2. Outline
1- What is magnetism?
2- Types of magnet
3- Theories of magnetism
4- Weber’s theory of magnetism
5- Main points of Weber’s theory
6- Explanation of Weber’s theory
3. What is magnetism?
• It the force of attraction or repulsion
due to electron arrangement.
• Magnetic forces are strongest at the
poles.
• Magnets have two poles: North and
South.
• When free to rotate, to come to rest
pointing in a north south direction.
• Like magnetic poles repel.
• Unlike magnetic poles attract.
4. Types of magnet
• Natural magnet: known as lodestone
which is a type of iron ore with magnetic
properties.
• Artificial magnet: materials in which
magnetic properties are produces
artificially such as iron, cobalt, nickel
steel.
• Temporary magnet: that loses their
magnet properties easily and quickly
such as soft iron.
5. Types of magnet
• Permanent magnet: which retain their
properties. More longer and difficult to
magnetize such as steel.
• Electromagnet: an electric current
produces magnetic effects and a coil of
wire carrying a current act as an
electromagnet.
7. Weber’s theory of magnetism
This theory was given by Weber in 1852. It
was further modified by Ewing in 1890.
8. The basic assumption of this theory is that
molecules of all substances are
inherently magnets in themselves, each
having a N and S pole. In unmagnetised
state, it is supposed that these smaller
molecular magnets lie in all sorts of
haphazard manner forming more or less
closed loop.
9. Main points of theory
1- Each and every molecule of a magnetic
substance is a complete magnet in itself, having a
north pole and a south pole of equal strength.
2- In an unmagnetised substance, the molecular
magnets are randomly oriented such that they form
closed chains.
3- When the substance is magnetized, the molecular
magnets are realigned so that north poles of all
molecular magnets point in one direction and south
poles of all molecular magnets point in the opposite
direction.
10. 4- When all the molecular magnets are fully aligned,
the substance is said to be saturated with
magnetism.
11. Explanation of Weber’s theory
A magnetized material will have most of its
molecular magnets lined up so that the north
pole of each molecule points in one direction,
and south pole faces the opposite direction.
A material with its molecules thus aligned will
then have one effective north pole, and one
effective south pole, where a steel bar is
magnetized by stroking.
12. When a steel bar is stroked several times in
the same direction by the magnet, the
magnetic force from the north pole causes
the molecules to align themselves.
During heating the magnetized specimen,
molecular magnets acquire some kinetic
energy. Some of the molecules may get back
to the closed chain arrangement. That is why
magnetism of the specimen would reduce on
heating.
