IB DP Physics: HL复习笔记11.1.2 Induced Emf

Induced Emf

  • 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 (ΔΦ)
    • 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

20.2-magnet-through-coil-experiment

A bar magnet is moved through a coil connected to a voltmeter 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 (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

20.2-magnet-through-coil-120.2-magnet-through-coil-2

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

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

20.2-Wire-through-magnets-experiment

A wire is moved between two magnets connected to a voltmeter to induce an e.m.f

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 magnet, 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

 

 

 

 

 

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