A level revision

Power

Power is rate at which energy is transferred, in a circuit this refers to the rate at which energy is transferred to a component. This depends on the potential difference across it (energy transferred from the electrons) and the current flowing through it (how fast the electrons are delivering their energy).

Power is given the symbol P and is measured in Watts (W). P can be calculated from the p.d. and the current using the 'power equation':

This equation can be combined with others, such as Ohm's law, V = IR, to give:

As mentioned power is a measure of how quickly energy is transferred so it can also be calculated using the equation:

With E being the energy in joules transferred in a time, t, in seconds. This can be combined with P = IV to given an equation for the amount of energy transferred in a circuit in a given time:

The power equation also gives another expression for potential difference:

Because potential difference (V) also equals energy (W) per unit charge (Q):