Edexcel A Level Physics

Revision Notes

7.17 Induced E.M.F in a Moving Coil

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Induced E.M.F in a Moving Coil

  • Electromagnetic induction is a phenomenon which occurs when an e.m.f is induced when a conductor moves through a magnetic field
  • If there is a change in magnetic flux Φ or magnetic flux linkage NΦ
    • Mechanical work (from moving the conductor in the field) is transformed into electrical energy
  • Therefore, if attached to a complete circuit, a current will be induced in the conductor
  • This is known as electromagnetic induction and is defined as:

The process in which an e.m.f is induced in a closed circuit due to changes in magnetic flux (linkage)

  • This can occur either when:
    • A conductor cuts through a magnetic field
    • The magnetic flux (linkage) through a coil changes, e.g. becomes more or less dense, or changes direction

  • Electromagnetic induction is used in:
    • Electrical generators which convert mechanical energy to electrical energy
    • Transformers which are used in electrical power transmission

  • This phenomenon can easily be demonstrated with a magnet and a coil, or a wire and two magnets

Relative Motion between a Coil and a Magnet

  • When a coil is connected to a sensitive voltmeter, a bar magnet can be moved in and out of the coil to induce an e.m.f in the coil

magnet through coil experiment, downloadable AS & A Level Physics revision notes

A bar magnet is moved through a coil connected to a voltmeter to induce an e.m.f

The observations are:

  • When the bar magnet is not moving, the voltmeter shows a zero reading
    • When the bar magnet is held still inside, or outside, the coil, the rate of change of flux is zero, so, there is no e.m.f induced

  • When the bar magnet begins to move inside the coil, there is a reading on the voltmeter
    • As the bar magnet moves, its magnetic field lines ‘cut through’ the coil, generating a change in magnetic flux (ΔΦ)
    • This induces an e.m.f within the coil, shown momentarily by the reading on the voltmeter

  • When the bar magnet is taken back out of the coil, an e.m.f is induced in the opposite direction
    • As the magnet changes direction, the direction of the current changes
    • The voltmeter will momentarily show a reading with the opposite sign

  • Increasing the speed of the magnet induces an e.m.f with a higher magnitude
    • As the speed of the magnet increases, the rate of change of flux increases

  • The direction of the electric current, and e.m.f, induced in the conductor is such that it opposes the change that produces it

magnet through coil (1), downloadable AS & A Level Physics revision notesmagnet through coil (2), downloadable AS & A Level Physics revision notes

An e.m.f is induced only when the bar magnet is moving through the coil

  • Factors that will increase the induced e.m.f are:
    • Moving the magnet faster through the coil
    • Adding more turns to the coil
    • Increasing the strength of the bar magnet

Rotating Coils 

  • When a coil rotates in a uniform magnetic field, the magnetic flux through the coil will vary as it rotates
  • Therefore, since the flux linkage through the coil also varies, this will induce an e.m.f that also varies 
    • The maximum e.m.f is when the coil cuts through the most field lines
    • The varying e.m.f induced is called an alternating voltage


Even though the flux linkage through the coil is maximum when θ = 0°, the change in flux linkage is minimal as the coil rotates, so the induced e.m.f is a minimum. The opposite is true when θ = 90°

  • Increasing the coil's frequency of rotation increases:
    • The frequency of the alternating voltage
    • The amplitude of the alternating voltage

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Author: Katie M

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.