CUET · PHYSICS · PYQ PAPER 2023

A short bar magnet placed with its axis at 30\(^\circ\) with an external field of 800G, experiences a torque of 0.016 Nm. Work done in moving it from its most stable to most unstable position is :

A 5.321 J
B 1.843 J
C 0.89 J
D 0.064 J

Verified Solution

Answer & Solution

Correct Answer

(D) 0.064 J

Step-by-step Solution

Detailed explanation

\(\tau = mB\sin\theta\) \(0.016 = m \times (800 \times 10^{-4}) \times \sin(30^\circ)\) \(0.016 = m \times 0.08 \times 0.5\) \(m = \frac{0.016}{0.04} = 0.4 Am^2\) \(W = U_{unstable} - U_{stable} = (-mB\cos(180^\circ)) - (-mB\cos(0^\circ))\) \(W = mB - (-mB) = 2mB\)…

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.

Same subject

Match List-I with List-II (Symbols have their usual meanings)

List-I (Physical Quantity)	List-II (Expression)
(A) Current density	(I) \( \frac{e E \tau}{m} \)
(B) Drift velocity	(II) \( \frac{n e^2 \tau}{m} \)
(C) Conductivity	(III) \( \frac{e \tau}{m} \)
(D) Mobility	(IV) \( n e v_d \)

Choose the correct answer from the options given below:

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List-I (Electromagnetic waves)	List-II (Wavelength range)
(A) X-rays	(I) 1 mm to 700 nm
(B) Radio waves	(II) 0.1 m to 1 mm
(C) Infrared waves	(III) 1 nm to \( 10^{-3} \) nm
(D) Microwaves	(IV) >0.1 m