CIE A Level Physics (9702) 2019-2021

Revision Notes

25.6.3 Principles of Electromagnetic Induction

Principles of Electromagnetic Induction

  • Electromagnetic induction is a phenomenon which occurs when an e.m.f is induced when a conductor moves through a magnetic field
  • When the conductor cuts through the magnetic field lines:
    • This causes a change in magnetic flux
    • Which causes work to be done
    • This work is then transformed into electrical energy
  • Therefore, if attached to a complete circuit, a current will be induced
  • 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

 

  • This can occur either when:
    • A conductor cuts through a magnetic field
    • The direction of a magnetic field through a coil changes
  • 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

Experiment 1: Moving a magnet through a coil

  • 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

The expected results 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
    • 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
  • 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

Experiment 2: Moving a wire through a magnetic field

  • When a long wire is connected to a voltmeter and moved between two magnets, an e.m.f is induced
  • Note: there is no current flowing through the wire to start with

The expected results are:

  • When the wire is not moving, the voltmeter shows a zero reading
    • When the wire is held still inside, or outside, the magnets, the rate of change of flux is zero, so, there is no e.m.f induced
  • As the wire is moved through between the magnets, an e.m.f is induced within the wire, shown momentarily by the reading on the voltmeter
    • As the wire moves, it ‘cuts through’ the magnetic field lines of the magnetic, generating a change in magnetic flux
  • When the wire is taken back out of the magnet, an e.m.f is induced in the opposite direction
    • As the wire changes direction, the direction of the current changes
    • The voltmeter will momentarily show a reading with the opposite sign
  • As before, the direction of the electric current, and e.m.f, induced in the conductor is such that it opposes the change that produces it
  • Factors that will increase the induced e.m.f are:
    • Increasing the length of the wire
    • Moving the wire between the magnets faster
    • Increasing the strength of the magnets

Applications of EM Induction

  • Electromagnetic (EM) induction is commonly used in power generation and power transmission, most commonly:
    • Electrical generators
    • Electric motors
    • Transformers

 

Generators

  • Generators are used inside power stations
    • It is a machine which converts mechanical energy to electrical energy for transmission over power lines and for electrical power required for transport
    • Sources of mechanical energy can be from steam turbines, water turbines, or wind turbines
  • An electrical generator consists of a rotor
    • This is an electromagnet made by coiling wires around two or more poles of a ferrous metal, ie. iron core
    • These are rotated between the poles of a magnet
    • As the coil rotates, the sides of the coil cut through the magnetic field lines, and current flows through the coil as an e.m.f is induced
    • This flows out from both sides of the coil through slip rings and produces an alternating current output

Motors

  • Electric motors work in a similar way to generators, except they convert electrical energy to mechanical energy
  • Motors also consist of a rotor between the poles of a magnet
    • The two poles of the permanent magnet remain the same, but the poles of the electromagnet reverse when the direction of electric current reverses
    • For example, the end of the electromagnet, ie. its north pole, will be attracted to the south pole of the permanent magnet
    • This pulls the electromagnet towards the permanent magnet
    • When the electric current changes direction, the same end of the electromagnet becomes its south pole
    • The electromagnet is now repelled by the south pole of the permanent magnet, and moves away from it

Transformers

  • Transformers are used to:
    • Increase the voltage of electricity generated in a power station so less energy is lost as heat when the current is transmitted down power lines
    • Decrease the voltage before it reaches buildings to be used in electrical appliances
  • A transformer is made up of two coils, the primary and secondary coil, each with a different number of coils wrapped around an iron core
    • The number of coils on each side determines whether the voltage will be stepped-up (increased) or stepped-down (decreased)
  • When A.C is applied through the first coil (called the primary coil), a varying magnetic flux is generated inside the iron core
  • The varying magnetic flux passes through the iron core to the secondary coil which induces an e.m.f in the second coil
  • From Faraday’s Law, the induced e.m.f in the secondary coil can be increased by the number of turns

Author: Katie

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.
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