question_answer

The resistance of the meter bridge AB in given figure is \[4\Omega .\] With a cell of emf \[\varepsilon =0.5V\]and rheostat resistance \[{{R}_{h}}=2\Omega \]the null point is obtained at some point J. When the cell is replaced by another one of emf \[\varepsilon ={{\varepsilon }_{2}}\]the same null point J is found for\[{{R}_{h}}=6\Omega .\]                          [JEE Main 11-Jan-2019 Morning]

The emf \[{{\varepsilon }_{2}}\] is

A) 0.5 V

B) 0.4 V

C) 0.5 V

D) 0.3 V

Correct Answer: D

Solution :

[d] In first case, current  flowing through secondary circuit

Now, once the balance point is achieved,

.(i)

Similarly, in the second case,

.(ii)

From eqn. (i)