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Wire resistivity using a metre bridge: NEET MCQs with answers

74 questions on this topic of Experimental Skills (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. In a metre bridge setup to determine the resistivity of a wire, which of the following practices is INCORRECT?

  1. The current is kept switched on continuously throughout the experiment to achieve steady thermal equilibrium.
  2. The jockey is gently tapped along the wire rather than pressed hard and slid.
  3. A shunted galvanometer is initially used to locate the approximate null point, and the shunt is removed for the final precise balance.
  4. The diameter of the resistance wire is measured at several points along its length in two mutually perpendicular directions.
Show answer and explanation

Answer: (A) The current is kept switched on continuously throughout the experiment to achieve steady thermal equilibrium.

Keeping the circuit switched on continuously produces Joule heating in the wire and the metre bridge wire, altering their resistances and introducing systematic errors. The key should only be closed while taking readings.

medium

2. In a metre-bridge experiment, a 5 Ω resistor is in the left gap and the null point is 40 cm from the left end. The resistance in the right gap is:

  1. 7.5 Ω
  2. 3.33 Ω
  3. 3 Ω
  4. 2 Ω
Show answer and explanation

Answer: (A) 7.5 Ω

At balance R/S = l/(100 − l), so S = R(100 − l)/l = 5 × 60/40 = 7.5 Ω.

hard

3. An unknown resistance X is connected in the left gap and a 20 Ω resistance in the right gap of a metre bridge. When X is shunted with another identical resistor of value X, by what distance does the balance point shift if the original balance point was at 40 cm?

  1. 25 cm towards the left
  2. 15 cm towards the right
  3. 15 cm towards the left
  4. 10 cm towards the right
Show answer and explanation

Answer: (C) 15 cm towards the left

Initially, X / 20 = 40 / 60 = 2 / 3, giving X = 40 / 3 Ω. When shunted by X, the new left gap resistance is X' = X / 2 = 20 / 3 Ω. Now, X' / 20 = l' / (100 - l') ⇒ (20/3) / 20 = 1 / 3 = l' / (100 - l'). Solving yields 100 - l' = 3l' ⇒ 4l' = 100 ⇒ l' = 25 cm. Thus, the shift is 40 cm - 25 cm = 15 cm towards the left.

medium

4. In a metre bridge experiment to determine the resistivity of a wire, a student measures the unknown resistance as R = (4.0 ± 0.1) Ω, length of the wire as L = (50.0 ± 0.1) cm, and diameter as d = (1.00 ± 0.02) mm. What is the maximum percentage error in the determination of resistivity ρ?

  1. 4.7%
  2. 3.5%
  3. 5.2%
  4. 6.7%
Show answer and explanation

Answer: (D) 6.7%

The formula for resistivity is ρ = R · πd² / (4L). The fractional error is Δρ/ρ = ΔR/R + 2(Δd/d) + ΔL/L. Substituting the values gives Δρ/ρ = (0.1/4.0) + 2(0.02/1.00) + (0.1/50.0) = 0.025 + 0.040 + 0.002 = 0.067, which corresponds to 6.7%.

easy

5. In a metre bridge experiment to determine the resistance of a wire, thick copper strips are used to connect the bridge elements on the wooden board. The main purpose of using thick copper strips is to:

  1. increase the sensitivity of the galvanometer
  2. minimise end-resistances at the junctions
  3. prevent the bridge wire from heating up
  4. provide mechanical strength to the setup
Show answer and explanation

Answer: (B) minimise end-resistances at the junctions

Thick copper strips have very large cross-sectional areas and low resistivity, which makes their resistance negligible. This minimises the undesirable end resistances introduced at the contact junctions.

hard

6. Statement I: To measure the resistance of a very low-resistance wire accurately, a metre bridge is usually not preferred. Statement II: For very low unknown resistances, the resistance of the end copper strips and connecting leads becomes comparable to the unknown resistance, causing large fractional errors. Choose the correct option:

  1. Both Statement I and Statement II are correct, and Statement II is the correct explanation of Statement I
  2. Both Statement I and Statement II are correct, but Statement II is not the correct explanation of Statement I
  3. Statement I is correct but Statement II is incorrect
  4. Statement I is incorrect but Statement II is correct
Show answer and explanation

