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An electric bell of resistance 5 ohm needs a current of 0.25 A to operate. The connecting wire has resistance 1 ohm per 15 m, and the bell push is 90 m away from the bell (so the wire runs there and back). The cells available each have emf 1.4 V and internal resistance 2 ohm. How many such cells, connected in series, are required to operate the bell?

1. 3
2. 4
3. 5
4. Can't be determined

Correct answer: 5

Solution

External resistance is 17 ohm; solving 1.4n/(17+2n) = 0.25 gives n = 4.7, so 5 cells are needed.

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