Lesson 8 of 12 · 10 min
Cells, emf and internal resistance
NCERT § "Cells, emf, Internal Resistance"
The power goes off. Riya's torch cell is marked 1.5 V, but with the bulb on a voltmeter across it shows only 1.25 V. Where did a quarter of a volt go?
The lesson in notes
In short
The emf ε of a cell is the potential difference between its terminals when no current is drawn; despite the name, it is a potential difference, not a force.
Charge moving through the electrolyte meets an internal resistance r.
When the cell drives a current I through an external resistor R, I = ε/(R + r) and the terminal voltage is V = ε − Ir, which is less than ε.
The terminal voltage equals ε only on open circuit (I = 0), and falls to zero when the terminals are shorted, where the maximum current ε/r flows.
Two readings with different external resistors give two equations that can be solved for both ε and r.
The internal resistance of a car battery is very small, so it can supply the large currents a starter motor needs; dry cells have larger internal resistance.