question_answer

A very small magnet is placed in the magnetic meridian with its south pole pointing north. The null point is obtained 20 cm away from the centre of the magnet. If the earth's magnetic field (horizontal component) at this point be 0.3 gauss, the magnetic moment of the magnet is [CPMT 1987; MNR 1978]

A)            \[8.0\times {{10}^{2}}e.m.u.\]                                         

B)            \[1.2\times {{10}^{3}}e.m.u.\]

C)            \[2.4\times {{10}^{3}}e.m.u.\]                                         

D)            \[3.6\times {{10}^{3}}e.m.u.\]

Correct Answer: B

Solution :

           At neutral point \[\left| B\, \right|=\left| {{B}_{H}} \right|\]\[\Rightarrow \frac{2M}{{{\left( 20 \right)}^{3}}}=0.3\] \[\Rightarrow M=1.2\times {{10}^{3}}\ emu.\]