Page 26 - Physics - XII

Q10. Consider a metre bridge whose length of wire is 2 m. A resistance of 10 Ω is connected across
one gap of the metre bridge and an unknown resistance is connected across the other gap.
When these resistances are interchanged, the balance point shifts by 50 cm. What is the value
of the unknown resistance?

(a) 250 Ω (b) 10 Ω (c) 16.7 Ω (d) None of these
Answer key

1. (a), 2. (b), 3. (a), 4. (c), 5. (d), 6. (c), 7. (b), 8. (a), 9. (a), 10. (c)

Case Study-based Questions

Read the paragraph and answer the questions that follow.

The potentiometer is an instrument that can be used to measure the emf of a source without drawing
any current from the source. It also has a number of other useful applications. Essentially, it balances an
unknown potential diff erence against an adjustable, measurable potential diff erence.

Potentiometer is based on the principle that when a ε 1 K
+
–
constant current fl ows through a wire of uniform area of Driver battery Rh
cross-section, the potential drop across any length of the A l Primary circuit B
wire is directly proportional to the length. A resistance + Secondary P J (Null defl ection position) –
wire AB of total resistance R is permanently connected circuit G
AB
to the terminals of a source of known e.m.f. ε . A sliding + ε –
1 2
jockey J is connected through the galvanometer G to a second source whose emf ε is to be measured.
2
As jockey J is moved along the resistance wire, the resistance R between points P and B varies; if the
PB
resistance wire is uniform, R is proportional to the length of wire between P and B. To determine the
PB
value of ε , J is moved until a position is found at which the galvanometer shows no defl ection; this
2
corresponds to zero current passing through ε .
2
The term potentiometer is also used for any variable resistor, usually having a circular resistance element
and a sliding contact controlled by a rotating shaft and knob.

Q1. Two cells of emfs approximately 5 V and 10 V are to be accurately compared using a
potentiometer of length 400 cm. Choose the correct option.
a. Potentiometer is usually used for comparing resistance and not voltages.
b. The fi rst portion of 50 cm of wire itself should have a potential drop of 10 V.
c. The battery of potentiometer can have a voltage of 15 V and R adjusted so that the potential

drop across the wire slightly exceeds 10 V.
d. The battery that runs the potentiometer should have voltage of 8 V.
Q2. AB is a potentiometer wire of potentiometer. If the value of R is E R

increased, in which direction will the balance point J shift?
(a) Towards A (b) Towards B A l B

(c) Same as initial point (d) None of these E′ G

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