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Potentiometers are an integral part of electronic equipment. This article covers the important multiple choice questions based on the Potentiometer from the CBSE Class 12 Physics Syllabus. MCQs based on working, construction, types, and applications of potentiometers are briefly covered below.
A potentiometer controls the flow of current. It is a three-terminal variable resistor in which controlling the resistance controls the current flow through the circuit. The potentiometer also acts as an adjustable voltage divider.
Potentiometer Design
Potentiometer works on the principle of the potential drop across a given cross-section of wire which is directly proportional to the wire length. This is true only when the cross-sectional area of the wire is uniform and the current flowing through the wire is also uniform. Potentiometers are of two types- rotary (resistance varies due to the angular movement) and linear (wiper moves along a linear path).
Read More: Current Electricity
Class 12 Potentiometer MCQs
Q.1. A potentiometer does not _________ .
- Measure the Emf of two cells
- Measure the unknown temperature
- Measure the cell Internal Resistance
- Compare the emf of two cells
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A.1. The correct option is C. Measure the cell Internal Resistance
Explanation: A potentiometer is helpful in measuring the emf between two cells, comparing the EMFs, and measuring the unknown temperature in the circuit. However, a potentiometer is not capable of measuring the internal resistance of the cells.
Q.2. The length of wire in the potentiometer is 100 cm with a resistance of 30 ohms. It is connected in series with a resistance of 20 ohms and an accumulator of emf 8V having negligible internal resistance. 1.2 V source balances it against the length ‘L’ of the potentiometer wire. Calculate the length ‘L’ of the wire.
- 30 cm
- 35 cm
- 20 cm
- 25 cm
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A.2. The correct option is D. 25 cm
Explanation: Current through the wire, I = \(\frac{8}{30+20} = \frac{8}{50}\)
= 0.16 A
Potential difference across the wire= 0.16 x 30
=4.8 V (V = current × resistance)
Length of the wire = 100 cm
Therefore, k = \(\frac{V}{I}\)
= 4.8/100
= 0.048
EMF 1.2V is balanced against the length L of the wire,
1.2 = kL
Substituting, 1.2/k= L
Hence,
L= 25 cm
Q.3. A potentiometer wire of length 20 m has a resistance of 50 ohms. It is connected in series with a resistance box and a 5 V storage cell. If the potential gradient along the wire is 0.5 mV/cm, what is the resistance unplugged in the box?
- 450 ohms
- 400 ohms
- 500 ohms
- 405 ohms
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A.3. The correct option is A. 450 Ohms
Explanation: Potential gradient along the wire= \(\frac{Potential difference}{Wire length}\)
= 0.5 x 10-3 = I x \(\frac{50}{1000}\)
I = 1/100
Calculating for R,
\(\frac{5}{50 + R}\)= \(\frac{1}{100}\)
R + 50 = 500
R = 450 ohms
Read More: NCERT Solutions for Class 12 Physics Current Electricity
Q.4. A potentiometer has 5 wires and the balance is obtained in the 2nd wire. If the balance is to be shifted to the 4th wire, the current should be reduced.
- True
- False
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A.4. The correct option is A. True
Explanation: When the balance has to be shifted to a higher length, the potential gradient should be reduced. This can be achieved by increasing the resistances (in series) with the potentiometer wire. This can thus be achieved by reducing the current flow in the circuit. Hence the above statement is true.
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Q.5. Which of the following is an application of the potentiometer?
- Compares two currents
- Measures current
- Measures internal resistance
- Measures external resistance
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A.5. The correct option is C. Measures internal resistance
Explanation: A potentiometer is a device used to measure the internal resistance in a circuit. It can also be used to measure the emf, comparing the EMFs of cells and variable resistors.
Q.6. What is the potential gradient?
- Reduction in potential per unit length of the wire
- Reduction in potential per unit area of wire
- Reduction in potential between the ends of the two wires
- Potential at any one end of the wire
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A.6. The correct option is A. Reduction in potential per unit length of the wire
Explanation: The fall in potential per unit length of the wire is called as the potential gradient. It is mathematically written as
k= V/I
Q.7. The potential gradient does not depend on ____________.
- Current flowing through the potentiometer
- Area of cross-section
- Specific resistance of the wire material
- Resistance per unit area of the wire
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A.7. The correct option is D. Resistance per unit area of the wire
Explanation: The potential gradient depends on the resistance per unit length of the wire and the cross-sectional area. The potential gradient has no relation to the resistance per unit area of the wire.
Q.8. What is the effect on the null point if the radius of the potentiometric wire is doubled?
- Increases
- Remains Unchanged
- Decreases
- Increases and then decreases
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A.8. The correct option is C. Decreases
Explanation: At balanced condition,
E= kL
Where E= emf
k= potential gradient
L= length of the potentiometric wire
Thus, emf of the battery depends on the resistance.
So when the radius of the wire is doubled, the resistance decreases due to inverse proportionality with the wire area. Thus the null point will decrease.
Q.9. The potential gradient of a potentiometer is 3 mV/cm. It measures the potential difference across a resistance wire of 20 Ω. If the potentiometric wire length is 60 cm to get the null point, what is the current passing through the resistor?
- 11 mA
- 10 mA
- 9 mA
- 8 mA
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A.9. The correct option is D. 9 mA
Explanation: Let x = 3 mV/ cm = 0.3 V/m
R= 20 Ω
L = 60cm = 0.6 m
If the current through the resistance is I
Potential difference across 20 Ω is V= IR — (1)
When the potential gradient is x, the null point comes at L.
Therefore, V=xL — (2)
Substituting, we get
I= xL/R
= 0.3 x 0.6 /20
=9 x 10-3 A
= 9 mA
Also Check:
| Important Concepts | ||
|---|---|---|
| Specific Resistance | Circuit Diagram | Unit of Voltage |
| Resistor Colour Codes | Resistor Types | Unit of Current |
| AC vs DC | Drift Velocity | Ampere |
Q.10. Potentiometer cannot measure _________ .
- Capacity
- Electric Current
- Potential
- Resistance
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A.10. The correct option is A. Capacity
Explanation: A potentiometer can measure the internal resistance, potential difference, and current. However, the capacity cannot be measured by the potentiometer.
Q.11. If the same current is allowed to flow but the potentiometric wire length is increased, the sensitivity ___________.
- Does not change
- Increases
- Decreases
- Becomes uncertain
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A.11. The correct option is B. Increases
Explanation: The smallest potential difference the potentiometer can measure is its sensitivity. This is typically increased by- increasing wire length or using a rheostat to reduce the current. Hence, the length of the wire is increased but the current allowed remains the same, sensitivity increases.
Q.12. A potentiometer measures the emf of a cell. At the null point, there is no current flow through_________
- Cell Circuit
- Main Circuit
- Both A and B
- Potentiometric Wire
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A.12. The correct option is A. Cell Circuit
Explanation: At the null point, the current flowing through the cell circuit will be zero.
Q.13. A cell is balanced on a potentiometric wire when the balancing length is 125 cm. Calculate the internal resistance if the resistance of 2 Ohms is connected with a balancing length of 100 cm.
- 0.25 Ohms
- 0.5 Ohms
- 0.05 Ohms
- 5 Ohms
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A.13. The correct option is B. 0.5 Ohms
Explanation: Let I1 = 125 cm, I2= 100 cm
Internal resistance, r =[ l1 – l2/ l2 ] R
= [125 – 100/100] x 2
=0.5 Ohms
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