question_answer

Direction: A potentiometer is device used for measuring emf and internal resistance of a cell. It consists of two circuits one is main circuit in which there is a cell of given emf\[\varepsilon \] and given resistance R which is connected across a wire of length 100 cm having resistance r. Another circuit having unknown emf\[\varepsilon \]and galvanometer. For a given potentiometer if \[\varepsilon '=30\,V,\,\,r=1\Omega \] and resistance R varies with time t given by\[R=2t\]. The jockey can move on wire with constant velocity 10 cm/s and switch S is closed at\[t=0\].

If balancing length is found to be 70 cm, then the time after which jockey starts moving from

A)  1 s                              

B)  2 s

C)  3 s                              

D)  4 s