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Course: Science — Third Preparatory
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Science — Third Preparatory

Science — Third Preparatory

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Science — Third Preparatory 2026/2027

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Unit 1 - Force, Motion, Acceleration and Physical Quantities

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Text lesson

Unit 2 — Electricity and Magnetism

Science — Third Preparatory

Unit 2 — Electricity and Magnetism

3.5 The electric generator

3.5 The electric generator

  • What if you turn the shaft of a motor by hand, instead of supplying it with current?
  • The process reverses and the motor produces electrical energy.
  • This is the principle of the generator.
  • Activity.
  • Connect a small DC motor to an LED (or small bulb), fix a small fan on its shaft and turn the fan by hand.
  • The lamp lights; turning faster makes it brighter.
  • (An LED lights only when current passes in one direction; if it does not light, reverse its connections.)
  • An electric generator converts mechanical energy into electrical energy.
  • Working principle.
  • A coil is rotated by some source of mechanical energy between the poles of a magnet.
  • As the coil cuts the magnetic field lines, an electromotive force (emf) is generated in it, driving a current through the external circuit.
  • The emf generated increases when the coil’s rotational speed increases (and also with a stronger magnet or more turns).
  • Life application bicycle dynamo.
  • A small generator pressed against the wheel is turned by friction as the wheel rotates; it gives electricity to the lamps without batteries.
  • The faster the wheel turns, the larger the emf and the brighter the lamp.
  • Uses:
Comparison aspectMain ideaKeywords
Power stations.
  • Turbines driven by water falls
  • steam or wind turn coils in large generators to power cities and factories. Power stations use generators producing alternating current (AC)
  • the type used in homes. (A generator with a split-ring commutator produces direct current.) Advantages of AC: it can be transmitted over long distances with small energy loss
  • and it can be converted to DC when needed
Turbinesdrivenwaterfalls
Emergency generators
  • in homes
  • hospitals and factories for power cuts
homeshospitalsfactoriespower
Comparison aspect Electric motor Electric generator
Input energy Electrical Mechanical
Output energy Mechanical Electrical
Principle Force on a current-carrying coil in a magnetic field (torque) Emf produced when a coil cuts magnetic field lines
Unit 2 — Electricity and Magnetism · 19 / 20