 💡Round sensibly and state the unit or symbol: efficiency has no unit, but a percentage answer needs the % sign.
- 💡Underline the words useful and total in the question before choosing numbers, because the total is often the larger value.
- 💡Show the substitution line, not just the final answer, so that a correct method can be followed even if the arithmetic slips.
- 💡Check whether the answer should be a decimal or a percentage, and give the unit only when the quantity has one; efficiency itself has no unit.
- 💡Write the equation first, then substitute, so the examiner can see the correct relationship even if the final value is wrong.
- 💡Check that the useful output is smaller than the total input; if it is not, you have probably swapped the values.
- 💡Give efficiency as a decimal or a percentage as requested, and never attach a unit to it.
- 💡Name the device and the intended useful energy transfer before describing the improvement, so your answer is clearly focused.
- 💡Use precise vocabulary such as lubricate, insulate and streamline rather than vague words like 'make it better'.
- 💡Link each method to the wasted transfer it reduces, then state the effect on the proportion of useful energy.
Common Mistakes
- Dividing total input by useful output instead of useful output by total input; correct by writing the equation first and checking that the answer is less than 1 or 100%.
- Using different units for the two energies, such as joules for input and kilojoules for output; correct by converting both to the same unit before dividing.
- Treating the wasted energy as the useful output; correct by identifying the intended useful transfer, for example light rather than heat from a filament lamp.
- Forgetting to convert a percentage answer back to a decimal when the question asks for efficiency as a number; correct by dividing the percentage by 100.
- Using the wasted energy instead of the total input energy as the denominator; correct by ensuring the denominator is the total input energy, which can be found by adding the useful and wasted energies if not given directly.
- Leaving the answer as a fraction without simplifying or converting; correct by giving a decimal or percentage as requested.
- Dividing total power input by useful power output instead of the other way round; correct this by checking that the fraction is less than 1 for any real device.
- Mixing units, such as using useful energy in joules with total power in watts; correct this by converting both quantities to the same type, either both energies or both powers.
- Forgetting to convert a decimal to a percentage when the question asks for a percentage; correct this by multiplying the decimal by 100 and adding the % sign.
- Using the wasted power as the numerator; correct this by remembering the numerator is the useful output, while wasted power is total input minus useful output.
- Adding a unit such as W or J to the efficiency value; correct this by cancelling the power units, since efficiency is a ratio with no unit.
- Treating a percentage answer as a decimal, for example writing 0.75% instead of 75%; correct this by multiplying the decimal by 100 before adding the % sign.
- Saying a machine can be made 100% efficient: correction — some energy is always dissipated to the surroundings, so efficiency stays below 100%.
- Confusing efficiency with power or with the total energy supplied: correction — efficiency is a ratio of useful output to total input, not a quantity of energy or a rate.
- Suggesting that adding more energy input increases efficiency: correction — adding input without changing useful output lowers efficiency; the wasted transfer must be reduced instead.