The energy stored in a system can be changed in three main ways: by heating, by work done by forces, and by work done when a current flows.
Heating. The thermal energy needed to warm a substance depends on its mass, its specific heat capacity and the change in temperature. Specific heat capacity is the energy needed to raise the temperature of 1 kg of a substance by 1 °C. Its unit is J/kg°C. The equation is: change in thermal energy = mass × specific heat capacity × change in temperature. For 2 kg of water, with specific heat capacity 4200 J/kg°C, a rise of 10 °C needs 2 × 4200 × 10 = 84000 J. If two objects of equal mass get the same energy and one warms less, that one has the higher specific heat capacity.
Work done by forces. When a force moves an object, the force does work and energy is transferred. Work done = force × distance moved in the direction of the force, measured in joules (J).
Work done by a current. When a current flows through a component, the charges do work and transfer energy, for example to the thermal store of a heater.