physics : class12 Null Deflection Method : Potentiometer : Current Electricity MCQ Questions and Answers
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Null Deflection Method
MCQ questions and answers.
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MCQ Questions
1. In the null deflection method using a potentiometer, the galvanometer shows zero deflection when
A.
The current through the potentiometer wire is zero
B.
The potential difference across the galvanometer is zero
C.
The resistance of the galvanometer is zero
D.
The battery is disconnected from the circuit
Correct Answer:
The potential difference across the galvanometer is zero
Explanation:
At the null point, the potential difference between the jockey contact and the test cell terminal is zero, so no current flows through the galvanometer and it shows no deflection.
2. The point on the potentiometer wire where the galvanometer shows zero deflection is called the
A.
Terminal point
B.
Balance point
C.
Charging point
D.
Resistance point
Correct Answer:
Balance point
Explanation:
The position of the jockey at which the galvanometer gives null deflection is known as the balance point or null point.
3. At the balance point in a potentiometer experiment, the potential difference across the test cell is
A.
Equal to the potential drop along the balancing length
B.
Always greater than the potential drop along the balancing length
C.
Always zero
D.
Independent of the balancing length
Correct Answer:
Equal to the potential drop along the balancing length
Explanation:
At null deflection, the emf of the cell is balanced by the potential drop across the corresponding length of the potentiometer wire.
4. In the null deflection method, the galvanometer is used to detect
A.
The temperature of the wire
B.
The balance point
C.
The length of the wire
D.
The current supplied by the battery directly
Correct Answer:
The balance point
Explanation:
The galvanometer indicates whether current flows in the secondary circuit and helps locate the point where the current becomes zero.
5. If the jockey is moved beyond the balance point in a potentiometer experiment, the galvanometer generally shows
A.
No change under all conditions
B.
A deflection in the opposite direction
C.
Permanent zero deflection
D.
An increase in wire length
Correct Answer:
A deflection in the opposite direction
Explanation:
On crossing the null point, the polarity of the potential difference between the potentiometer wire and the test cell reverses, causing the galvanometer deflection to reverse.
6. For comparing the emfs of two cells by the null deflection method, the cells are connected
A.
In opposition in the secondary circuit
B.
Only in parallel with the galvanometer
C.
Directly across the potentiometer battery
D.
Without any connection to the galvanometer
Correct Answer:
In opposition in the secondary circuit
Explanation:
For emf comparison, the two cells are connected in opposition so that their balance lengths can be measured using the same potential gradient.
7. If two cells have balance lengths l1 and l2 on the same potentiometer wire, their emfs E1 and E2 are related by
A.
E1 divided by E2 equals l2 divided by l1
B.
E1 divided by E2 equals l1 divided by l2
C.
E1 plus E2 equals l1 plus l2
D.
E1 minus E2 equals l1 minus l2
Correct Answer:
E1 divided by E2 equals l1 divided by l2
Explanation:
On the same potentiometer wire, the potential gradient is the same, so emf is proportional to the balance length: E1/E2 = l1/l2.
8. Why is the null deflection method considered accurate for measuring emf?
A.
The cell supplies a large current at balance
B.
The cell does not supply current at the balance point
C.
The potentiometer wire has no resistance
D.
The galvanometer has no resistance
Correct Answer:
The cell does not supply current at the balance point
Explanation:
At null deflection, no current is drawn from the test cell, so its emf is measured without the usual error caused by internal resistance and loading.
9. To obtain a null point for a cell in a potentiometer, the cell emf should be connected
A.
Opposite to the potential drop along the selected wire length
B.
In the same direction as the potential drop
C.
Only across the galvanometer
D.
With both terminals connected to the same point
Correct Answer:
Opposite to the potential drop along the selected wire length
Explanation:
For cancellation at the balance point, the emf of the cell must oppose the potential difference developed along the potentiometer wire.
10. At the null point, the current through the galvanometer in a potentiometer circuit is
A.
Maximum
B.
Equal to the main circuit current
C.
Zero
D.
Greater than the current in the wire
Correct Answer:
Zero
Explanation:
Null deflection means there is no potential difference across the galvanometer, so no current passes through it at the balance point.