Question:

A short bar magnet is placed in the magnetic meridian of the earth with north pole pointing north. Neutral points are found at a distance of $30 \,cm$ from the magnet on the East - West line, drawn through the middle point of the magnet. The magnetic moment of the magnet in $Am^2$ is close to : (Given $\frac{\mu_{0}}{4\pi}=10^{-7}$ in SI units and $B_H$ = Horizontal component of earth's magnetic field = $3.6 \times 10^{-5}$ Tesla.)

Updated On: Aug 17, 2024
  • 9.7
  • 4.9
  • 19.4
  • 14.6
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The Correct Option is A

Solution and Explanation

$|\bar{B}|$ at an equatorial point
$=\frac{\mu_{o}}{4 \pi} \frac{|\bar{m}|}{r^{3}}=B_{ H }$
$\Rightarrow|\bar{m}|=\frac{B_{ H } r^{3}}{10^{-7}}$
$=\frac{3.6 \times 10^{-5} \times(0.3)^{3}}{10^{-7}}= 9.7$
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Magnetism & Matter

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.