Question

If the galvanometer G does not show any deflection in the circuit shown, the value of R is given by:

• A. 400Ω
• B. 200Ω
• C. 50Ω
• D. 100Ω

Answer 1

Answer: (D)

100Ω

Answer 2

The correct option is (D): 100Ω
Applying Kirchhoff's law for the whole circuit:
400i=10−2
⇒ $i=\frac{8}{400}=\frac{1}{50}$
when the galvanometer shows zero deflection then no current flow through it.
now for loop (first box) : 400i+Ri=10
⇒ $i=\frac{10}{(R+400)}$
substituting the value of i,
$\frac{1}{50}=\frac{10}{(R+400)}$
or R+400=500
⇒R = 100Ω