Magnetism And Matter

							Science and Technology Workbook for Class X                                                                       9

                                 Chapter-5
                            MAGNETISM AND MATTER
Q.1 A magnet of magnetic moment M is rotated through 360° from it’s stable equilibrium position in a magnetic
    field B . What will be the work done?                                                                (2 marks)
1.  The required work done will be
                                 W = MB (cosθ1 – cosθ2 ) = MB (cos 0° – cos 360°) = 0
    Where θ1 = 0° is initial stable equilibrium position of the magnet and θ2 = 360° is the final displaced position
    of magnet.
Q.2 A bar magnet of magnetic moment 200 Am2 is suspended in a magnetic field of 0.25 T. What is the torque
    required to deflect it through 30° ?                                                                 (2 marks)
2.  In stable equilibrium position, the magnet is aligned in the magnetic field direction. The torque required to
    deflect the magnet through θ = 30° is equal but opposite to the magnetic torque on it.
                                  τ = MB sinθ = 200 × 0.25 × sin 30° = 25 Nm
Q.3 Name the materials with which the following hysteresis loops A and B are associated, respectively ? What
    do you conclude on the basis of the graph ?
                                                              A
                                                      I           B




                                                                  H




                                                                                                     (2 marks)
3.  The hysteresis loop A, being tall and narrow, is for the soft iron and shows the low retentivity, low
    coercivity and low hysteresis loss. The hysteresis loop B, being tall and wide, is for the steel and shows
    the high retentivity, high coercivity and high hysteresis loss.
Q.4 Draw a graph between susceptibility and temperature for a paramagnetic substance ?               (2 marks)

4.    χ




       O
                                T

      The magnetic susceptibility χ of a paramagnetic is positive and inversely proportional to the absolute
      temperature T, i.e.,
                                  1
                             χ∝
                                  T
10                                               Science and Technology Workbook for Class X

    and hence, the χ- versus T curve is a rectangular hyperbola.
Q.5 Why will a magnet attract an ordinary nail or paper clip but not a wooden pencil ?           (2 marks)
5.  An ordinary nail or paper clip contains magnetic domains that can be induced into alignment when a
    magnet is brought near them and hence they are attracted by the magnet. But a wooden pencil doesn’t
    have magnetic domains that can be induced into alignment. Therefore, it can’t be attracted by a magnet.