current-electricity Question 53
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]
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Solution:
Ans. Try yourself Similar to Q. 5 (a), Short Answer Type Questions-II.
Factors : There are two factors on which sensitivity of potentiometer depends :
(a) length of potentiometer $(l)$
(b) potential drop $(E)$
(i) If resistance $R$ is increased, current through the potentiometer wire will decrease, as a result of which, potential gradient of potentiometer wire also decreases. In such case the balance point $X$ will shift towards point $B$.
(ii) If resistance $S$ increases, keeping resistance $R$ constant, in such case the balance point $X$ will shift towards $A$.
[CBSE Marking Scheme 2013]
