>
Home · IGCSE 0625 · Electricity and magnetism · The a.c. generator · Practice
Core · Practice questions · The a.c. generator

One turn, one wave.

Six original Cambridge-style questions on the a.c. generator: the parts and their jobs, the shape of the output, where the e.m.f. is maximum and zero, the effect of spinning faster, and why slip rings give a.c.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
What the examiner wants

The shape, the positions, the speed.

01
Recall
[3 marks]

Describe the basic structure of a simple a.c. generator, and state the job of the slip rings.

  • A coil that rotates in a magnetic field between the poles of a magnet. ✓
  • Slip rings and brushes connect the spinning coil to the external circuit. ✓
  • They allow continuous rotation and give an alternating output. ✓
02
Application
[2 marks]

Sketch the shape of the output voltage from an a.c. generator as the coil makes one full turn, and state how many times the voltage reaches zero in that turn.

  • A smooth wave: up to a peak, down through zero to a negative peak, then back to zero (one full cycle). ✓
  • The voltage is zero twice in one full turn. ✓
03
Analysis
[2 marks]

State the position of the coil when the induced e.m.f. is at its maximum, and explain why it is largest there.

  • When the coil plane is parallel to the field (the sides moving straight across the field lines). ✓
  • The sides cut the field lines at the fastest rate there, giving the largest e.m.f. ✓
04
Analysis
[2 marks]

At one instant the induced e.m.f. is zero, even though the coil is still spinning quickly. Explain how this is possible.

  • At that instant the coil sides are moving along the field lines, not across them. ✓
  • So they cut no field lines, and the induced e.m.f. is zero even though the coil is moving fast. ✓
05
Application
[3 marks]

A generator coil is made to spin faster. Describe the effect on the peak voltage and on the frequency of the output, and state one other change that would increase the peak without changing the frequency.

  • The peak voltage increases. ✓
  • The frequency increases (more cycles each second). ✓
  • Using more turns, or a stronger magnet, raises the peak without changing the frequency. ✓
06
Application
[2 marks]

A power station and a wind turbine both supply electricity. Explain what they have in common in terms of how the electricity is generated.

  • Both spin a coil (or magnet) to produce a changing magnetic field through a coil. ✓
  • This induces an alternating e.m.f. by electromagnetic induction; only the source of the spinning (steam, wind) differs. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.