← Electricity and electromagnetism

Generators and transformers

The generator (a motor run in reverse) and the transformer are the two electromagnetism topics beyond the basic motor effect that appear most often in Question Three.

A motor and a generator are the same physical setup, opposite direction

  • MOTOR: electrical energy in (current from a supply) → a force on the conductor (F = BIL) → mechanical motion out.
  • GENERATOR: mechanical energy in (something moves the conductor) → an induced EMF (from the changing flux as it moves through the field) → electrical energy out.
  • The formula for the induced EMF of a straight conductor moving through a field at constant speed: V = BvL (B = field strength, v = speed, L = length in the field).
Worked example A conductor moving through a field A 0.95 m conducting axle moves at 3.10 m s⁻¹ through a magnetic field of 8.10×10⁻³ T, perpendicular to the field. Find the EMF induced. Identify the formula: V = BvL, since a straight conductor is moving at constant speed perpendicular to a uniform field. V = 8.10×10⁻³ × 3.10 × 0.95 = 0.0239 V. ✓ V ≈ 0.024 V (24 mV) — a small but genuine induced voltage from mechanical motion alone, no battery involved.

The transformer equation, and why it needs AC

  • V_s / V_p = N_s / N_p — the secondary-to-primary voltage ratio equals the secondary-to-primary turns ratio.
  • A transformer works only on AC, because Faraday's law needs a CHANGING flux to induce an EMF — a constant DC current produces a constant, unchanging flux, so nothing is induced in the secondary coil after the initial switch-on moment.
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A generator producing maximum voltage needs a specific ORIENTATION, not just motion

The induced EMF is maximum when the conductor moves PERPENDICULAR to the field (cutting the most field lines per second), and drops to ZERO when moving PARALLEL to the field (cutting none). A question asking for "the position that produces maximum voltage" wants this orientation stated explicitly — simply saying "when it moves fastest" misses the geometric condition the question is actually testing.

Finding the negative end of a moving conductor

  • Use the right-hand SLAP rule (a generator variant): thumb = motion (velocity), First finger = field, seCond finger points to the induced CURRENT direction inside the conductor, which flows from the negative end to the positive end internally.
  • The end the current flows TOWARD inside the conductor is the positive terminal; the end it flows FROM is the negative terminal.
  • Reversing either the direction of motion or the field direction reverses which end is negative.

Practise just this