What you'll learn
Energy is the opening unit of AQA GCSE Combined Science: Trilogy physics, and it introduces a way of thinking that runs through the whole course. Energy is not a substance and it is not used up; it is transferred between stores, and the total is always conserved. This unit teaches you to describe changes in terms of stores and transfers, to calculate the energy in a moving object, in a raised object and in a stretched spring, to work with specific heat capacity and power, to calculate efficiency, and to compare the energy resources available to a country. By the end you should be able to name the energy stores, apply the kinetic energy, gravitational potential energy and specific heat capacity equations, explain dissipation and how it is reduced, calculate efficiency as a decimal or a percentage, and evaluate renewable and non-renewable resources. This unit is assessed on Physics Paper 1 and includes the required practical on thermal insulation.
Key terms and definitions
Energy store — a way of accounting for energy in a system: kinetic, thermal, chemical, gravitational potential, elastic potential, nuclear, magnetic and electrostatic
System — an object or group of objects being considered, which may be open or closed
Closed system — a system in which no energy is transferred in or out, so the total energy is constant
Conservation of energy — the principle that energy can be transferred, stored or dissipated, but cannot be created or destroyed
Dissipation — the transfer of energy to the surroundings by heating, where it is spread out and no longer useful
Work done — energy transferred when a force moves an object through a distance, measured in joules
Power — the rate of transferring energy or the rate of doing work, measured in watts, where one watt is one joule per second
Specific heat capacity — the energy required to raise the temperature of one kilogram of a substance by one degree Celsius
Efficiency — the proportion of the energy supplied that is transferred usefully
Lubricant — a substance that reduces friction between surfaces, reducing the energy dissipated
Core concepts
Energy stores and transfers
Energy is described using stores. The stores you need are kinetic, thermal, chemical, gravitational potential, elastic potential, nuclear, magnetic and electrostatic.
Energy is transferred between these stores in four ways: mechanically, by a force doing work; electrically, by a current doing work; by heating; and by radiation, including light and sound.
The correct language matters in exams. A ball thrown upwards does not lose energy; energy is transferred from its kinetic store to its gravitational potential store. Describing a change as energy being used up or produced is penalised.
Kinetic energy
An object that is moving has energy in its kinetic store. The kinetic energy is given by one half multiplied by mass multiplied by speed squared, with mass in kilograms, speed in metres per second and energy in joules.
The squared term is important and is frequently tested. Doubling the mass doubles the kinetic energy, but doubling the speed multiplies the kinetic energy by four. This is why stopping distances increase so sharply with speed.
Gravitational potential energy
When an object is raised, energy is transferred to its gravitational potential store. The energy is given by mass multiplied by gravitational field strength multiplied by height, with mass in kilograms, gravitational field strength in newtons per kilogram — about 9.8 on the Earth's surface — and height in metres.
The height used must be the vertical height gained, not the distance travelled along a slope. This is a standard trap in questions about ramps and hills.
Elastic potential energy
Stretching or compressing a spring transfers energy to its elastic potential store. Provided the limit of proportionality has not been exceeded, the energy stored is one half multiplied by the spring constant multiplied by the extension squared, with the spring constant in newtons per metre and extension in metres.
Again the squared term matters: doubling the extension stores four times as much energy.
Specific heat capacity
Different materials need different amounts of energy to warm them by the same amount. The change in thermal energy is given by mass multiplied by specific heat capacity multiplied by temperature change, with specific heat capacity measured in joules per kilogram per degree Celsius.
Water has an unusually high specific heat capacity of about 4,200, which is why it is used in central heating systems and why coastal areas have milder climates than inland ones. A high specific heat capacity means a substance takes a lot of energy to warm up and releases a lot as it cools.
Note that a temperature change is used, not a temperature. A rise from 20 to 50 degrees Celsius is a change of 30, and using 50 in the calculation is a common and expensive error.
Power
Power is the rate at which energy is transferred, or the rate at which work is done. It equals energy transferred divided by time, or work done divided by time, with energy in joules, time in seconds and power in watts.
Two devices can transfer the same total amount of energy while having different powers; the more powerful one simply does it in less time. Questions often present exactly this comparison.
Dissipation and reducing unwanted transfers
In every system, some energy is dissipated to the surroundings, usually by heating, and once it has spread out it is no longer useful. This is why no machine is perfectly efficient.
Unwanted transfers can be reduced in two main ways. Lubrication reduces friction between moving surfaces, so less energy is dissipated by heating. Thermal insulation reduces the rate at which energy is transferred through the walls of a building.
The rate of energy transfer through a wall depends on its thickness and on the thermal conductivity of the material. A thicker wall or a material of lower thermal conductivity gives a lower rate of transfer, so the building cools more slowly.
Efficiency
Efficiency is the useful output energy transfer divided by the total input energy transfer. It can equally be calculated as useful power output divided by total power input.
The answer is a decimal between 0 and 1, or a percentage if multiplied by 100. An efficiency above 1, or above 100 per cent, is impossible and always indicates an arithmetic error — a useful check on any answer.
