Generators

Overview

This lesson introduces electromagnetic induction, which is one of the most important ideas in the topic. The key pattern is simple: if the magnetic field linking a conductor changes, an e.m.f. is induced. A generator is then just a device that keeps creating that change again and again.

What You Need to Know

• An induced e.m.f. is produced when a conductor moves across a magnetic field or when the magnetic field linking the conductor changes.
• You can demonstrate induction by moving a magnet into and out of a coil connected to a galvanometer.
• A larger induced e.m.f. is produced by a stronger magnetic field, faster movement, or more turns in the coil.
• The induced e.m.f. acts in a direction that opposes the change causing it. This is the key idea of Lenz’s law.
• In a simple a.c. generator, a rotating coil in a magnetic field produces an alternating e.m.f.
• Slip rings and brushes allow contact to be maintained while the coil rotates.
• The e.m.f.-time graph rises to a peak, falls to zero, reaches a trough, and returns to zero over one full turn.

How to Work Through It

1. Start with a magnet-and-coil demonstration so you can see induction happen before naming it.
2. Identify which changes increase the size of the galvanometer deflection.
3. Transfer the same idea to a rotating coil and label the parts of a simple a.c. generator.
4. Match positions of the coil to the peaks, troughs, and zeros on the output graph.

Check Your Understanding

• What must change for an e.m.f. to be induced?
• How can you increase the size of the induced e.m.f.?
• Why does the induced e.m.f. oppose the change that causes it?
• At which coil positions is the generated e.m.f. zero and when is it greatest?

Common Mistakes

• Thinking a stationary magnet near a coil always induces a current. There must be a change in the field linking the coil.
• Mixing up split-ring commutators and slip rings. Generators use slip rings for alternating output.
• Treating the graph as separate from the coil motion. Each peak and zero matches a specific coil position.

Next Steps

• Use the slides and Lenz’s law questions to practise explaining induction with clear cause-and-effect language.
• Keep the idea of alternating output secure because it feeds directly into the next lesson on transformers.