Energy, Year 10: efficiency and energy resources

They are lubricated with oil or grease

提示Think what a squeaky hinge needs.

为什么Where two surfaces rub, friction transfers energy to thermal stores that are of no use. A film of lubricant lets the surfaces slide past each other more easily, so more of the energy supplied goes where it is wanted. Lubrication is one of the two methods AQA names for reducing unwanted transfers.

higher

提示Compare how quickly a metal spoon and a wooden spoon warm up in the same hot drink.

为什么Thermal conductivity describes how readily a material passes energy on by conduction. Metals have large values; brick, wood and trapped air have small ones. AQA does not ask for a definition of the term, only this link to the rate of transfer.

The one with the thicker walls

提示Think of the difference between a thin jumper and a chunky one made of the same wool.

为什么Energy escaping from the house has to be conducted through the walls. The further it has to travel through the material, the lower the rate of transfer, so the house stays warm for longer. AQA expects pupils to describe how wall thickness affects the rate of cooling of a building.

Material B

提示You want the one that passes energy through itself reluctantly.

为什么A low thermal conductivity means a low rate of energy transfer by conduction, so less energy leaks out through the wall each second and the house cools slowly. This is why cavity walls are filled with foam and lofts are lined with mineral wool.

A thermal insulator

提示Loft wool, foam and a tea cosy are all examples.

为什么Most of these materials work by trapping air, which conducts very poorly. Wrapping a hot-water tank or lining a loft with one cuts the rate at which energy escapes, so less fuel is needed to keep things warm.

20

提示Compare the useful output with the total supplied, then scale to a hundred.

为什么Efficiency = useful output energy transfer ÷ total input energy transfer, so 10 ÷ 50 = 0.2, which is 20%. The other 80% heats the lamp and the room. AQA expects the equation to be recalled.

useful

提示Only the part of the output that does the job you wanted is counted on top.

为什么The same equation works with powers: useful power output ÷ total power input. Both versions must be recalled. The answer can be left as a decimal, such as 0.25, or multiplied by 100 to give a percentage.

0.75

提示Put the smaller figure over the larger one.

为什么Efficiency = useful power output ÷ total power input = 300 ÷ 400. AQA expects this power form of the equation to be recalled too. As a percentage the same answer is 75%.

1200 J

提示The efficiency is the fraction of the input that comes out usefully.

为什么Rearranging the efficiency equation gives useful output = efficiency × total input = 0.6 × 2000. The remaining 800 J is dissipated, mostly heating the motor and its surroundings.

A machine cannot give out more useful energy than it is supplied with

提示Ask what a value above 1 would say about the output compared to the input.

为什么Energy is conserved, so the useful output is at most equal to the total input, and in practice always less because some is dissipated. Efficiency is therefore never above 1, or 100%. The usual cause of an answer like 1.2 is dividing the input by the output.

Insulate the tank

提示Slow down the escape of energy from the hot metal to the air.

为什么Wrapping the tank in a material of low thermal conductivity cuts the rate at which energy leaks out, so a bigger share of the energy supplied stays in the water. Efficiency is raised by reducing the wasted transfer, not by supplying more energy. Describing ways to increase efficiency is Higher tier content in AQA's specification.

A renewable energy resource

提示Wind, waves and sunlight all belong to this group.

为什么The test is whether the resource is replaced at least as fast as we use it. Wind, hydro-electricity, the tides, the Sun, water waves, geothermal and bio-fuel all pass it. Coal, oil, gas and nuclear fuel do not.

transport; electricity; heating

提示Think of a car, a power station and a gas boiler.

为什么Different resources suit different uses. Oil products are used mostly for moving vehicles, gas for warming buildings and for power stations, and wind and nuclear almost entirely for power stations. AQA asks pupils to compare how resources are used under these three headings.

It only generates when the wind is blowing

提示Consider what each one needs in order to run, and who controls the supply of it.

为什么A reliable resource can deliver energy whenever it is needed. Gas can be burned on demand, day or night, but nobody can switch the weather on. Solar and wave power have the same weakness, whereas the tides, although they come and go, are predictable.

Geothermal

提示The name is built from the Greek words for 'Earth' and 'heat'.

为什么AQA lists nine main resources: fossil fuels, nuclear fuel, bio-fuel, wind, hydro-electricity, the tides, water waves, the Sun and this one. It is reliable, because the rocks stay hot whatever the weather, but it is only practical in certain places.

The fuel is used up and is not replaced

提示Go back to what 'renewable' actually tests for — it is not about pollution.

为什么Nuclear fuel such as uranium is mined from the ground and there is a fixed amount of it. Once consumed it is not replenished, which is the definition of non-renewable. Being low in carbon and being renewable are two separate questions.

carbon dioxide

提示It is the gas you breathe out, and plants take in.

为什么Coal, oil and gas are all made largely of carbon, which joins with oxygen when the fuel burns. The extra gas in the atmosphere traps more energy and warms the planet. This is the main environmental cost of the resources the world has relied on most.

Radioactive waste that stays hazardous for a very long time

提示Think about what is left when the fuel rods are finished with.

为什么Used nuclear fuel goes on emitting radiation for thousands of years, so it has to be stored securely and kept away from people and water supplies. Building and taking apart the stations is also costly. Weighing this against the lack of carbon emissions is the kind of comparison AQA asks for.

Flooding a valley destroys habitats

提示Picture what happens to the land behind a new dam.

为什么The reservoir behind the dam drowns farmland, woodland and sometimes villages, and changes the river downstream. Tidal barrages do something similar to estuaries, and wind farms change the landscape. No energy resource is entirely free of environmental cost.

To cut carbon dioxide emissions

提示Ask what coal gives out, and wind does not, as electricity is produced.

为什么Coal releases more greenhouse gas for each unit of electricity than any other major resource, so replacing it is the quickest way to reduce a country's contribution to global warming. Falling costs of wind turbines and solar panels helped too. Explaining a pattern like this in how resources are used is a skill AQA lists.

Science can identify the problem, but the decision depends on social, economic, political and ethical factors

提示Measuring emissions is one kind of question; deciding what a community should pay for or put up with is another.

为什么Scientists are good at measuring the environmental effects of energy resources and predicting what will follow from each choice. Whether to act, and who should bear the cost, are matters of money, fairness, law and public opinion. AQA expects pupils to recognise this limit.