Magnetism, Motors and Generators
Magnetic Fields
A magnetic field is the region around a magnet where a force acts on another magnetic material. The field is strongest at the poles and weakens with increasing distance. We represent magnetic fields with field lines — these lines run from the north pole to the south pole outside the magnet, and the closer together they are, the stronger the field.
Permanent magnets keep their magnetism at all times. Magnets made from iron, nickel, cobalt or steel can all attract magnetic materials. Induced magnets are materials that become temporarily magnetic when placed in an external field. Unlike permanent magnets, induced magnets are always attracted to the source of the field — they are never repelled.
Magnetic field lines run from north pole to south pole outside the magnet. The closer the lines, the stronger the field. Like poles repel; unlike poles attract. The Earth itself behaves like a giant magnet — magnetic compasses align because the Earth has a magnetic core.
Electromagnetism
When an electric current flows through a wire, it creates a magnetic field around the wire. The field lines form concentric circles around the conductor. Reversing the current reverses the direction of the field.
Winding the wire into a solenoid greatly strengthens the magnetic field because the fields of each turn add together inside the coil. The field pattern of a solenoid resembles that of a bar magnet, with a north end and a south end that depend on the direction of current flow. Placing a soft iron core inside the solenoid further increases the field strength, creating an electromagnet.
Electromagnets can be switched on and off, making them far more versatile than permanent magnets. The development of electromagnets during the 19th century had a transformative impact on society — enabling the electric telegraph (revolutionising long-distance communication), the electric bell, the relay switch, MRI scanners in hospitals, and cranes in scrapyards that lift and release steel on demand. This is a clear example of how a scientific discovery leads to technological changes that improve people's lives.
In loudspeakers and headphones, a current-carrying coil is placed inside the field of a permanent magnet. The solenoid generates a magnetic field that interacts with the permanent magnet's field; varying the current in the coil causes the coil to vibrate at the corresponding frequency, pushing a cone to create sound waves that match the original electrical signal.
The Motor Effect
When a current-carrying conductor is placed inside a magnetic field, the conductor experiences a force — this is called the motor effect. The interaction between the magnetic field of the conductor and the external magnetic field creates a force on the wire. The force is greatest when the current is perpendicular to the field, and zero when they are parallel. Reversing either the current direction or the field direction reverses the force direction.
Fleming's left-hand rule: hold the left hand with the thumb, first finger and second finger at right angles to each other. The First finger points in the direction of the Field (north to south); the seCond finger points in the direction of Conventional current; the thuMb shows the direction of Motion (force). This rule shows the relative orientations of the force, the conductor and the magnetic field.
The size of the force can be calculated using:
$$F = BIl$$
where \(F\) is force in newtons (N), \(B\) is magnetic flux density in tesla (T), \(I\) is current in amps (A), and \(l\) is the length of conductor in the field in metres (m).
A wire of length 0.25 m carries a current of 4 A perpendicular to a magnetic field of flux density 0.30 T. Calculate the force on the wire.
\(F = BIl = 0.30 \times 4 \times 0.25 = 0.30\) N
The direction of the force would be found using Fleming's left-hand rule — the thumb indicates which way the wire is pushed.
The Electric Motor
A simple electric motor uses the motor effect to produce continuous rotation. A rectangular coil of wire is placed between the poles of a magnet. When current flows, the two sides of the coil that lie at right angles to the magnetic field experience opposite forces — one side is pushed up, the other down — creating a turning effect. As the coil rotates, this continuous force produces sustained rotation, converting electrical energy into kinetic energy.
Electromagnetic Induction and Generators
If a conducting wire moves through a magnetic field — or if the magnetic field around a stationary wire changes — a potential difference is induced across the wire's ends. This is electromagnetic induction. If the circuit is complete, a current will flow.
The direction of the induced potential difference is such that it drives a current which would create a magnetic field opposing the original change — this is a consequence of energy conservation.
In a generator, a magnet or electromagnet is rotated inside a coil of wire. As the magnet rotates, the magnetic field through the coil continuously changes, inducing a potential difference. This produces an alternating current (a.c.) — the voltage reverses direction every half turn, so one revolution of the magnet gives one complete cycle.
A transformer works by induction: alternating current in the primary coil creates a changing magnetic field in the iron core, which induces an alternating potential difference in the secondary coil. The ratio of voltages equals the ratio of turns:
$$\frac{V_p}{V_s} = \frac{N_p}{N_s}$$
Energy conservation means \(V_p I_p = V_s I_s\) — stepping up voltage reduces current, cutting heat losses in transmission cables (since \(P_{\text{loss}} = I^2 R\)). This discovery fundamentally changed how humanity generates and distributes power.