12 physics Activity: To Study the Variation of Potential Drop
Activity: To Study the Variation of Potential Drop Along a Uniform Wire Using a Potentiometer
Aim
To study the variation of potential drop with length of a uniform wire and hence verify the principle of a potentiometer.
Apparatus Required
Potentiometer, driver cell (battery), rheostat, galvanometer, jockey, given cell, key, voltmeter (optional), connecting wires.
Principle
A potentiometer works on the principle that the potential drop across a uniform wire carrying steady current is directly proportional to its length.
Mathematically:
V ∝ l
or
V = k l
where:
V = potential drop
l = length of wire
k = potential gradient (potential drop per unit length)
Theory
When a steady current flows through a uniform potentiometer wire, the potential drop along the wire is uniform. Using the null deflection method, the potential difference across a cell can be measured accurately without drawing any current from it. This ensures high accuracy.
The principle is verified by showing that the ratio of potential drops across two lengths of wire is equal to the ratio of their lengths:
V₁ / V₂ = l₁ / l₂
Circuit Diagram
(Neatly draw the standard potentiometer circuit showing driver cell, rheostat, potentiometer wire AB, galvanometer, given cell, key, and jockey.)
Procedure
Arrange the circuit as shown in the diagram.
Connect the driver cell, rheostat, key, and potentiometer wire in series.
Adjust the rheostat to allow a steady current through the potentiometer wire.
Connect the given cell in the secondary circuit with the galvanometer and jockey.
Touch the jockey at different points along the wire to obtain null deflection.
Note the balancing length l corresponding to the EMF of the given cell.
Change the potential difference by adjusting the rheostat or using another cell.
Again note the new balancing length.
Repeat the process for at least three different readings.
Observations
| S.No | Potential Difference (V) | Balancing Length l (cm) | Potential Gradient k = V/l (V/cm) |
|---|---|---|---|
| 1 | |||
| 2 | |||
| 3 |
Calculations
For each observation:
Potential Gradient,
k = V / l
Example Calculation:
If: V = 1.5 V l = 150 cm
Then:
k = 1.5 / 150 = 0.01 V/cm
Similarly calculate k for other observations.
Graph
Plot a graph between potential difference (V) on Y-axis and balancing length (l) on X-axis. A straight-line graph passing through the origin verifies V ∝ l.
Result
The graph between potential drop and length of the potentiometer wire is a straight line. Hence, the potential drop along a uniform wire is directly proportional to its length, verifying the principle of the potentiometer.
Precautions
Connections should be tight and clean.
The jockey should be pressed lightly on the wire.
Current through the potentiometer wire should be steady.
The potentiometer wire must be uniform and straight.
Sources of Error
Non-uniform wire.
Fluctuation in current.
Contact resistance at the jockey.
Advantages of Potentiometer
Highly accurate measurement of potential difference.
No current is drawn from the source.
Free from errors due to internal resistance.
Viva Voce Questions
What is the principle of a potentiometer?
What is potential gradient?
Why is the potentiometer more accurate than a voltmeter?
What kind of wire is used in a potentiometer?
Conclusion
The activity successfully verifies that the potential drop along a uniform wire is directly proportional to its length, which is the fundamental principle of a potentiometer.
Variation of Potential Drop & Principle of Potentiometer – MCQs
1. The principle of a potentiometer is that the potential drop across a wire is proportional to its:
A. Thickness
B. Length
C. Material
D. Temperature
✔ Answer: B
2. A potentiometer works on the method of:
A. Maximum deflection
B. Null deflection
C. Least count
D. Resonance
✔ Answer: B
3. The potential gradient of a potentiometer wire depends on:
A. Length of wire only
B. Current through the wire
C. Resistance of wire
D. All of these
✔ Answer: D
4. The potential drop along a potentiometer wire is uniform because the wire is:
A. Thick
B. Short
C. Uniform in cross-section
D. Bent
✔ Answer: C
5. If the current in a potentiometer wire is increased, the potential gradient will:
A. Decrease
B. Become zero
C. Increase
D. Remain constant
✔ Answer: C
6. In a potentiometer, the balancing point is obtained when:
A. Voltmeter shows maximum reading
B. Galvanometer shows zero deflection
C. Current is maximum
D. Resistance is minimum
✔ Answer: B
7. The variation of potential drop along a potentiometer wire is:
A. Random
B. Exponential
C. Linear
D. Parabolic
✔ Answer: C
8. The EMF of a cell can be measured accurately using a potentiometer because it:
A. Has very low resistance
B. Draws no current from the cell
C. Uses digital display
D. Has short wire
✔ Answer: B
9. The potential drop per unit length of the potentiometer wire is called:
A. Resistance
B. EMF
C. Potential gradient
D. Current density
✔ Answer: C
10. For greater sensitivity of a potentiometer, the wire should be:
A. Short and thick
B. Long and uniform
C. Non-uniform
D. Bent in shape
✔ Answer: B
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