Page 112 - Physics_Form_2

Physics for Secondary Schools
The current is switched on for a short time
only because overheating can damage the
coil. Moreover, the bar does not become
more strongly magnetised if the current
is left on for a long time. The steel bar
attracts the iron filings, showing that it has
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been magnetised. In each of the methods
mentioned, if the object is composed of
ferromagnetic materials, it will remain
magnetised for a while after removing the
external field. However, if it is composed
of paramagnetic materials, it will lose
its magnetism immediately when the
magnetising field is switched off.
Demagnetisation
Demagnetisation is the process of
destroying the magnetic property of a
material.
The process involves the destruction of
the dipole alignment in the magnetised
material. This process can be done in the
following ways:
1. If a permanent magnet is heated or
vibrated in the absence of an external
field, the magnetisation can be
destroyed.
2. Randomly stroking one magnet with
another can demagnetise the magnet
being stroked.
3. Wrapping a wire coil around the
magnet and connecting the coil to
a source of alternating current will
eliminate the magnetic alignment.
4. Repeated hammering or dropping
the magnet in the absence of external
magnetic field can also distort dipoles
alignment.
Note; During heating, the dipoles
gain thermal energy and start to
106
Physics Form 2 Final.indd 106
vibrate randomly losing alignment.
Randomly stroking makes the
individual dipoles orient in different
directions. Alternating current
repeatedly realign the dipoles in
different directions making them settle
in random orientations. Hammering or
dropping induce physical force into
dipoles that disaligns them.
Activity 3.6
Aim: To demagnetise a bar
magnet using alternating
current and a rheostat
Materials: bar magnet, solenoid wire,
rheostat, iron filings, AC
power source
Procedure
1. Test a bar magnet for magnetism
using iron filings.
2. Set up an electric circuit by
connecting the solenoid wire to a low
voltage AC power source, ensuring
the rheostat is connected in series to
control the current flow
3. Adjust the rheostat to regulate the
current to a safe and effective level
for demagnetisation.
4. Place the solenoid with its axis
pointing in an east-west direction
as shown in Figure 3.24.
East
West
Figure 3.24
Student’s Book Form Two
25/10/2025 10:26