Internal resistance
The resistance within a power source (such as a battery or cell) due to the resistance of the electrolyte and electrodes. Denoted by r, internal resistance causes a drop in terminal potential difference (V = ε − Ir) when current I flows. It is measured in ohms and is typically small for batteries but becomes significant at high currents.
Formula
r = (ε − V) / I
Real World
A degraded car battery may still show 12 V open-circuit but fail to start the engine because its increased internal resistance causes a large voltage drop the moment the starter motor draws hundreds of amperes.
Exam Focus
In graph questions, plot V against I: the gradient gives −r and the y-intercept gives ε; state both clearly for full marks.
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