Electromagnetic induction is the reverse of the motor effect: instead of a current producing motion, motion produces a current. Whenever the magnetic field passing through a coil changes, a voltage is induced across the coil, and if the circuit is complete, a current flows.
A generator makes this happen continuously. A coil is spun between the poles of a magnet, so the amount of field threading through it rises and falls with each turn. This constant change induces an alternating voltage, and the faster the coil spins or the stronger the magnet, the larger the induced voltage.
This is how power stations generate electricity, whether the coil is turned by steam, flowing water at a dam, or the wind. A bicycle dynamo lighting a lamp is the same idea on a small scale.
In SPM you should explain induction as a changing magnetic field through a coil and relate the induced voltage to speed and field strength.
Common misconceptions
- A current is induced just by holding a coil near a magnet -> There must be a change in the field through the coil; a stationary coil in a steady field induces nothing.
- A generator makes electric charge from nothing -> It converts mechanical energy of the spinning coil into electrical energy; charge is not created.
- Induction and the motor effect are unrelated -> They are opposite processes: motors turn current into motion, generators turn motion into current.
The physics behind it
A generator relies on electromagnetic induction. Whenever the magnetic field passing through a coil changes, a voltage, called an induced electromotive force (e.m.f.), appears across the coil, measured in volts (V).
If the circuit is complete, this e.m.f. drives a current in amperes (A).
The amount of field threading a coil is the magnetic flux, and the induced e.m.f. is larger when that flux changes more quickly. In an a.c. generator a coil is spun steadily between the poles of a magnet, so the flux through it rises and falls with each half-turn, giving an alternating e.m.f.
Four things raise the induced e.m.f.: using more turns on the coil, spinning the coil faster, using a stronger magnet (greater magnetic flux density, in tesla, T), and using a larger coil area. The direction of the induced current always opposes the change producing it, which is Lenz’s law, a statement of energy conservation.
See it in daily life
Almost all the electricity you use starts with a spinning coil. In a power station the coil is turned by high-pressure steam, and at a hydroelectric dam it is turned by falling water.
In each case something forces the coil to rotate in a magnetic field, and induction does the rest, converting movement into electrical energy.
A small, clear example is a bicycle dynamo. As the wheel turns, it spins a small magnet near a coil, and the changing field induces a current that lights the lamp.
Pedal faster and the lamp glows brighter, because faster rotation means a faster change of flux and a larger induced e.m.f.
The same principle appears in a wind turbine, where moving air turns the blades, and even in the tiny generator that charges some hand-cranked torches. Wherever a conductor and a magnetic field move relative to each other, a voltage can be induced.
How this comes up in SPM
In Paper 2 you are often asked to explain how an e.m.f. is induced when a coil rotates in a magnetic field, and to state the factors that affect the size of the induced current. Recall these factors yourself, as there is no formula sheet.
Questions may ask you to describe how an a.c. generator produces an alternating output, or to sketch the shape of the output against time. You may also be asked to apply Lenz’s law to give the direction of an induced current, and to explain why the induced current opposes the motion.
This concept sits beside the motor effect and the transformer, so be ready to contrast a generator (motion producing current) with a motor (current producing motion), and to link induction to how electricity is generated and transmitted.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)