Magnetism & Electromagnets
Magnetic Poles and Materials
Every magnet has two ends called magnetic poles. One is a north-seeking pole (north pole) and the other is a south-seeking pole (south pole). The fundamental rule of magnetism is simple: unlike poles attract and like poles repel. Two north poles push each other away, while a north and a south pole pull towards each other.
Only certain materials are magnetic. The four you need to know are iron, steel, cobalt, and nickel. These are called magnetic materials. Non-magnetic materials such as wood, plastic, copper, and aluminium are not attracted to magnets and cannot be magnetised.
Unlike poles attract, like poles repel. The four magnetic materials are iron, steel, cobalt, and nickel.
Permanent and Induced Magnets
A permanent magnet produces its own magnetic field all the time. Bar magnets and horseshoe magnets are common examples. Once magnetised, they keep their magnetism unless heated strongly or struck repeatedly.
An induced magnet is a material that becomes magnetic only when it is placed in a magnetic field. When a piece of iron is brought close to a permanent magnet, the iron becomes an induced magnet — it is attracted to the permanent magnet. However, when the permanent magnet is removed, the iron loses most of its magnetism. This makes iron a soft magnetic material.
Steel, by contrast, is a hard magnetic material — it is more difficult to magnetise than iron, but once magnetised it keeps its magnetism. This is why permanent magnets are often made from steel or special alloys.
Permanent magnets are used in compasses, fridge magnets, loudspeakers, electric motors, and generators. They are chosen when a constant magnetic field is needed without an electricity supply.
Temporary (induced) magnets are used in electromagnets, scrapyard cranes, electric bells, and magnetic door catches. They are ideal when the magnetism needs to be switched on and off.
Magnetic Fields
A magnetic field is the region around a magnet where a force acts on another magnet or on a magnetic material. The field is represented using field lines. These lines always point from the north pole to the south pole outside the magnet. Where the lines are close together, the field is strong. Where the lines are far apart, the field is weak.
Around a bar magnet, the field lines curve outwards from the north pole and loop back into the south pole. A uniform field has parallel, equally spaced field lines — for example, the field between two flat, opposite poles placed close together.
Magnetic field lines go from north to south outside the magnet. Closer lines mean a stronger field; wider spacing means a weaker field.
Plotting Compasses and the Earth’s Magnetic Field
A plotting compass is a small compass that can be placed around a magnet to map out the shape and direction of the field. The compass needle always points along the field line at that position, from north to south.
The Earth itself behaves as if it has a giant bar magnet inside it. The geographic North Pole is actually near a magnetic south pole — this is why the north-seeking end of a compass needle points towards geographic north. The fact that compass needles point north everywhere on Earth is evidence that the Earth’s core must be magnetic.
A compass needle is a small bar magnet that is free to rotate. It aligns itself with the Earth’s magnetic field. Because the Earth’s magnetic south pole is near the geographic North Pole, the north-seeking pole of the compass is attracted towards it. This behaviour — consistent at every location on the planet — is strong evidence that the Earth’s iron-nickel core is magnetic.
Electromagnetism: Current and Magnetic Fields
When an electric current flows through a conductor, it creates a magnetic field around the wire. This can be demonstrated by placing plotting compasses around a wire carrying a current — the compasses will deflect, showing circular field lines around the wire.
The magnetic field around a long straight conductor forms concentric circles centred on the wire. The direction of the field depends on the direction of the current. You can use the right-hand grip rule: point your right thumb in the direction of the current, and your fingers curl in the direction of the magnetic field.
The strength of this field depends on two factors: the size of the current (a larger current produces a stronger field) and the distance from the conductor (the field is strongest close to the wire and gets weaker further away).
A current-carrying wire produces circular magnetic field lines around it. The field strength increases with a larger current and decreases with greater distance from the wire.
Solenoids and Electromagnets
A solenoid is a long coil of wire. When a current passes through a solenoid, each turn of the coil produces its own circular magnetic field. Inside the solenoid, these individual fields add together to create a very strong, almost uniform magnetic field along the centre. Outside the solenoid, the fields partially cancel each other out, producing a much weaker field.
The field pattern of a solenoid is very similar to that of a bar magnet — it has a north pole at one end and a south pole at the other. The strength of the solenoid’s magnetic field can be increased by:
- Increasing the current flowing through the coil
- Adding more turns of wire to the coil
- Placing an iron core inside the solenoid
An electromagnet is a solenoid with a soft iron core. The iron core becomes an induced magnet when the current flows, greatly strengthening the field. When the current is switched off, the iron loses its magnetism almost immediately, making electromagnets very useful for applications where the magnetic field needs to be controlled — such as scrapyard cranes, electric bells, circuit breakers, and MRI scanners.
Inside a solenoid, the fields from individual coils add together to form a strong, nearly uniform field. Outside, they cancel to give a weaker field. Adding an iron core creates an electromagnet.