Discussion Forum



1)

The electric resistance of a certain wire of iron is R. If its length and radius are both doubled, then


A) the resistance and the specific resistance, will both remain unchanged

B) the resistance will be doubled and the specific resistance will be halved

C) the resistance will be halved and the specific resistance will remain unchanged

D) the resistance will be halved and the specific resistance will be doubled

Answer:

Option C

Explanation:

R1 = $\frac{\rho l_{1}}{A_{1}}$ , now l2 = 2l1  r2 = 2r1

A1 = πr12

A2 = π(r2)2 = π(2r1)2 = 4πr1 2= 4A1

R2 = $\frac{\rho \left(2l_{1}\right)}{4A_{1}} = \frac{\rho l_{1}}{2A_{1}}$

= $\frac{R_{1}}{2}$

.'. Resistance is halved, but specific resistance remains the same.