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What is power, how do we measure efficiency, and how can we reduce wasted energy?

Power as the rate of energy transfer, the equations for power, efficiency as the fraction of energy transferred usefully, the dissipation of energy, and ways to reduce unwanted energy transfers such as lubrication and insulation.

A focused answer to the OCR Gateway GCSE Combined Science A topic P5 on power and efficiency, covering power as the rate of energy transfer, the power equations, efficiency and how to calculate it, the dissipation of energy, and reducing unwanted energy transfers.

Generated by Claude Opus 4.89 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this topic is asking
  2. Power
  3. Efficiency
  4. Reducing wasted energy

What this topic is asking

OCR wants you to define power as the rate of energy transfer, use the power equations, calculate efficiency, explain how energy is dissipated, and describe ways to reduce unwanted energy transfers.

Power

A more powerful device transfers the same amount of energy in less time, or more energy in the same time. The two equations are:

P=EtandP=WtP = \frac{E}{t} \quad \text{and} \quad P = \frac{W}{t}

where EE is the energy transferred (in joules), WW is the work done (in joules) and tt is the time (in seconds). For example, a 20002000 W kettle transfers 20002000 J every second. Power lets you compare devices: a powerful motor or heater does its job faster, but also draws energy faster.

Efficiency

In any real device some of the input energy is transferred to non-useful stores, almost always the thermal store (waste heat). For example, a filament lamp transfers most of its input energy to heat and only a small fraction to useful light, so it has a low efficiency, while an LED transfers far more of its input to light and is much more efficient. Efficiency questions are a frequent calculation, so practise switching between decimals and percentages.

Reducing wasted energy

Because wasted energy is usually dissipated as heat, reducing unwanted transfers improves efficiency. The main methods OCR expects are:

  • Lubrication reduces friction between moving parts, so less energy is transferred to the thermal store as the parts rub.
  • Thermal insulation (such as loft insulation, cavity wall insulation and double glazing) reduces the rate of heat loss from a building; thicker insulation and materials with a lower thermal conductivity lose heat more slowly.
  • Streamlining reduces air resistance (drag) on vehicles, so less energy is wasted pushing air aside.

These reduce the energy dissipated and so increase the proportion transferred usefully, which is the practical point of studying efficiency.

Exam-style practice questions

Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

OCR 20184 marksAn electric motor transfers 6000 J of energy in 30 s. Calculate its power output, and state what power means.
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A Physics Paper 6 calculation. Method: power =energy transferredtime=600030=200= \dfrac{\text{energy transferred}}{\text{time}} = \dfrac{6000}{30} = 200 W. Markers credit the correct substitution and the answer with the unit watts. Power means the rate of energy transfer (or the rate of doing work), that is the energy transferred per second; one watt equals one joule per second. Markers want the calculation and a correct definition of power as energy transferred per unit time. A common slip is to leave out the unit or to multiply instead of divide.

OCR 20214 marksA lamp is supplied with 500 J of energy and transfers 100 J of this usefully as light. Calculate its efficiency as a percentage, and suggest where the rest of the energy goes.
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A P5 efficiency calculation. Method: efficiency =useful energy outputtotal energy input=100500=0.2= \dfrac{\text{useful energy output}}{\text{total energy input}} = \dfrac{100}{500} = 0.2, which as a percentage is 0.2×100=20%0.2 \times 100 = 20\%. Markers credit the correct ratio and the conversion to a percentage. The rest of the energy (400400 J) is dissipated (wasted) to the thermal energy store, heating the lamp and the surroundings, because no device is perfectly efficient. Markers want the 20%20\% and the point that the wasted energy is transferred to the thermal store (as heat).

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