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A magnetised needle in a uniform magneti...

A magnetised needle in a uniform magnetic field experiences a torque but no net force. An iron nail near a bar magnet, however, experiences a force of attraction in addition to a torque. Why ?

Text Solution

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No force if the field is uniform. The iron nail experiences a non-uniform field due to the bar magnet. There is induced magnetic moment in the nail, therefore, it experiences both force and torque. Then net force is attractive because the induced south pole (say) in the nail is closer to the north pole of magnet than induced north pole.
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Knowledge Check

  • A bar magnet placed in a uniform magnetic field making an angle theta with the field experiences a torque. If the angle made by the magnet with the field is doubled, the torque experienced by the magnet increases by 41.4% . The initial angle made by the magnet with the magnetic field is

    A
    `60^(@)`
    B
    `30^(@)`
    C
    `90^(@)`
    D
    `45^(@)`

  • The magnetic force on a current carrying wire placed in uniform magnetic field if the wire is oriented perpendicular to magnetic field is

    A
    0
    B
    ILB
    C
    2ILB
    D
    `ILB//2`

  • A magnetic needle lying parallel to a magnetic field is turned through 60^(@) . The work done on it is W. The torque required to maintain the magnetic needle in the position mentioned above is

    A
    `sqrt3w`
    B
    `(sqrt3)/(2)W`
    C
    `(W)/(2)`
    D
    `2w`

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