Answer: (A) Both Statement I and Statement II are correct, and Statement II is the correct explanation of Statement I

A metre bridge is not suitable for measuring very low resistances (e.g., less than 1 Ω) because the end resistances of the copper strips and connecting wires introduce significant systematic error, making the bridge ratio inaccurate. Thus, both statements are correct and Statement II explains Statement I.

medium

7. Consider the following statements regarding the experimental procedure of determining the resistivity of a wire using a metre bridge: Statement I: A high resistance box connected in series with the galvanometer protects it from large initial currents and does not change the position of the null point. Statement II: The null point is shifted toward the right end if the electromotive force (emf) of the auxiliary driving cell is doubled. Choose the correct option:

  1. Both Statement I and Statement II are correct
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
Show answer and explanation

Answer: (B) Statement I is correct but Statement II is incorrect

Statement I is correct because at the balance point, zero current flows through the galvanometer branch, so adding a series resistor does not alter the null position. Statement II is incorrect because the Wheatstone condition (P/Q = R/S) depends purely on the ratio of resistances and is independent of the driving cell's emf.

medium

8. In a metre-bridge experiment, a 6 Ω resistor is in the left gap and the null point is 75 cm from the left end. The resistance in the right gap is:

  1. 2 Ω
  2. 4.5 Ω
  3. 18 Ω
  4. 1.5 Ω
Show answer and explanation

Answer: (A) 2 Ω

At balance R/S = l/(100 − l), so S = R(100 − l)/l = 6 × 25/75 = 2 Ω.

medium

9. In a metre-bridge experiment, a 6 Ω resistor is in the left gap and the null point is 60 cm from the left end. The resistance in the right gap is:

  1. 9 Ω
  2. 4 Ω
  3. 3.6 Ω
  4. 2.4 Ω
Show answer and explanation

Answer: (B) 4 Ω

At balance R/S = l/(100 − l), so S = R(100 − l)/l = 6 × 40/60 = 4 Ω.

easy

10. Consider the following statements regarding the metre bridge wire used in finding resistivity: Statement I: The wire is made of alloys like manganin or constantan because they possess high resistivity and a nearly zero temperature coefficient of resistance. Statement II: The wire should be of non-uniform cross-section along its length to improve precision. Choose the correct option:

  1. Both Statement I and Statement II are correct
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
Show answer and explanation

Answer: (B) Statement I is correct but Statement II is incorrect

Statement I is correct because manganin/constantan wires prevent resistance changes due to Joule heating and provide sufficient resistance per unit length. Statement II is incorrect because the bridge principle assumes resistance per unit length is strictly uniform along the entire length.

easy

11. Statement I: In a metre bridge experiment, interchanging the positions of the known resistance and the unknown resistance wire helps in eliminating errors arising from end-resistances. Statement II: The end-resistance arises because the resistance of the copper strips and soldered joints at the two ends cannot be exactly zero.

  1. Both Statement I and Statement II are correct.
  2. Statement I is correct but Statement II is incorrect.
  3. Statement I is incorrect but Statement II is correct.
  4. Both Statement I and Statement II are incorrect.
Show answer and explanation

Answer: (A) Both Statement I and Statement II are correct.

Both statements are correct. Non-zero resistance of metallic end strips and contact resistance introduce end errors, which are minimized by interchanging the resistances in the two gaps and taking the mean of the two determinations.

medium

12. While performing a metre bridge experiment, a student finds that the galvanometer shows deflection in the same direction at both ends of the bridge wire (at 0 cm and 100 cm). Which of the following is a possible cause for this defect?

  1. The jockey is being slid along the wire instead of being tapped gently
  2. The connections at the resistance gaps are perfectly balanced
  3. The emf of the driving cell is much smaller than 2 V
  4. The battery circuit is open or disconnected at one terminal
Show answer and explanation

Answer: (D) The battery circuit is open or disconnected at one terminal

One-sided deflection at both extremities occurs when current does not flow through the main bridge wire (e.g., disconnected or loose battery terminals), or if the galvanometer path is improperly connected such that the potential difference across it does not reverse sign across the bridge length.

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