Efficiency is increased by reducing the wasted energy, for example by lubricating moving parts or insulating a hot component.
Energy resources
The resources available for use on Earth are fossil fuels, which are coal, oil and gas; nuclear fuel; biofuel; wind; hydroelectricity; geothermal; the tides; the Sun; and water waves. They are used for transport, for electricity generation and for heating.
A renewable resource is one that is being, or can be, replenished as it is used. Wind, hydroelectricity, geothermal, tidal, solar, water waves and biofuel are renewable. Fossil fuels and nuclear fuel are not.
Comparisons should be made on reliability, environmental impact and cost. Fossil fuels are reliable and provide power on demand, but they release carbon dioxide contributing to climate change, and sulfur dioxide causing acid rain, and they are finite. Nuclear fuel produces no carbon dioxide during generation but produces radioactive waste that must be stored for a very long time, and the plants are expensive to build and decommission. Wind and solar produce no emissions in use but are not reliable, since they depend on the weather. Tidal power is highly predictable but the barrages affect estuary habitats. Hydroelectricity is reliable and responds quickly to demand, but flooding a valley destroys habitats and may displace people.
Political, social, ethical and economic considerations mean that changing to renewable resources is slower than the science alone would suggest, and questions frequently ask you to explain why.
Worked examples
Example 1: Kinetic energy and the effect of speed (4 marks)
A car of mass 1,200 kilograms travels at 10 metres per second. Calculate its kinetic energy, and state what happens to the kinetic energy if the speed doubles.
Kinetic energy is one half multiplied by mass multiplied by speed squared. That gives 0.5 multiplied by 1,200 multiplied by 10 squared, which is 0.5 multiplied by 1,200 multiplied by 100, giving 60,000 joules or 60 kilojoules. If the speed doubles to 20 metres per second, the speed squared term becomes four times larger, so the kinetic energy becomes four times greater, at 240,000 joules.
Example 2: A specific heat capacity calculation (4 marks)
Calculate the energy needed to raise the temperature of 2.5 kilograms of water from 18 degrees Celsius to 68 degrees Celsius. The specific heat capacity of water is 4,200 joules per kilogram per degree Celsius.
The temperature change is 68 minus 18, which is 50 degrees Celsius. The energy is mass multiplied by specific heat capacity multiplied by temperature change, which is 2.5 multiplied by 4,200 multiplied by 50. That gives 525,000 joules, or 525 kilojoules.
Example 3: Calculating efficiency (3 marks)
A motor is supplied with 800 joules of energy and transfers 260 joules usefully. Calculate its efficiency as a percentage and suggest how it could be improved.
Efficiency is useful output divided by total input, which is 260 divided by 800, giving 0.325. As a percentage that is 32.5 per cent. The efficiency could be improved by lubricating the moving parts, which reduces friction so that less energy is dissipated to the surroundings by heating.
Common mistakes and how to avoid them
The most common error in this unit is using a temperature rather than a temperature change in the specific heat capacity equation. Subtract first, every time.
Students frequently forget to square the speed in the kinetic energy equation, or square the whole expression including the one half. Only the speed is squared.
In gravitational potential energy questions, the distance along a slope is often used instead of the vertical height. Look for the vertical measurement in the question.
Another routine slip is describing energy as being used up, lost or created. Energy is transferred between stores and some is dissipated to the surroundings; the total is conserved.
Finally, many answers describe a resource as renewable without saying why. Renewable means it is replenished as it is used, and that phrase is what earns the mark.
Exam technique for "Physics: Energy"
Write the equation before substituting, every time. Method marks are awarded for a correct equation even when the arithmetic goes wrong, and they are lost entirely if only a number appears.
Check units before calculating. Mass must be in kilograms, not grams; time in seconds, not minutes. Converting at the start prevents most errors in this unit.
When asked to describe an energy change, name both stores and the transfer route: energy is transferred mechanically from the chemical store of the fuel to the kinetic store of the car, with some dissipated to the thermal store of the surroundings.
For comparison questions on energy resources, use the same three criteria for each resource — reliability, environmental impact and cost — and finish with a judgement that refers to the situation described in the question.
Quick revision summary
Energy is held in stores and transferred mechanically, electrically, by heating or by radiation, and the total in a closed system is conserved. Kinetic energy is one half times mass times speed squared, so doubling speed quadruples it. Gravitational potential energy is mass times gravitational field strength times vertical height. Elastic potential energy is one half times spring constant times extension squared. Change in thermal energy is mass times specific heat capacity times temperature change, and water's high value of 4,200 makes it useful for heating systems. Power is energy transferred divided by time, in watts. Energy dissipated to the surroundings is reduced by lubrication and by thermal insulation, with the rate through a wall depending on thickness and thermal conductivity. Efficiency is useful output divided by total input and can never exceed 1. Resources are compared on reliability, environmental impact and cost, with renewables replenished as they are used.