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Two cells of e.m.f. 12 V and 13 V having internal resistances 1 ohm and 2 ohm respectively are joined in parallel and connected to an external load of 10 ohm. Within which range does the potential difference across the load fall?

1. between 11.5 V and 11.6 V
2. between 11.4 V and 11.5 V
3. between 11.7 V and 11.8 V
4. between 11.6 V and 11.7 V

Correct answer: between 11.5 V and 11.6 V

Solution

Using Millman's theorem the load voltage is V = (12/1 + 13/2)/(1/1 + 1/2 + 1/10) = 18.5/1.6 = 11.56 V, which lies between 11.5 V and 11.6 V.

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