Energy Stores and Transfers Notes

A system is an object or group of objects. When it changes, energy moves between energy stores: kinetic, thermal, gravitational potential, elastic potential, chemical and nuclear.

For everyday changes, name every store involved. Marks are given for the store that decreases and the stores that increase.

Energy can be transferred to or from a system by heating, by work done by a force, or by work done when a current flows.

A Sankey-style bar shows how the total is redistributed. The total energy before and after a change is always the same.

Kinetic energy is the energy of a moving object: $E_k = \frac{1}{2}mv^2$ (J, with mass in kg and speed in m/s).

Elastic potential energy is stored in a stretched or compressed spring, if the limit of proportionality is not exceeded: $E_e = \frac{1}{2}ke^2$ (spring constant in N/m, extension in m).

Gravitational potential energy depends on height: $E_p = mgh$, with $g$ given in the exam.

The change in thermal energy depends on mass, specific heat capacity and temperature change: $\Delta E = mc\Delta\theta$. Specific heat capacity is the energy needed to raise 1 kg of a substance by 1 °C.

Power is the rate of energy transfer or the rate of doing work, measured in watts. Two motors lifting the same weight through the same height transfer the same energy, but the one that does it faster is more powerful.