Magnetism and Matter

Magnetism and Matter

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  1. A bar magnet, of magnetic moment M', is placed in a magnetic field of induction B'. The torque exerted on it is​​








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    ​We know that when a bar magnet is placed in the magnetic field at an angle θ, then torque acting on the bar magnet (τ) ​= MB sin θ = M→ × B→. ​
    Note : This torque τ has a tendency to make the axis of the magnet parallel to the direction of the magnetic field.

    Correct Option: C

    ​We know that when a bar magnet is placed in the magnetic field at an angle θ, then torque acting on the bar magnet (τ) ​= MB sin θ = M→ × B→. ​
    Note : This torque τ has a tendency to make the axis of the magnet parallel to the direction of the magnetic field.

  1. Current i is flowing in a coil of area A and number of turns N, then magnetic moment of the coil, M is








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    Magnetic moment linked with one turn = iA ​Magnetic moment linked with N turns = iNA amp-m2.  Here, A = Area of current loop.

    Correct Option: A

    Magnetic moment linked with one turn = iA ​Magnetic moment linked with N turns = iNA amp-m2.  Here, A = Area of current loop.

  1. A magnetic needle suspended parallel to a magnetic field requires J of work to turn it through 60°. The torque needed to maintain the needle in this position will be :








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    According to work energy theorem ​W =Ufinal – Uinitial = MB (cos 0 – cos 60°)

    W =
    MB
    		= √3J    ...(i)
    2

    τ = M→ × B→ = MBsin60° =
    MB√3
    		= √3J    ...(ii)
    2

    From equation (i) and (ii)
    τ =
    2√3 × √3
    		= 3J
    2

    Correct Option: B

    According to work energy theorem ​W =Ufinal – Uinitial = MB (cos 0 – cos 60°)

    W =
    MB
    		= √3J    ...(i)
    2

    τ = M→ × B→ = MBsin60° =
    MB√3
    		= √3J    ...(ii)
    2

    From equation (i) and (ii)
    τ =
    2√3 × √3
    		= 3J
    2

  1. A bar magnet of length ‘l’ and magnetic dipole moment ‘M’ is bent in the form of an arc as shown in figure. The new magnetic dipole moment will be​​








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    Magnetic dipole moment
    ​M = m × l 
    M' = m × r ​
    From figure

    l =πror r = 3l
    3π

    so. M' = m × r =m × 3l= 3 M
    ππ

    Correct Option: A

    Magnetic dipole moment
    ​M = m × l 
    M' = m × r ​
    From figure

    l =πror r = 3l
    3π

    so. M' = m × r =m × 3l= 3 M
    ππ

  1. A bar magnet of magnetic moment M is placed at right angles to a magnetic induction B. If a force F is experienced by each pole of the magnet, the length of the magnet will be








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    FL = MB (= Torque) ⇒ L =
    MB
    F

    Correct Option: B

    FL = MB (= Torque) ⇒ L =
    MB
    F