Magnets, Electromagnetism and the Motor Effect
Permanent and Induced Magnets
A permanent magnet produces its own magnetic field. It always has a north pole and a south pole. Like poles repel; unlike poles attract. The magnetic materials are iron, steel, cobalt and nickel. Permanent magnets are made from steel or magnetic alloys, because soft iron loses its magnetism as soon as it leaves a field.
An induced magnet is a material that becomes magnetic only when placed inside a magnetic field. When removed from the field, it quickly loses its magnetism. An induced magnet is always attracted to the permanent magnet, never repelled.
A magnetic field is the region around a magnet where a force acts on another magnet or magnetic material. Field lines go from north to south outside the magnet. The closer together the field lines, the stronger the field.
Place a compass near a bar magnet. Mark the position of the compass needle. Move the compass so the tail of the needle is at the mark you just made. Repeat to trace a complete field line. Alternatively, place the magnet under a sheet of paper and sprinkle iron filings on top — they align with the field lines.
Electromagnetism
When a current flows through a wire, it produces a magnetic field around the wire. The field lines form concentric circles around the wire. The direction of the field can be found using the right-hand rule: point your thumb in the direction of current, and your fingers curl in the direction of the field.
A solenoid is a coil of wire. When current flows through it, the magnetic field inside is strong and uniform (like a bar magnet). The strength of the field can be increased by: increasing the current, increasing the number of turns, or placing an iron core inside the solenoid.
An electromagnet is a solenoid with a soft iron core. It can be switched on and off by controlling the current. This makes electromagnets useful in scrapyard cranes, circuit breakers, electric bells, and MRI scanners.
The Motor Effect
When a current-carrying wire is placed in a magnetic field, it experiences a force. This is called the motor effect. The force is at its maximum when the wire is perpendicular to the magnetic field and zero when the wire is parallel to the field.
The size of the force is given by:
$$F = BIl$$
Where \(F\) is force in newtons, \(B\) is magnetic flux density in tesla (T), \(I\) is current in amps, and \(l\) is the length of wire in the field in metres.
Fleming’s left-hand rule predicts the direction of the force. Hold your left hand with the thumb, first finger, and second finger all at right angles. The first finger points in the direction of the magnetic field (north to south), the second finger points in the direction of conventional current, and the thumb shows the direction of the force (motion).
Electric Motors
A simple d.c. motor uses the motor effect to produce rotation. A coil of wire is placed between the poles of a magnet. When current flows, one side of the coil is pushed up and the other is pushed down (by the motor effect), creating a turning force.
A split-ring commutator reverses the direction of current in the coil every half turn. Without it, the coil would rotate 180° and then stop. The commutator ensures the forces always act in the right direction to keep the coil spinning continuously.
The speed of a motor can be increased by: increasing the current, using a stronger magnet, or adding more turns to the coil.