Question:

A bar magnet has a magnetic moment of $200\, A \,m^2$. The magnet is suspended in a magnetic field of $0.30\, N\, A^{-1}\, m^{-1}$. The torque required to rotate the magnet from its equilibrium position through an angle of $30^{\circ}$, will be

Updated On: Jul 29, 2024
  • $30 \,N\,m$
  • $30 \sqrt{3} \,N \,m$
  • $60\, N\, m$
  • $60 \sqrt{3} \,N \,m$
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The Correct Option is A

Solution and Explanation

Given, $M=200 A-m^{2} B=0.30 NA ^{-1} M^{-1}$
and $\theta=30^{\circ}$
We know that the Torque,
$\tau = M \times B $
$ \Rightarrow |\tau| =M B \sin \theta=200 \times 0.3 \times \frac{1}{2} $
$=100 \times 0.3=30 N - m $
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Magnets are used in many devices like electric bells, telephones, radio, loudspeakers, motors, fans, screwdrivers, lifting heavy iron loads, super-fast trains, especially in foreign countries, refrigerators, etc.

Magnetite is the world’s first magnet. This is also called a natural magnet.  Though magnets occur naturally, we can also impart magnetic properties to a substance. It would be an artificial magnet in that case.

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Some of the properties of the magnetic field lines are:

  • The lines and continuous and outside the magnet, the field lines originate from the North pole and terminate at the South pole
  • They form closed loops traversing inside the magnet. 
  • But here the lines seem to originate from the South pole and terminate at the North pole to form closed loops.
  • More number of close lines indicate a stronger magnetic field
  • The lines do not intersect each other
  • The tangent drawn at the field line gives the direction of the field at that point.