What is efficiency, and how do renewable and non-renewable energy resources compare?
Efficiency as the fraction of energy usefully transferred, the efficiency equation, ways of reducing wasted energy, and the comparison of renewable and non-renewable energy resources with their trade-offs.
A focused answer to OCR Gateway GCSE Physics A on efficiency and energy resources, covering efficiency as the fraction of energy usefully transferred, the efficiency equation, reducing wasted energy, and comparing renewable and non-renewable resources with their reliability, cost and environmental trade-offs.
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What this topic is asking
OCR wants you to define efficiency and use the efficiency equation, describe ways of reducing wasted energy, and compare renewable and non-renewable energy resources with their trade-offs. This draws on topic P7.2 (efficiency) and P8.2 (energy resources) of the OCR Gateway Physics A (J249) specification.
Efficiency
No real device is perfectly efficient, because some energy is always dissipated to the surroundings, usually as thermal energy. The useful output plus the wasted output equals the total input, so you can find one from the other two.
Reducing wasted energy
The energy wasted by a device can be reduced in several ways:
- Lubrication reduces friction between moving parts, so less energy is dissipated as heat.
- Thermal insulation (such as loft insulation or cavity wall foam) reduces unwanted heat transfer, keeping useful thermal energy where it is wanted.
- Streamlining reduces air resistance (drag), so vehicles waste less kinetic energy heating the air.
Each measure increases the fraction of energy transferred usefully, raising the efficiency.
Energy resources
The resources differ in reliability, cost and environmental impact. Fossil fuels and nuclear are reliable and can meet demand on demand, but fossil fuels pollute and nuclear produces radioactive waste. Renewables are clean but many are intermittent (solar needs daylight, wind needs wind), so a mix of resources is used to balance reliability against environmental impact.
Comparing resources
When an exam asks you to evaluate or compare resources, weigh up: whether it is renewable, its reliability (can it meet demand at any time?), its start-up and running costs, and its environmental impact (carbon dioxide, pollution, waste, visual and noise effects). A strong answer gives both sides for each resource and reaches a balanced judgement.
Try this
Q1. A device wastes out of every supplied. Calculate its efficiency as a percentage. [2 marks]
- Cue. Useful output ; efficiency .
Q2. State one reason a country uses a mix of energy resources rather than relying on renewables alone. [1 mark]
- Cue. Many renewables are intermittent (unreliable), for example solar needs daylight and wind needs wind, so other resources are needed to meet demand at all times.
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 20193 marksAn electric motor is supplied with of energy and transfers usefully to a kinetic store. Calculate the efficiency of the motor as a percentage.Show worked answer →
A P7 Calculate question on the given efficiency equation. Efficiency . Substitute: efficiency (2 marks for the equation and the fraction). As a percentage, multiply by : (1 mark). Markers reward the correct ratio and the conversion to . A common error is to use the wasted energy () on top, or to forget to multiply by for a percentage. Efficiency can also be found as a ratio of useful power output to total power input.
OCR 20216 marksCompare the advantages and disadvantages of generating electricity using wind power and using a fossil fuel such as coal. Refer to reliability, cost and environmental impact.Show worked answer →
A P8 six-mark level of response question, marked on levels, needing a balanced comparison. Wind power: it is renewable (will not run out), produces no carbon dioxide when generating, and has low running costs; but it is unreliable (only works when the wind blows), has a high initial cost, and some object to the appearance or noise of turbines. Coal (fossil fuel): it is reliable and can be turned up to meet demand, and the stations are established; but it is non-renewable (finite), releases carbon dioxide (contributing to climate change) and other pollutants such as sulfur dioxide. Top answers give advantages and disadvantages of both, explicitly comparing reliability, cost and environmental impact (especially carbon dioxide). Markers credit a balanced, comparative judgement rather than a one-sided list.
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