Current Electricity

Three voltmeters, all having different internal resistances are joined as shown in figure. When some potential difference is applied across A and B, their readings are V1, V2 and V3. Choose the correct option. [figure]

1. V1=V2
2. V1 V3-V2
3. V1+V2=V3
4. V1+V2>V3

Answer

(3) V1+V2=V3

Solution

Upper branch: V1 and V2 in series; lower branch: V3 alone; both branches are across A and B.

V1+V2=VAB and V3=VAB (KVL round the loop: V1+V2-V3=0)

V1+V2=V3

(A): V1=V2 needs equal resistances (series, same current) (A) is wrong

(B): V1=V3-V2 actually holds (B) is wrong

(C): V1+V2=V3 (C) is correct

(D): V1+V2 equals V3, not greater (D) is wrong

Answer: (3)