Question

Two batteries, one of emf 18V and internal resistance 2Ω and the other of emf 12 V and internal resistance 1Ω, are connected as shown. The volt meter V will record a reading of :

(A) 15V 

(B) 30 V

(C) 14 V

(D) 18 V

Answer 1

Correct Answer is: (C) 14 V

It is clear that the two cells oppose each other hence, the effective emf in closed circuit is 18 – 12 = 6V and net resistance is 1 + 2 = 3Ω (because in the closed circuit the internal resistances of two cells are in series).

The current in circuit will be in direction of arrow shown

in figure.

The potential difference across V will be same as the terminal voltage of either cell.

Since, current is drawn from the cell of 18 volt,

hence,

V₁ = E₁ – ir₁

= 18 – (2 × 2) = 18 – 4 = 14 V

Similarly, current enters in the cell of 12V hence

V₂ = E₂ + ir₂

= 12 + 2 × 1

= 12 + 2 = 14 V

Hence, V = 14V