Magnetic Fields

Magnetic Fields Basics

A magnetic field is the region around a magnet where magnetic forces can be felt. It is an invisible field that exerts a force on magnetic materials and moving charges.

Magnetic fields are represented by field lines. These lines:

• Show the direction of the magnetic force: they go from the north pole to the south pole outside the magnet.
• Are closer together where the magnetic field is stronger.
• Never cross each other.

Around a bar magnet, the magnetic field lines form closed loops from the north pole to the south pole outside the magnet, and through the magnet from south to north inside it.

The field is strongest at the poles where the lines are closest together.

For instance, if you sprinkle iron filings around a bar magnet, the filings align along the magnetic field lines, showing the pattern clearly.

Permanent & Induced Magnets

Permanent magnets produce their own persistent magnetic field. They are made from materials like steel or alloys that keep their magnetic properties for a long time.

Inside permanent magnets, tiny regions called magnetic domains are groups of atoms with magnetic moments aligned in the same direction. In a permanent magnet, most domains are aligned, producing a strong overall magnetic field.

Induced magnets are materials that become magnetic only when placed in a magnetic field. For example, soft iron is not a permanent magnet but becomes magnetised when near a permanent magnet.

When an induced magnet is removed from the magnetic field, it usually loses most or all of its magnetism because its domains return to random orientations.

The process of magnetism in induced magnets happens because the external magnetic field causes the magnetic domains inside the material to align temporarily.

Magnetic Forces

Magnetic materials experience a force when placed in a magnetic field. For example, a piece of iron is attracted towards a magnet because the magnet induces magnetism in the iron, causing attraction.

A current-carrying wire in a magnetic field also experiences a force. This force is the basis for many electric motors and devices.

The direction of the force on a current-carrying wire can be found using Fleming's left-hand rule:

• First finger points in the direction of the magnetic field (from north to south).
• Second finger points in the direction of the current (positive to negative).
• Thumb points in the direction of the force on the wire.

This rule helps predict the motion of wires in motors and other devices where magnetic forces act on currents.

For example, if a wire carrying current flows upwards and is placed in a magnetic field directed from left to right, Fleming's left-hand rule shows the force acts either towards or away from you depending on the current and field directions.

Magnetic Field Patterns

A magnetic field is also created around a current-carrying straight wire. The field lines form concentric circles around the wire, with the direction given by the right-hand grip rule (thumb in direction of current, fingers curl in direction of magnetic field).

Inside a solenoid (a coil of wire), the magnetic field lines are nearly parallel and close together inside, creating a strong, uniform magnetic field similar to that of a bar magnet. Outside the solenoid, the field is weaker and spreads out.

The Earth itself acts like a giant magnet with a magnetic field similar to a bar magnet. The Earth's magnetic field lines run from the magnetic south pole near the geographic north to the magnetic north pole near the geographic south.

This magnetic field protects the Earth from solar wind and helps compasses point north.

For example, the magnetic field around a straight wire carrying current upwards can be visualised by placing a compass near the wire; the compass needle will point tangentially to the circular field lines.