The energy stored in a system can be changed in three main ways. The first is heating. When a substance is heated, its temperature rises, and the energy needed depends on its mass, the temperature change and its specific heat capacity. This is the energy needed to raise the temperature of 1 kg of the substance by 1 °C, measured in J/kg °C. The energy transferred is mass × specific heat capacity × change in temperature, or ΔE = m c Δθ. For example, heating 2 kg of a metal with c = 400 J/kg °C by 10 °C needs 2 × 400 × 10 = 8000 J.
A substance with a low specific heat capacity, such as copper, warms up quickly because little energy is needed for each degree. Water has a high specific heat capacity, so it needs much more energy for the same rise.
The second way is work done by forces. When a force moves an object, the force does work and energy is transferred. If the work is done against friction, the energy goes to the thermal store and the surfaces get warmer. If an object is lifted, the energy goes to its gravitational potential store.
The third way is work done when a current flows. Charge is pushed through a component by the potential difference, which does work on it. In a resistor this transfers energy to the thermal store, so the temperature rises.