Edexcel GCSE Physics (1PH0) — Electromagnetic induction
Every time you turn on a light at home, you are relying on giant magnets spinning inside power stations. Electromagnetic induction is the production of a potential difference across a conductor when it moves relative to a magnetic field. This phenomenon is known as the generator effect.
For induction to happen, the moving conductor must literally "cut" through the magnetic field lines. If the conductor moves exactly parallel to the magnetic field, no lines are cut, and the induced potential difference is . The maximum potential difference is induced when the conductor moves exactly perpendicular () to the field lines, as this gives the highest rate of cutting.
It is important to note that moving a wire in a magnetic field only induces a potential difference. An electric current will only actually flow if the conductor is part of a complete closed circuit.
Understanding how we generate electricity explains why the mains supply in our homes constantly changes direction. An alternator generates electricity by rotating a coil of wire inside a fixed magnetic field. As the coil spins, its sides move up through the field and then down through the field, causing the induced potential difference to reverse direction every (every half-turn).
To transfer this electricity to an external circuit, the ends of the coil are connected to two continuous metal slip rings. These rings rotate alongside the coil while rubbing against fixed carbon brushes.
Because each slip ring is permanently connected to one specific end of the spinning coil, the reversing potential difference is passed directly into the circuit. This produces an alternating current (a.c.), where the current continuously changes direction.
While power stations use alternators for the national grid, some smaller devices require a steady, one-way flow of electricity. A dynamo is a generator that also rotates a coil to cut magnetic field lines, but it produces a unidirectional output. Instead of continuous rings, it uses a split-ring commutator.
This commutator acts as a mechanical rotary switch. It is a single metal ring split into two halves that reverses the connections between the spinning coil and the external circuit every half-turn. This reversal is timed to happen exactly when the potential difference drops to zero.
By swapping the external contacts every , the split-ring commutator ensures the current in the external circuit always flows in the same direction. This produces a direct current (d.c.) output.
How can a generator be modified to produce a larger electrical output? The size of the induced potential difference depends entirely on the rate at which magnetic field lines are cut.
You can increase the output magnitude by:
Even when a generator is spinning at a constant speed, its electrical output drops to zero twice every single rotation. When the rotating coil is perfectly horizontal, its sides move perpendicular to the magnetic field. This provides the maximum rate of cutting, generating a peak potential difference.
Conversely, when the coil reaches a vertical position, its sides move parallel to the magnetic field. During this brief moment, no field lines are cut, resulting in zero potential difference.
The output graph of an alternator forms a sinusoidal wave that alternates between positive and negative voltage values. The output graph of a dynamo forms a series of positive peaks (bumps) that never drop below the zero-volt line. If the speed of rotation is doubled, the peak potential difference doubles in height, and the frequency also doubles, making the peaks twice as close together on the time axis.
Students often state that a generator 'creates a current'. You must say it 'induces a potential difference', as current only flows if the circuit is complete.
In 6-mark explanation questions, Edexcel examiners specifically look for the phrase 'cutting magnetic field lines' — do not just say the coil 'moves in a magnetic field'.
Students think the induction inside a dynamo coil is d.c. The induction inside the coil is actually a.c., but the split-ring commutator reverses the external connections to convert it to a d.c. output.
If you are asked to sketch a graph for a generator rotating at twice the speed, you must draw the wave with peaks twice as high AND twice as close together on the x-axis.
Electromagnetic induction
The production of a potential difference across a conductor when it moves through a magnetic field and cuts magnetic field lines.
Generator effect
The use of relative motion between a conductor and a magnetic field to induce a potential difference and generate electricity.
Magnetic field lines
Imaginary lines showing the strength and direction of a magnetic field, which must be cut by a conductor to induce a potential difference.
Alternator
A type of generator that uses continuous slip rings to produce an alternating current (a.c.).
Slip rings
Circular metal conductors that provide a permanent, continuous connection between a rotating coil and a fixed external circuit.
Alternating current (a.c.)
An electric current that continuously changes its direction of flow.
Dynamo
A type of generator that uses a split-ring commutator to produce a direct current (d.c.).
Split-ring commutator
A metal ring divided into two halves that reverses the circuit connections every half-turn to maintain a unidirectional current.
Direct current (d.c.)
An electric current that always flows in the same direction.
Put your knowledge into practice — try past paper questions for Physics
Electromagnetic induction
The production of a potential difference across a conductor when it moves through a magnetic field and cuts magnetic field lines.
Generator effect
The use of relative motion between a conductor and a magnetic field to induce a potential difference and generate electricity.
Magnetic field lines
Imaginary lines showing the strength and direction of a magnetic field, which must be cut by a conductor to induce a potential difference.
Alternator
A type of generator that uses continuous slip rings to produce an alternating current (a.c.).
Slip rings
Circular metal conductors that provide a permanent, continuous connection between a rotating coil and a fixed external circuit.
Alternating current (a.c.)
An electric current that continuously changes its direction of flow.
Dynamo
A type of generator that uses a split-ring commutator to produce a direct current (d.c.).
Split-ring commutator
A metal ring divided into two halves that reverses the circuit connections every half-turn to maintain a unidirectional current.
Direct current (d.c.)
An electric current that always flows in the same direction.
