Identify the correct statements among the following:
(A) The torque acting on a planar current loop in a magnetic field changes, when its shape is changed without changing its area.
(B) Two parallel wires carrying currents in opposite directions attract each other.
(C) An ideal ammeter has zero resistance, and an ideal voltmeter has infinite resistance.
(D) A galvanometer can be converted into voltmeter by connecting a high resistance in series.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (C) and (D) only
(A) The torque acting on a planar current loop in a magnetic field changes, when its shape is changed without changing its area. — Incorrect. The torque is \( \tau = NIAB\sin\theta \), set by the magnetic moment \( m = NIA \). It depends on the area of the loop, not on its shape, so the torque stays the same.
(B) Two parallel wires carrying currents in opposite directions attract each other. — Incorrect. Currents in the same direction attract; currents in opposite directions repel.
(C) An ideal ammeter has zero resistance, and an ideal voltmeter has infinite resistance. — Correct. An ammeter is joined in series, so it must add no resistance to the circuit; a voltmeter is joined in parallel, so it must draw no current.
(D) A galvanometer can be converted into voltmeter by connecting a high resistance in series. — Correct. A large series resistance R lets the full-scale current \( I_g \) flow only when the voltage across the combination reaches the required range V.
\( \displaystyle R = \frac{V}{I_g} - G \)
Note: A similar question was also asked in CUET (UG) 2026 on 24-05-2026 (Slot 2).
NCERT (2026-27):
Summary – “… 9. Parallel currents attract and anti-parallel currents repel. 10. A planar loop carrying a current I, having N closely wound turns, and an area A possesses a magnetic moment m where, m = N I A … When this loop is placed in a uniform magnetic field B, the force F on it is: F = 0 And the torque on it is,
\( \displaystyle \boldsymbol{\tau} = \mathbf{m} \times \mathbf{B} \)”
The Moving Coil Galvanometer – “… The galvanometer can also be used as a voltmeter to measure the voltage across a given section of the circuit. … To ensure this, a large resistance R is connected in series with the galvanometer.”