P1.3 Energy · KS3 Science

Energy stores

Energy is measured in joules (J). 1 kilojoule (kJ) = 1000 J.
Energy stores: kinetic (moving objects), gravitational potential (objects raised up), elastic potential (stretched or squashed objects), thermal (hot objects), chemical (food, fuels and batteries), nuclear, magnetic and electrostatic.

Energy is transferred between stores mechanically (by a force), electrically (by a current), by heating, and by radiation (such as light and sound).
Energy can't be created or destroyed, only transferred from one store to another. This is the law of conservation of energy.
Food labels show energy in kJ. The body uses energy from food to move, grow, repair itself and keep warm.

Energy that isn't transferred usefully is wasted. It usually spreads out (dissipates) into the surroundings as thermal energy, and sometimes as sound.
Efficiency = useful energy out ÷ total energy in. Multiply by 100 for a percentage.
A bulb that takes in 100 J and gives out 20 J of light is 20% efficient; the other 80 J heats the surroundings.
Lubricating moving parts reduces friction, and insulation reduces heat loss, so less energy is wasted.

Temperature (in °C) is how hot something is. It depends on how fast its particles are moving.
Thermal energy depends on the mass, temperature and material. A warm bath has more thermal energy than a cup of boiling water, because it has much more mass.
Energy flows from hotter objects to cooler ones until they reach the same temperature.
Conductors (such as metals) let thermal energy pass through quickly. Insulators (such as plastic, wood, wool and air) let it pass slowly. Trapped air is a good insulator.

Energy: What is needed to make things happen, measured in joules (J).
Joule: The unit of energy.
Energy store: A way that energy is held, such as a chemical or kinetic store.
Kinetic energy: Energy stored in a moving object.
Gravitational potential energy: Energy stored in an object that is raised up.
Chemical energy: Energy stored in food, fuels and batteries.
Thermal energy: Energy stored in a hot object.
Conservation of energy: Energy can't be created or destroyed, only transferred.
Dissipation: Energy spreading out into the surroundings, becoming less useful.
Efficiency: The proportion of energy that is transferred usefully.
Temperature: How hot something is, measured in degrees Celsius (°C).
Conductor: A material that lets thermal energy (or electricity) pass through easily.
Insulator: A material that doesn't let thermal energy (or electricity) pass through easily.