CP3 · Conservation of energy · Topic 3 — Conservation of energy | GCSE Edexcel Combined Science

Revise the key ideas

Energy is measured in joules (J). It cannot be created or destroyed: total energy in a closed system remains constant.
The eight stores are kinetic, thermal/internal, chemical, gravitational potential, elastic potential, nuclear, magnetic and electrostatic.
Energy transfers occur mechanically (forces doing work), electrically (moving charges), by heating and by radiation such as light or infrared. Sound can also carry energy away.
A battery-powered motor transfers energy electrically from the battery’s chemical store to a moving object's kinetic store and thermal stores.
A falling object transfers energy from its gravitational potential store to its kinetic store. Air resistance also transfers energy to thermal stores in the object and surroundings.
Useful transfers achieve the intended purpose. Unwanted transfers often heat the surroundings or produce sound. Dissipated energy is spread out and becomes less useful, but has not been destroyed.
A Sankey diagram represents transfers with arrow widths proportional to energy. Input equals total useful and unwanted output; read values and units carefully.
Energy conservation: 100 J = 25 J + 75 J. Arrow shafts show the proportions.

Change in gravitational potential energy = mass × gravitational field strength × change in vertical height, ΔGPE = mgΔh. Use kg, N/kg and m to obtain J.
Use the vertical height change, not the length of a slope.
A 2 kg object raised 3 m in g = 10 N/kg gains 60 J. The reference level is chosen for the calculation; energy depends on a change in height.
Kinetic energy = ½mv², with mass in kg and speed in m/s. Squaring speed means doubling speed quadruples kinetic energy at unchanged mass.
A 4 kg object moving at 5 m/s has kinetic energy ½ × 4 × 25 = 50 J.
Efficiency = useful energy transferred ÷ total energy supplied. It has no unit and is a fraction between 0 and 1. Multiply by 100 to express it as a percentage.
A lamp supplied 100 J and transferring 25 J by useful light has efficiency 0.25, or 25%. The remaining 75 J is transferred in unwanted ways.
Lubrication reduces friction and unwanted heating in moving machinery. Insulation reduces unwanted thermal transfers. Both can improve efficiency for the intended task.

Heating normally transfers energy from a hotter region to a cooler region. Temperature is not the same quantity as energy; heating can raise temperature or cause a change of state.
In conduction, energy passes between neighbouring particles; free electrons also transfer energy efficiently in metals. Material does not flow along the conductor as a whole.
Convection happens in fluids (liquids and gases). A warmer region usually becomes less dense and rises. Cooler, denser fluid sinks to take its place, setting up a convection current.
Convection transfers energy by bulk movement of fluid.
Infrared radiation transfers energy without needing a material to travel through (a medium). It can travel through a vacuum, unlike conduction and convection.
A low thermal conductivity makes a material a good thermal insulator. For otherwise identical walls, greater thickness reduces the rate of conduction.
A larger temperature difference across a wall increases the rate of energy transfer; a higher thermal conductivity also increases that rate.
Trapped air in cavity-wall insulation and double glazing reduces conduction; small pockets limit convection. Unfilled large air spaces can still allow convection.
Small air pockets inhibit large convection currents.
Loft insulation, thick low-conductivity walls and double glazing reduce cooling. Draught proofing reduces transfers by moving air. Shiny surfaces can reduce infrared absorption and emission.

Fossil fuels are coal, oil and natural gas. They form over very long times and are used faster than they are replaced, making them non-renewable.
Combustion transfers energy from chemical stores; electricity generation often uses heating to make steam that drives turbines and generators.
Burning fossil fuels releases carbon dioxide, contributing to climate change. Some fuels also cause air pollution and habitat damage during extraction.
Nuclear fuels such as uranium are non-renewable. Nuclear reactions release energy for heating and electricity generation with low operational carbon dioxide emissions.
Nuclear generation produces radioactive waste requiring secure management; accidents, decommissioning costs and lifecycle emissions must be considered. Low operational emissions do not mean no environmental impacts.

Renewable resources are replaced naturally quickly enough for continued use. Examples include wind, solar, tides, hydroelectricity, geothermal energy and biofuels grown and replaced sustainably.
Wind turbines transfer kinetic energy of air; hydroelectricity uses water dropping from a higher level; tidal systems use tidal water movements. These can drive generators.
Solar cells transfer energy electrically from sunlight; solar heating warms water. Their output varies with sunlight and weather.
Geothermal heating uses energy from hot rocks. Biofuels release energy by combustion; plant growth can take up carbon dioxide, but farming, processing, transport and land-use changes affect net emissions.
Resource reliability differs: wind and solar are variable, tides are predictable, stored-water hydroelectricity can respond quickly, and geothermal can provide steadier output in suitable locations.
Compare cost, start-up time, availability, land use, habitat impacts, pollution, waste and reliability. No single resource is best in every location.
Use several criteria rather than a single advantage.
Energy use changes with demand, price, technology and environmental priorities. Use the data in a question to describe trends without inventing causes or treating a correlation as proof.