GCSE Physics · AQA 8463 · Topic 4.1.1

Energy changes in a system and the ways energy is stored

Key points

A system is an object or group of objects. When a system changes, energy moves between its stores.
Kinetic energy: $E_k = \frac{1}{2}mv^2$ (mass in kg, speed in m/s). Doubling the speed makes the kinetic energy four times as big.
Gravitational potential energy: $E_p = mgh$ , with $g$ about 9.8 N/kg on Earth.
Elastic potential energy of a stretched spring: $E_e = \frac{1}{2}ke^2$ (only while the limit of proportionality is not passed).
Specific heat capacity $c$ is the energy needed to raise the temperature of 1 kg of a substance by 1 °C: $\Delta E = mc\Delta\theta$ .
Power is the rate of energy transfer or of doing work: $P = \frac{E}{t} = \frac{W}{t}$ , measured in watts (1 W = 1 J/s).
A common exam point: when an object falls, the decrease in $E_p$ equals the increase in $E_k$ if air resistance is ignored.

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Physics describes every change, from a ball being thrown to a kettle boiling, as energy moving between stores. Once you can name the stores and calculate how much energy is in each, you can predict how fast things move, how high they rise…

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