Magnetism

Permanent and Induced Magnets

Permanent magnets are objects that produce their own persistent magnetic field. They have magnetic domains 6tiny regions where groups of atoms have aligned magnetic poles that remain aligned even without an external magnetic field. Common examples include bar magnets and fridge magnets.

Induced magnets are materials that become magnetic only when placed in a magnetic field. When near a permanent magnet, their magnetic domains align temporarily, creating a magnetic effect. Once removed from the magnetic field, they usually lose their magnetism.

Materials that can be magnetised (turned into magnets) are usually ferromagnetic materials such as iron, cobalt, and nickel. These materials have domains that can be aligned by an external magnetic field.

Magnetisation is the process of aligning the magnetic domains within a material, turning it into a magnet. This can happen naturally (in permanent magnets) or by placing a material in a magnetic field (induced magnetism).

Demagnetisation occurs when the magnetic domains become misaligned again, causing the material to lose its magnetism. This can happen by heating the magnet, hitting it, or placing it in an opposing magnetic field.

For example, if you stroke a piece of iron with a permanent magnet, the iron can become an induced magnet. When you remove the permanent magnet, the iron loses most or all of its magnetism.

Magnetic Fields

A magnetic field is the region around a magnet where magnetic forces can be detected. Magnetic fields are represented by magnetic field lines, which:

• Show the direction of the magnetic force (from north to south outside the magnet)
• Are closer together where the field is stronger
• Never cross each other

Around a bar magnet, the field lines curve from the north pole to the south pole. The field is strongest at the poles where the lines are closest.

A current-carrying wire also produces a magnetic field. The field lines form concentric circles around the wire. The direction of the field depends on the direction of the current and can be found using the right-hand grip rule (thumb points in current direction, fingers curl in field direction).

The Earth 27s magnetic field acts like a giant bar magnet tilted slightly from the planet 27s axis. It causes compass needles to point north and protects the Earth from solar wind.

For example, the magnetic field around a bar magnet looks like this:

Field lines emerge from the north pole, curve around the magnet, and enter the south pole, showing the direction a north pole would move.

Magnetic Forces

Magnets exert forces on each other and on magnetic materials. These forces can be:

• Attractive between opposite poles (north attracts south)
• Repulsive between like poles (north repels north, south repels south)

Magnetic materials placed in a magnetic field experience a force that pulls them into the field. This is why iron filings gather around the poles of a magnet.

A current-carrying wire in a magnetic field experiences a force. The direction of this force depends on the current direction and the magnetic field direction. This principle is used in devices like loudspeakers and electric motors (see The Motor Effect topic for more).

Applications of magnetic forces include:

• Magnetic separation to remove magnetic materials from mixtures
• Electric motors where forces on current-carrying wires cause rotation
• Loudspeakers where magnetic forces move a cone to produce sound

For example, two bar magnets placed close together will either attract or repel depending on the poles facing each other.