Power of the source

Consider a source of electromotive force $\mathcal{E}$ carrying a current $i$. The electromotive force measures the work per unit charge that the source does to push the charges; multiplying it by the charge per second (i.e. the current) gives a work per second, that is, a power. If $i$ flows internally from pole $-$ to pole $+$ (the natural direction), the source delivers energy to the circuit:

Key formula

$\ev{P_\text{gen} = \mathcal{E}\cdot i \;\geq 0}$

The sign matters. If the current flows in the opposite direction — a situation typical of charging a battery, in which an external source forces current against the EMF — then $P_\text{gen} < 0$: the source is not delivering, but absorbing energy from outside and storing it.

Sign of the source power

$P_\text{gen} = \mathcal{E}\cdot i$ is positive when the source delivers; negative when it is being charged (current forced in the opposite direction).

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Topics: Electric circuits Concepts: Power · Electromotive force

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