OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(c) efficiency of a mechanical system; efficiency = (useful output energy…
Full specification wording
Power: (c) efficiency of a mechanical system; efficiency = (useful output energy / total input energy) × 100%
Efficiency compares the useful output energy from a mechanical system with the total input energy supplied to it.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 3 of 6
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Efficiency compares the useful output energy from a mechanical system with the total input energy supplied to it. It is calculated as efficiency = (useful output energy / total input energy) × 100%. Because energy is conserved, the useful output energy is always less than the total input energy in a real system, so efficiency is less than 100%. For example, an electric motor may be supplied with 500 J of electrical energy and transfer 350 J as useful kinetic energy; its efficiency is (350 / 500) × 100% = 70%. The remaining 150 J is dissipated, often as thermal energy due to friction or resistance. When comparing power instead of energy, the same ratio applies if the time interval is the same.
What earns the marks
- Efficiency is the ratio of useful output energy to total input energy, expressed as a percentage.
- The equation is efficiency = (useful output energy / total input energy) × 100%.
- Useful output energy is the energy transferred to the intended form, such as kinetic energy or gravitational potential energy.
- Total input energy is the total energy supplied to the system, including energy eventually dissipated as heat or sound.
- Efficiency is always less than 100% for real systems because some energy is dissipated to the surroundings.
- Efficiency can also be calculated using power values if the time interval is the same: efficiency = (useful output power / total input power) × 100%.
Common mistakes and how to put them right
Confusing useful output energy with total input energy.
Put it right by identifying which energy transfer is intended and which is wasted.
Forgetting to multiply by 100% and leaving the answer as a decimal.
Put it right by converting the ratio to a percentage as required by the equation.
Using the wrong units or mixing energy and power in the same calculation.
Put it right by ensuring both quantities are either energies in joules or powers in watts.
Assuming efficiency can be greater than 100% if the output seems larger.
Put it right by recognising that energy is conserved and some is always dissipated in real systems.
Exam tips
- Write the efficiency equation clearly and substitute values with units before calculating.
- If the question gives power rather than energy, check that the time interval is the same for both input and output; if so, use power values directly.
- Express the final answer as a percentage, and include the % symbol.
- In multiple-choice questions, eliminate options greater than 100% immediately for real systems.
Try the lesson
The start of the Power: (c) efficiency of a mechanical system; efficiency = (useful output energy / total input energy) × 100% lesson, made from checked teaching notes for this specification point.
Listen
Efficiency compares the useful output energy from a mechanical system with the total input energy supplied to it. It is calculated as efficiency = (useful output energy / total input energy) × 100%. Because energy is conserved, the useful output energy is always less than the total input energy in a real system, so efficiency is less than 100%. For example, an electric motor may be supplied with 500 J of electrical energy and transfer 350 J as useful kinetic energy; its efficiency is (350 / 500) × 100% = 70%. The remaining 150 J is dissipated, often as thermal energy due to friction or resistance. When comparing power instead of energy, the same ratio applies if the time interval is the same. A strong answer earns marks for these points. Efficiency is the ratio of useful output energy to total input energy, expressed as a percentage. The equation is efficiency = (useful output energy / total input energy) × 100%. Useful output energy is the energy transferred to the intended form, such as kinetic energy or gravitational potential energy. Total input energy is the total energy supplied to the system, including energy eventually dissipated as heat or sound. A common slip: Confusing useful output energy with total input energy: the useful output is always smaller. Instead, identifying which energy transfer is intended and which is wasted. Exam tip: Write the efficiency equation clearly and substitute values with units before calculating.
See it
Key words
Say what each word means before you turn the card.
Efficiency
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Useful output energy
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Total input energy
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See every definition
- Efficiency
- The ratio of useful output energy to total input energy, expressed as a percentage using efficiency = (useful output energy / total input energy) × 100%.
- Useful output energy
- The energy transferred to the intended form, such as kinetic energy or gravitational potential energy.
- Total input energy
- The total energy supplied to the system, including energy eventually dissipated as heat or sound.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
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