A changing magnetic field can induce a voltage in a wire and this causes current to flow. A potential difference can also be induced if a wire is moved in a magnetic field and, if this wire is in a circuit, this can allow for a current to flow. The size of the induced potential difference depends on the number of turns in a coil of wire, the strength of the magnetic field and on how fast the magnetic field changes or moves past the coil. Reversing the direction of the change reverses the directions of the induced p.d.
If the p.d. causes a current to flow, the magnetic field of the current opposes the original change
A generator consists of a coil of wire rotated inside a magnetic field, meaning its field lines are cut. As the coil turns, a voltage is induced in the wire. The ends of the coil are connected to slip rings which stop the coil getting tangled. Electrical contact with an external circuit is made using carbon brushes. The induced voltage is alternating as one side of the coil moves up through the magnet then down half a turn later, and so ac is produced.
Generators used in power stations and the alternators in cars use the same principles but have a rotating electromagnet surrounded by coils of wire
Dynamos produces direct current using a commutator. The commutator switches over the connections every half-turn of the coil, and so produces a form of direct current
Microphones convert the pressure variations in sound waves into variations in current in electrical circuits. The sound waves cause variations in air pressure which make a diaphragm vibrate. The diaphragm moves a coil of wire backwards and forwards along a permanent magnet and this coil is attached to connecting wires which are part of the circuit.
Loudspeakers convert variations in an electrical circuit into sound waves. Loudspeakers have similar components to a microphone. The varying current flows through a coil that is in a magnetic field, causing a force on the coil which moves backwards and forwards as the current varies. The coil is connected to a diaphragm, and the movements of the diaphragm produce sound waves
Electricity is transferred around the country by the national grid. The voltage is stepped up to 400kV to transport the electricity long distances because this steps down the current. Less current means less power is lost due to heating (P=I2R). Voltage is then stepped down to reduce the voltage before it reaches the destination. It is stepped down to 33kV for heavy industry or 11kV for light industry, while it is stepped down further to 230V for homes. To step the voltage up or down, a transformer is used in local sub-stations and at the power station.
Transformers are made using two coils of insulated wire wound onto a soft iron core. There is no electrical connection between the two coils of wire. A transformer can change the size of an alternating voltage
- Transformers only work with a.c., when the direction of the p.d., and also the current, changes many times per second
- The a.c. in the primary coil creates a continuously changing magnetic field, and the iron core of the transformer carries this magnetic field to the secondary coil
- The changing magnetic field induces a changing p.d. in the secondary coil
- The p.d. is greater in the secondary coil is there are more turns than in the primary coil
Use the equation where V = Voltage and N = Number of turns in coil
Energy is neither created nor destroyed, so, assuming that the transformer is 100% efficient, the power supplied to the primary coil must be equal to the power transferred away from the secondary coil.
Use the equation 
where V = Voltage (V) and I = Current (A)
You can carry out calculations to explain why transmitting power at high voltages is more efficient.
- Current required = Power supplied ÷ Voltage of supply
- Power Transferred by heating in the wires to the substation = (Current in i)2 × Resistance of wire
- Energy Transferred per unit of time = (Power in ii) × Time (in s but for unit specified)