Question: Q. 5. (i) State the underlying principle of a potentiometer. Why is it necessary to (a) use a long wire, (b) have uniform area of cross-section of the wire and (c) use a driving cell whose emf is taken to be greater than the emf of the primary cells?

(ii) In a potentiometer experiment, if the area of the cross-section of the wire increases uniformly from one end to the other, draw a graph showing how potential gradient would vary as the length of the wire increases from one end.

R U[Comptt. O.D. I, II, III 2014]

ns. (i) Principle of potentiometer :

The potential drop across the length of a steady current carrying wire of uniform cross-section is proportional to the length of the wire.

$1 / 2$

(a) We use a long wire to have a lower value of potential gradient (i.e., a lower ’least count’ or greater sensitivity of the potentiometer.) $\quad 1 / 2$

(b) The area of cross-section has to be uniform to get a ‘uniform wire’ as per the principle of the potentiometer.

$1 / 2$

(c) The emf of the driving cell has to be greater than the emf of the primary cells as otherwise no balance point would be obtained.

$$ 1 / 2 $$

(ii) Potential gradient, $k=\frac{V}{L}$

$\therefore$ The required graph is as shown below :

1

[CBSE Marking Scheme 2014]

Q. 6. State the underlying principle of a potentiometer. Write two factors on which the sensitivity of a potentiometer depends.

In the potentiometer circuit shown in the figure, the balance point is at $X$. State, giving reason, how the balance point is shifted, when :

(i) resistance $R$ is increased ?

(ii) resistance $S$ is increased, keeping $R$ constant ?

U] [Delhi Comptt. I, II, III, 2013]