Edexcel A Level Physics:复习笔记7.17 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

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

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 [popover id="QczwWzRh7Txmc1KJ" label ="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