Question

A bar magnetic having a magnetic moment of 2.0 × 105 JT-1, is placed along the direction of uniform magnetic field of magnitude B = 14 × 10-5 T. The work done in rotating the magnet slowly through 60° from the direction of field is:

• A. 14 J
• B. 8.4 J
• C. 4 J
• D. 1.4 J

Answer 1

Answer: (A)

14 J

Answer 2

$U =$_ -$\stackrel{\rightarrow}{M}$ . _$\stackrel{\rightarrow}{B}$

$So,$ $U_f$ – $U_i$ $= –MB(1 – cosθ)$

$W=2 \times 10^5 \times 14 \times 10^{-5} (1 - \frac1{2})$

$So, \,W = – ΔU = 14J$

$\therefore$ The correct option is (A): 14 J