The magnetisation at the centre of a bar magnet (magnetic length 10 cm), having pole strength 6.3 A m, and area of cross-section \( 0.70\,\mathrm{cm^{2}} \) is
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \( 9.0 \times 10^{4}\,\mathrm{A\,m^{-1}} \)
Magnetisation is the magnetic moment per unit volume, \( M = m_{\mathrm{total}}/V \). For a bar magnet of pole strength \( q_m \), magnetic length \( 2l \) and cross-section \( A \):
\( \displaystyle m_{\mathrm{total}} = q_m \times 2l \qquad V = A \times 2l \)
\( \displaystyle M = \frac{q_m \times 2l}{A \times 2l} = \frac{q_m}{A} \)
The magnetic length cancels. With \( A = 0.70\,\mathrm{cm^{2}} = 0.70\times10^{-4}\,\mathrm{m^{2}} \):
\( \displaystyle M = \frac{6.3}{0.70\times10^{-4}} = 9.0\times10^{4}\,\mathrm{A\,m^{-1}} \)