Level 1: An Old Cell Reads 1.5 V but the Torch Stays Dark: Why?
Measure the resistance inside it.
A cell of EMF 1.5 V gives a terminal voltage of only 0.5 V when a 1 Ω bulb is connected across it. Its internal resistance is:
The bulb has 0.5 V across 1 Ω, so the current in the circuit is 0.5 A. The rest of the EMF, 1.0 V, is lost across the internal resistance.
The internal resistance is 2 Ω, twice the bulb's resistance, so two thirds of the EMF is wasted inside the cell. This is what an old cell looks like from the outside.
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