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Lesson: Chapter 13 — Electromagnetism and electromagnetic induction

Putting the chapter together 1 of 3

Chapter summary

The whole chapter in one list.

A current and a field

  • The direction of the magnetic field around a conductor carrying a current can be found with the corkscrew rule.
  • When a corkscrew is turned so that it moves in the direction of the current, the way it turns is the direction of the field lines.
  • A force acts on a conductor carrying a current placed in a magnetic field.
  • That force is directly proportional to three factors: the current in the conductor, the length of the conductor, and the strength of the magnetic field.
  • Fleming's left-hand rule gives the direction of the force: with the thumb, first finger and middle finger of the left hand at right angles, the middle finger along the current and the first finger along the field, the thumb points along the force on the conductor.
  • Devices such as the DC motor and the loudspeaker work using the force on a current-carrying conductor in a magnetic field.
  • In a working motor, electrical energy is converted into mechanical energy.

Induction

  • An emf set up in a closed circuit by a changing magnetic field is called electromagnetic induction.
  • The size of the induced emf depends on the number of turns of the coil, the strength of the magnet and the speed at which the magnet is moved.
  • Fleming's right-hand rule gives the direction of the current caused by the induced emf: with the thumb, first finger and middle finger of the right hand at right angles, the first finger along the field and the thumb along the motion, the induced current flows the way the middle finger points.
  • The bicycle dynamo, the moving-coil microphone and the transformer are practical uses of electromagnetic induction.

DC, AC and transformers

  • A current whose direction does not change with time is a direct current.
  • A current whose direction changes with time is an alternating current.
  • Cells and solar cells give direct current; an AC dynamo gives alternating current.
  • Transformers change an alternating voltage from one value to another.
  • The primary and secondary coils of a transformer are related by VS/VP = NS/NP and VPIP = VSIS.