Topic 7: Magnetism and electromagnetism

Magnetism explains how some materials attract or repel without touching, while electromagnetism links electricity and magnetic fields. This topic builds from simple bar magnets up to the motors, generators and transformers that run the modern world. Several of the most demanding ideas, marked HT, deal with how movement and magnetism generate electricity.

Permanent and induced magnets

A magnet has two poles, north and south, where the magnetic effect is strongest. The rule of attraction is simple: like poles repel and unlike poles attract. These pushes and pulls are non-contact forces, meaning the magnets do not need to touch. A permanent magnet, such as a steel bar magnet, produces its own magnetic field all the time. An induced magnet is a material, usually iron, that becomes magnetic only when it is placed inside another magnetic field. Induced magnetism always causes attraction, never repulsion, which is why a magnet picks up iron nails. When an induced magnet is removed from the field it quickly loses most or all of its magnetism. The magnetic materials you must know are iron, steel, cobalt and nickel.

Magnetic fields

A magnetic field is the region around a magnet where another magnet or a magnetic material will feel a force. Field lines are drawn to show this field. By convention the lines always point from the north pole of a magnet to its south pole. The closer together the lines are, the stronger the field, so the field is strongest at the poles. A compass contains a tiny bar magnet that lines up with the field it sits in, so it can be used to plot field lines. Because a compass needle points roughly north when far from other magnets, this is evidence that the Earth itself has a magnetic field, generated by its molten iron core.

Fields around wires and solenoids

When a current flows through a straight wire it creates a magnetic field around it. The field lines form concentric circles centred on the wire. Reversing the current reverses the direction of the field, and increasing the current makes the field stronger. The field also gets weaker further from the wire. Shaping the wire into a coil called a solenoid concentrates the field. Inside a solenoid the field lines are parallel, evenly spaced and strong, producing a uniform field, while outside the solenoid the field looks just like that of a bar magnet. Adding an iron core turns the solenoid into an electromagnet, which is much stronger and can be switched on and off. This control makes electromagnets useful in devices such as scrapyard cranes, relays and electric bells.

The motor effect and Fleming's left-hand rule

When a current-carrying conductor is placed in a magnetic field, the field of the wire and the field of the magnet interact, exerting a force on the wire. This is the motor effect. The force is largest when the wire is at 90 degrees to the field. The size of the force is given by F = B I l, where F is the force in newtons, B is the magnetic flux density in tesla, I is the current in amperes and l is the length of wire in the field in metres. The direction of the force is found using Fleming's left-hand rule: point the First finger along the Field (N to S), the seCond finger along the Current (conventional, + to -), and the thuMb shows the Motion or force. Worked example: a wire of length 0.05 m carries a current of 3 A in a field of flux density 0.2 T. The force is F = B I l = 0.2 x 3 x 0.05 = 0.03 N.

Electric motors

An electric motor uses the motor effect to produce continuous rotation. A rectangular coil of wire sits between the poles of a magnet. Because current flows in opposite directions along the two sides of the coil, the motor effect pushes one side up and the other side down, making the coil spin. A device called a split-ring commutator swaps the connections to the coil every half turn. This reverses the current direction in the coil at just the right moment, so the forces keep turning the coil the same way instead of stopping. Larger forces, and so faster motors, come from a bigger current, a stronger magnetic field, or more turns on the coil.

HT: The generator effect

The generator effect is the reverse of the motor effect: moving a conductor through a magnetic field, or changing the field through a coil, induces a potential difference. If the conductor is part of a complete circuit, an induced current flows. The size of the induced p.d. is increased by moving the wire faster, using a stronger magnet, or adding more turns to the coil. An important rule is that the induced current always acts to oppose the change that produced it, so an induced current creates its own magnetic field that pushes back against the original movement. The direction of the induced current is found using Fleming's right-hand rule, used in the same way as the left-hand rule but with the right hand.

HT: Alternators and dynamos

Generators turn the generator effect into a useful supply. An alternator produces alternating current (a.c.). It uses slip rings and brushes so that, as the coil spins, the output reverses direction smoothly, giving a sine-shaped voltage that swaps polarity twice per turn. A dynamo produces direct current (d.c.). It uses a split-ring commutator, just like a motor, which flips the connections every half turn so the output always pushes the same way, giving a series of bumps that never go negative. On a graph the alternator output crosses zero and goes negative, while the dynamo output stays on one side of the axis.

HT: Loudspeakers, microphones and transformers

A loudspeaker uses the motor effect. An a.c. signal in a coil attached to a paper cone sits in a magnet's field, so the changing current makes the cone move in and out, pushing the air to create sound waves. A microphone works the opposite way using the generator effect: sound waves vibrate a diaphragm and a coil, inducing a changing current that copies the sound. A transformer changes the size of an a.c. voltage. It has a primary and a secondary coil wound on an iron core. The alternating current in the primary creates a changing magnetic field in the core, which induces a p.d. in the secondary. The transformer equation is Vp / Vs = np / ns, where V is voltage and n is the number of turns. For an ideal transformer, power in equals power out, so Vp x Ip = Vs x Is. Worked example: a transformer has 1000 turns on the primary and 50 on the secondary, with a primary voltage of 230 V. Then Vs = Vp x ns / np = 230 x 50 / 1000 = 11.5 V, a step-down transformer.