Question

Calculate the number of dry cells, each of emf 2V and internal resistance 1V that is joined in series with a resistance of 30 ohms so that a current of 0.8A passes through it.

(a) 20

(b) 10

(c) 30

(d) 40

The question was posed to me in class test.

This interesting question is from Current Electricity topic in chapter Current Electricity of Physics – Class 12

Answer 1

Correct answer is (a) 20

The explanation is: Emf = 2V; r = 1 ohm; I = 0.8A; R = 30 ohms

The required equation: I = \(\frac {(n \times e)}{ [(n \times r) + R] }\)

0.8 = \(\frac {(n \times 2)}{ [(n \times 1) + 30] }\)

0.8n + 24 = 2n

1.2n = 24

n = \(\frac {24}{1.2}\) = 20

Therefore, the number of dry cells required are 20.