physics : class12 Internal Resistance Measurement : Potentiometer : Current Electricity MCQ Questions and Answers
Practice important
Internal Resistance Measurement
MCQ questions and answers.
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MCQ Questions
1. In the potentiometer method for measuring the internal resistance of a cell, what is measured first when the cell is not supplying current?
A.
Balance length for the emf of the cell
B.
Current through the external resistance
C.
Resistance of the potentiometer wire
D.
Temperature of the cell
Correct Answer:
Balance length for the emf of the cell
Explanation:
When the cell is not supplying current, the potentiometer measures its emf. The corresponding balance length is usually denoted by l₁.
2. In the internal resistance experiment, a known resistance is connected across the cell after measuring its emf. What does the second balance length represent?
A.
The terminal potential difference of the cell
B.
The internal resistance of the cell directly
C.
The resistance of the external circuit
D.
The current in the potentiometer wire
Correct Answer:
The terminal potential difference of the cell
Explanation:
When a known external resistance is connected, the cell supplies current and its terminal voltage is measured by the second balance length, usually denoted by l₂.
3. If l₁ is the balance length for the emf E and l₂ is the balance length for the terminal voltage V, which relation is correct?
A.
E/V = l₂/l₁
B.
E/V = l₁/l₂
C.
E/V = l₁ + l₂
D.
E/V = l₁ - l₂
Correct Answer:
E/V = l₁/l₂
Explanation:
The potential difference measured by a potentiometer is proportional to its balance length. Therefore, E/V = l₁/l₂.
4. A cell of internal resistance r is connected to an external resistance R. If l₁ and l₂ are the balance lengths for emf and terminal voltage respectively, which formula gives r?
A.
r = Rl₂/(l₁ - l₂)
B.
r = Rl₁/l₂
C.
r = R(l₁ - l₂)/l₂
D.
r = R(l₁ + l₂)/l₂
Correct Answer:
r = R(l₁ - l₂)/l₂
Explanation:
Using E/V = l₁/l₂ and E/V = (R + r)/R, the internal resistance is r = R(l₁ - l₂)/l₂.
5. Why is the balance length for the terminal voltage smaller than the balance length for the emf of a cell supplying current?
A.
The potentiometer wire becomes shorter
B.
The terminal voltage is less than the emf due to the internal voltage drop
C.
The external resistance increases the emf
D.
The cell stops producing potential difference
Correct Answer:
The terminal voltage is less than the emf due to the internal voltage drop
Explanation:
When current flows, a voltage drop occurs across the internal resistance of the cell, so its terminal voltage is less than its emf. Hence, l₂ is less than l₁.
6. A cell has emf balance length l₁ = 100 cm and terminal-voltage balance length l₂ = 80 cm. If an external resistance of 4 ohms is used, what is the internal resistance of the cell?
A.
0.5 ohm
B.
1 ohm
C.
2 ohms
D.
4 ohms
Correct Answer:
1 ohm
Explanation:
Using r = R(l₁ - l₂)/l₂, r = 4(100 - 80)/80 = 1 ohm.
7. A cell gives balance lengths of 120 cm without an external load and 100 cm with a 5-ohm resistance connected across it. What is the internal resistance of the cell?
A.
0.5 ohm
B.
1 ohm
C.
2 ohms
D.
5 ohms
Correct Answer:
1 ohm
Explanation:
The internal resistance is r = R(l₁ - l₂)/l₂ = 5(120 - 100)/100 = 1 ohm.
8. In the potentiometer method, the known resistance connected across the cell is used mainly to:
A.
Increase the emf of the cell
B.
Draw current from the cell and produce a terminal voltage
C.
Remove the internal resistance of the cell
D.
Increase the length of the potentiometer wire
Correct Answer:
Draw current from the cell and produce a terminal voltage
Explanation:
The known external resistance makes the cell supply current. The resulting terminal voltage is compared with the emf to determine the internal resistance.
9. If the external resistance connected across a cell is R and the internal resistance is r, which expression gives the ratio of emf to terminal voltage?
A.
E/V = R/(R + r)
B.
E/V = r/R
C.
E/V = (R + r)/R
D.
E/V = R + r
Correct Answer:
E/V = (R + r)/R
Explanation:
For a cell supplying current, E = I(R + r) and V = IR. Therefore, E/V = (R + r)/R.
10. If the balance lengths for the emf and terminal voltage of a cell are equal, what is the indicated internal resistance of the cell?
A.
Zero
B.
Equal to the external resistance
C.
Twice the external resistance
D.
Infinite
Correct Answer:
Zero
Explanation:
Equal balance lengths mean the emf equals the terminal voltage. Therefore, there is no internal voltage drop, indicating zero internal resistance.