Energy is not a physical substance, although we describe it as being transferred between stores. It is a number that can be calculated before a change and again afterwards, and the total comes out the same. What changes is which store the energy is held in. Physical processes such as heating, forces and electric currents are what transfer it.
Energy is measured in joules (J). Because it is a quantity, the amount a system has can be added up at the start and at the end, and for a closed system those totals are equal. This is conservation of energy: energy is never made or destroyed, only shifted between stores.
The useful way to describe a change is to compare the start and the finish and say which stores went up and which went down. When a ball is dropped, the gravitational store decreases and the kinetic store increases. When a car brakes to a stop, the kinetic store decreases and thermal stores in the brakes, tyres and air increase.
At KS3, the main energy stores are kinetic (movement), gravitational potential (height), elastic potential (stretched or squashed), chemical (fuel and food), and thermal (hot things).
Energy tells you how much, not why. A ball rolls down a slope and speeds up because gravity pulls it, not because energy pushed it. A kettle boils because energy is transferred electrically to the heating element, and then by heating to the water. Energy is not a substance that flows or pushes.
An explanation of the steps of a change should name the physical process: a force acting, a current flowing, a temperature difference, a fuel burning. Use the energy account to say how much shifted and between which stores, and use the process to say what made it happen. The energy account and the process are different things: energy does not itself make something move.
Energy is transferred between stores in four ways, and these are the same four used at GCSE:
Two things that are often listed alongside these are not pathways. Speeding an object up increases its kinetic store, but that is a description of which store changed, not of how the energy got there. It arrived mechanically, through whatever force did the work. Burning a fuel is a chemical reaction, which then transfers energy by heating and by radiation.
Whenever two objects are at different temperatures, there is a net transfer of energy from the hotter object to the cooler one. The temperature difference shrinks until both are at the same temperature, thermal equilibrium, at which point there is no net transfer between them.
There are three mechanisms. In conduction, energy is passed on by collisions between neighbouring particles. It works best in solids, where the particles are strongly coupled, and best of all in metals, where free electrons also carry energy very effectively. In convection, a warmed fluid usually expands and becomes less dense, so it rises while cooler fluid sinks to replace it, setting up a circulation. It needs a density difference and gravity, so it cannot happen in a solid. In radiation, energy travels as electromagnetic waves, which need no particles at all. At everyday temperatures the radiation is mainly infrared. The Sun reaches us across empty space this way, sending visible light as well as infrared.
An insulator slows the transfer. Materials that trap air work well, because air conducts badly and trapped air cannot circulate to convect. That is how loft insulation, double glazing and a foam cup all work.
This is also why a metal handrail feels colder than a wooden one in the same room. They are at the same temperature; the metal simply conducts energy away from your hand faster.
Power is how fast energy is transferred, measured in watts (W). One watt is one joule every second, so a 60 W lamp transfers 60 J each second.
Appliances carry a power rating, and comparing them tells you which shifts energy fastest. A 2 kW kettle transfers energy twice as fast as a 1 kW toaster, so over the same time it transfers twice as much.
Power and energy are different quantities: power is a rate, energy is a total. Energy transferred = power × time.
The joule is small for everyday purposes, so larger units are used. A kilojoule (kJ) is 1000 J. A kilowatt-hour (kWh) is the energy transferred by a 1 kW appliance running for one hour, which is much bigger again, at 3 600 000 J.
To find energy in kilowatt-hours, multiply the power in kilowatts by the time in hours. A 2 kW heater left on for 3 hours uses 2 × 3 = 6 kWh.
Watch the units in the question: a power in watts must be divided by 1000 to give kilowatts, and a time in minutes must be divided by 60 to give hours.
Electricity and gas are sold by the kilowatt-hour, which suppliers often call a "unit". A meter records the number of units used, and the bill is the number of units multiplied by the price per unit, plus a fixed standing charge.
So the cost of running an appliance is: energy in kWh × price per kWh. A 2 kW heater run for 3 hours uses 6 kWh, and at 30p per kWh that costs £1.80.
This is why high-power appliances used for long periods, such as heaters, tumble dryers and immersion heaters, dominate a bill, while a low-power device left on standby costs very little even though it is never switched off.
Food labels give the energy available from the food, in kilojoules and often also in kilocalories. This is the size of the chemical store the food supplies, and it is what respiration releases for your body to use.
Comparing labels is a matter of comparing like with like: the figure is usually quoted per 100 g as well as per portion. Use the per-100 g figure to compare equal masses of different foods, and the per-portion figure to compare typical servings. Fats supply roughly twice the energy per gram that carbohydrates or proteins do.
A fuel holds a chemical store that is released by burning. Non-renewable resources are used up faster than they form. Coal, oil and natural gas all release carbon dioxide when burned; nuclear fuel does not. Renewable resources are naturally replenished on a human timescale: wind, solar, hydroelectric, tidal, wave, geothermal and biofuel.
The trade-offs matter as much as the list. Fossil fuels are reliable and can be turned up on demand, but they are finite and release carbon dioxide. Most renewables release no carbon dioxide in use, but wind and solar are variable, since wind needs wind and solar needs daylight, so their output follows the conditions rather than the demand. Nuclear is reliable and carbon-free in use but produces radioactive waste that must be stored for a very long time.
A simple machine such as a lever, a ramp or a pulley system lets a small force do a job that would otherwise need a large one. It does not give you energy for nothing. The same work is done either way: a smaller force must act over a greater distance.
Pushing a load up a long gentle ramp needs less force than lifting it straight up, but you push it much further, and the work done, which is force multiplied by distance, comes out about the same. A crowbar multiplies your force at the tip, but the tip moves only a short way while your hands move a long way.
In a real machine the two are not exactly equal, because friction transfers some energy to thermal stores. In an ideal machine with no friction, the work input would equal the useful work output.
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