Question

How many dry cells, each of emf 1.5V and internal resistance 0.5Ω, much be joined in series with a resistor of 20Ω to give a current of 0.6A in the circuit? 

(A) 2 

(B) 8 

(C) 10 

(D) 12

Answer 1

The correct option (C) 10

Explanation:

I = [nE/{R + nr}]

I = current = 0.6A

E = external resistor = 20Ω

r = internal resistor = 0.5Ω

∴ 0.6 = {(1.5n)/(20 + 0.5 n)}

12 + 0.3n = 1.5n

12 = 1.2n

n = 10