Electromotive Force

We define the electromotive force (e.m.f.) $\mathcal{E}$ of the source as the work done by the non-electrostatic force per unit charge carried from the $-$ pole to the $+$ pole inside the source:

$\mathcal{E} = \frac{L_{\text{non el.}}}{q} \qquad [\text{V}]$

Despite the name, it is not a force: it is an energy per unit charge, and so is measured in volts. It represents the maximum potential difference the battery can offer, the one measured between the terminals when no current is flowing.

Key formula — Electromotive force

$\ev{\mathcal{E}=\dfrac{L_{\text{non el.}}}{q}}$ Under open-circuit conditions (zero current), the potential difference between the terminals of the source coincides with $\mathcal{E}$.

The e.m.f. depends only on the chemistry

The e.m.f. of a cell depends only on the chemical species involved, not on the geometry.

• Daniell cell: $\approx 1{,}1$ V
• Alkaline cell: $1{,}5$ V
• Lithium cell: $3{,}6$ V

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Topics: Circuiti elettrici Concepts: Forza elettromotrice

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