Electromagnetic Induction and Transformers
Electromagnetic Induction
When a conductor moves through a magnetic field, or when a magnetic field changes around a conductor, a potential difference (voltage) is induced across the conductor. If the conductor is part of a complete circuit, a current flows. This process is called electromagnetic induction.
The induced potential difference can be increased by: moving the magnet (or conductor) faster, using a stronger magnet, or increasing the number of turns on the coil. Reversing the direction of movement or reversing the polarity of the magnet reverses the direction of the induced potential difference and current.
A potential difference is only induced while the magnetic field is changing. If the magnet is held stationary inside the coil, no potential difference is induced — there must be relative movement between the magnet and the conductor.
Fleming’s left-hand rule is used for the motor effect — when a current-carrying conductor is placed in a magnetic field and experiences a force. The thumb represents the direction of force (motion), the first finger represents the magnetic field, and the second finger represents the current.
Electromagnetic induction is essentially the reverse of the motor effect. Instead of a current causing motion, motion (or a changing field) causes a current. Generators work on this principle — a coil rotates in a magnetic field, inducing a continuously changing potential difference.
Generators: AC and DC
An alternator (AC generator) produces alternating current. A coil rotates between the poles of a magnet, and the induced potential difference reverses direction every half turn. This produces a sinusoidal (wave-shaped) output. The coil is connected to the external circuit via slip rings and brushes, which allow continuous rotation.
A dynamo (DC generator) uses a split-ring commutator instead of slip rings. The commutator swaps the connections every half turn, so the current always flows in the same direction in the external circuit, though it still varies in magnitude.
Increasing the speed of rotation increases both the peak voltage and the frequency of the output from a generator. A stronger magnet, or more turns on the coil, raises the peak voltage but does not change the frequency.
Transformers
A transformer changes the voltage of an alternating current (AC) supply. It consists of two coils of wire (the primary and secondary) wrapped around a soft iron core. When AC flows through the primary coil, it creates a changing magnetic field in the iron core. This changing field induces a potential difference in the secondary coil.
The transformer equation relates the voltages and number of turns:
$$\frac{V_p}{V_s} = \frac{N_p}{N_s}$$
where \(V_p\) and \(V_s\) are the primary and secondary voltages, and \(N_p\) and \(N_s\) are the number of turns on the primary and secondary coils.
A step-up transformer has more turns on the secondary coil than the primary, so it increases the voltage. A step-down transformer has fewer turns on the secondary, so it decreases the voltage. In an ideal (100% efficient) transformer, electrical power is conserved:
$$V_p \times I_p = V_s \times I_s$$
This means that if the voltage is stepped up, the current is stepped down by the same factor, and vice versa. Real transformers are not perfectly efficient because some energy is lost as heat in the coils (due to resistance) and in the core (due to eddy currents and magnetisation/demagnetisation losses).
The National Grid
The National Grid transmits electricity from power stations to consumers across the country. Electricity is transmitted at very high voltages (up to 400,000 V) and correspondingly low currents. Since power lost as heat in the cables is given by \(P = I^2 R\), transmitting at low current greatly reduces energy losses.
Step-up transformers at power stations increase the voltage for efficient long-distance transmission. Step-down transformers near homes and businesses reduce the voltage to safe, usable levels (230 V for domestic supply in the UK).
Transformers require a continuously changing magnetic field to induce a voltage in the secondary coil. Direct current (DC) produces a constant magnetic field (after the initial switch-on), so no voltage is induced in the secondary coil. Alternating current continuously reverses direction, creating the constantly changing magnetic field needed for electromagnetic induction. This is a key reason why AC is used for mains electricity distribution.