A current produces a magnetic field, and a changing magnetic field can induce a potential difference. If the conductor forms part of a complete circuit, that can produce a current. Motors depend on the first effect; generators and transformers depend on the second.
Every magnet has a north-seeking and a south-seeking pole. Like poles repel, unlike poles attract, and both are non-contact forces.
A permanent magnet produces its own field all the time. An induced magnet becomes magnetic only while it is in a magnetic field, and loses most of its magnetism when removed, and induced magnetism always causes attraction, never repulsion.
A magnetic field is the region around a magnet where another magnet or a magnetic material feels a force. It is strongest at the poles, and its strength falls with distance.
Field lines are drawn from north to south outside the magnet, and the closer together they are the stronger the field. A plotting compass shows the direction at a point, and can be used to demonstrate the direction of the Earth's own magnetic field. Evidence from geophysics supports the model that the Earth's field is generated by the motion of electrically conducting molten metal there, the geodynamo, not by a permanent magnet.
A current in a wire produces a magnetic field around it, in concentric circles whose direction is given by the right-hand rule and whose strength depends on the current and the distance from the wire.
Shaping the wire into a solenoid lines the field up so that it is strong and uniform inside, and looks like a bar magnet's field outside. Adding an iron core makes an electromagnet, which is far stronger and can be switched off.
A current-carrying conductor placed in a magnetic field experiences a force, provided the conductor is not parallel to the field. This is the motor effect.
For a conductor at right angles to the field the size of the force is \(F = BIl\), where \(B\) is the magnetic flux density in tesla; at any other angle that equation does not apply. The direction is given by Fleming's left-hand rule: first finger field, second finger current, thumb force, with the three held at right angles.
In a motor, a coil carrying a current sits in a magnetic field. The forces on the two sides of the coil act in opposite directions, so the coil turns.
A split-ring commutator reverses the current every half turn, so the force on each side always pushes the same way round and the rotation continues rather than reversing.
A loudspeaker uses the motor effect. Variations in an alternating current through a coil produce a varying force on it, which moves a cone back and forth and creates pressure variations in the air, sound.
Moving a magnet relative to a conductor, or changing the magnetic field through it, induces a potential difference across the conductor. If the conductor is part of a complete circuit, a current flows. This is the generator effect.
The induced potential is larger if the movement is faster, the magnet stronger or the coil has more turns. Reversing the direction of motion, or the poles of the magnet, reverses the induced potential. The induced current always creates a magnetic field opposing the change that produced it.
An alternator uses slip rings, so the connections never swap and the induced potential difference reverses every half turn, giving alternating current.
A dynamo uses a split-ring commutator instead, which swaps the connections every half turn so the output never reverses, giving direct current.
A moving-coil microphone is a loudspeaker in reverse. Pressure variations in a sound wave move a diaphragm attached to a coil in a magnetic field; the movement induces a varying current whose pattern matches the sound.
A transformer has two coils wound on an iron core. An alternating current in the primary produces a continually changing magnetic field in the core, which induces an alternating potential difference in the secondary.
It only works with alternating current: a steady direct current would give a steady field and induce nothing after the first instant.
\(\frac{V_p}{V_s} = \frac{N_p}{N_s}\). More turns on the secondary steps the potential difference up; fewer steps it down.
Treating a transformer as 100% efficient, \(V_p I_p = V_s I_s\), so stepping the potential difference up steps the current down in the same ratio.
The Grid therefore transmits at very high voltage. Power wasted heating the cables is \(P = I^2R\), so a ten-fold reduction in current cuts the wasted power by a factor of one hundred.
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