[Physics] When potential difference is equal to the emf, isn’t the current 0

Question

I have been told that the $emf$ is equal to the potential difference across the terminals of a cell when no current is flowing.

Does that mean that the current is zero ($I=0$)?

Answer 1

Real chemical cells in batteries produce a potential difference which is determined by the chemistry of the cell. A lead/sulfuric acid cell will have an chemical potential ($\epsilon$, the emf), of slightly greater than 2.0 V. Couple six of those cells in series (like a car battery) and you get about 12 V.

Those cells, however, have an effective internal resistance ($r_i$). If current $I$ flows through the cell, the resistance drops the total potential difference to $\epsilon - Ir_i$. If one can minimize the current by having an extremely high external resistance (a good quality voltmeter), the reading of the meter will approach $\epsilon$.

$$V_{meter}=\epsilon-Ir_i$$ $$I=\frac{\epsilon}{r_{meter}+r_i}$$

If $r_{meter}\to\infty$, you can see what happens to $V_{meter}.$