Question

if the length of potentiometer wire is increased by keeping constant potential difference across the wire then​

Answer 1

Sensitivity of a potentiometer can be increased by decreasing its potential gradient. This can be done

- by increasing the length of potentiometer wire.

- if the potentiometer wire is of fixed length, the potentiometer gradient can be decreased by reducing the current in the potentiometer wire circuit with the help of rheostat and using a single cell.

Answer 2

answer : option (a) null point is obtained at larger distance.

you should apply relation of potential gradient and length.

i.e., k = V/L , where V is potential difference , k is potential gradient and L is length of wire.

now if length of wire is increased, potential gradient will decrease. it means, null point is obtained at longer distance.

analytical method : Let voltage of driving source is V, cell voltage is E (initial potential) , L1 is initial length of wire and l is balancing length.

then, E/V = l/L1 .......(1)

case 2 : after changing the length of wire of potentiometer = L2, balancing length = l'

then, E/V = l'/L2......(2)

from equations (1) and (2),

l' = l(L2/L1)

as it is given, L2 > L1

so, l' > l

hence null point is obtained at larger distance.

[ some more points : sensitivity is inversely proportional to potential gradient. it means that smallest potential difference that can be measured with help of potentiometer. ]