A short bar magnet placed with its axis at \(30^{\circ}\) and a uniform external magnetic field of 0.5 T experience torque of magnitude equal to \(4.5 \times 10^{-2} J\). Then the magnitude of the magnetic moment will be _________.

A \(3.6 \times 10^2 J T ^{-1}\)
B \(36 \times 10^{-2} J T ^{-1}\)
C \(1.8 \times 10^2 J T ^{-1}\)
D \(18 \times 10^{-2} J T ^{-1}\)

Verified Solution

Answer & Solution

Correct Answer

(D) \(18 \times 10^{-2} J T ^{-1}\)

Step-by-step Solution

Detailed explanation

\(\begin{array}{l}\tau=m B \sin \theta \\ 4.5 \times 10^{-2}=m \times 0.5 \times \sin 30^{\circ} \\ 4.5 \times 10^{-2}=m \times \frac{1}{2} \times \frac{1}{2} \\ m=4.5 \times 10^{-2} \times 2 \times 2 \\ m=18 \times 10^{-2} J T ^{-1}\end{array}\)

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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