Question

STATEMENT-1
In a Meter Bridge experiment, null point for an unknown resistance is measured. Now, the unknown resistance is put inside an enclosure maintained at a higher temperature. The null point can be obtained at the same point as before by decreasing the value of the standard resistance.
STATEMENT-2
Resistance of a metal increases with increase in temperature.

• A. STATEMENT-1 is True, STATEMENT-2 is True; STATEMENT-2 is a correct explanation for STATEMENT-1
• B. STATEMENT-1 is True, STATEMENT-2 is True; STATEMENT-2 is NOT a correct explanation for STATEMENT-1
• C. STATEMENT -1 is True, STATEMENT-2 is False
• D. STATEMENT -1 is False, STATEMENT-2 is True

Answer 1

Answer: (D)

STATEMENT -1 is False, STATEMENT-2 is True

Resistance of a metal increases with increase in temperature as the Brownian motion of atoms presents more obstacles in the movement of charge carriers. Therefore statement 2 is correct.

For statement 1 , Assume initially $l=20$. Therefore, $R_{unknown}=\frac{R(100-l)}{l}=\frac{R80}{20}=4R$.

Now if the unknown resistance is heated, and the standard resistance is decreased to get the same null point, then, $R_{unknown}^{\prime }=\frac{R^{\prime }(100-80)}{20}=4R^{\prime }$.

But as $R^{\prime }<R\Rightarrow R_{unknown}^{\prime }<R_{unknown}$ which is incorrect as unknown resistance has increased due to heating. Therefore statement 1 is false.