Question

The resistance of wire is 20 ohm. The wire is stretched to three times of its length. Then the resistance will be :-

• A. $6.67\Omega$
• B. $60\Omega$
• C. $120\Omega$
• D. $180\Omega$

Answer 1

The correct option is D $180\Omega$

Resistance, $R=\frac{\rho l}{A}$
When length l increases three times, the cross section $A$ will decease three times of previous. So length becomes $3l$ and cross section becomes $A/3$.
Now resistance becomes ,$R^{\prime }=\frac{\rho 3l}{\pi A/3}=9R=9\times 20=180\Omega$