Electromagnetic Induction

Electromagnetic Induction

1. ac generator

   • structure
     • rectangular coil that spins in a uniform magnetic field
   • function
     • coil spins at steady rate
       • flux linkage changes continuously
         • emf induced
       • sides of coil move parallel to field lines
         • induced emf is zero
         • rate of change of flux is zero
         • sides of coil don't cut the field lines
       • sides of coil cut at right angles across the field lines
         • induced emf is a maxiumum
           • can be increased
             • increasing speed of rotation of coil
             • using stronger magnet
             • bigger coil
             • coil with more turns
   • alternators
     • 3 sets of coils at 120 degrees to one another
       • each set produces an alternating emf 120 degrees out of phase with each of the other two emfs
       • electromagnet spins between them
         • rotor
         • supplied with current from dc generator
         • induces an alternating emf in each set of stator coils

2. generating electricity

   • when magnet moved near a wire
     • current passes through it
       • electromotive force is induced in the wire
         • this is electromagnetic induction
           • occurs when a wire cuts across the lines of a magnetic field
   • induced emf
     • increasing induced emf
       • move wire faster
       • use a stronger magnet
       • make wire into coil
         • push magnet in or out coil
     • wire parallel to magnetic field lines
       • no emf induced
     • wire must cut along lines of magnetic field
     • generating induced emf
       • electric motor in reverse
         • falling weight makes motor coil turn between poles of the magnet in the motor
           • emf induced in coil forces current round the circuit
           • faster coil turns more current flows
       • cycle dynamo
         • magnet in dynamo spins
           • induces emf in coil
             • forces current round circuit
         • energy transferred from coil is equal to the work done on the coil to keep it spinning
           • assuming no energy is wasted
       • must be relative motion between coil and magnet
         • becomes zero when relative motion ceases
     • source emf
       • rate of energy transfer from source to other components is equal to product of induced emf and current

3. transformers

   • function
     • changes an alternating pd to a different peak value
   • structure
     • two coils
       • primary coil
       • secondary coil
       • same iron core
   • how it works
     • primary coil connected to a source of alternating pd
       • alternating magnetic field produced in core
         • field passes through secondary coil
           • alternating emf induced in secondary coil
             • magnetic flux produced by primary coil passes through to secondary coil
   • step up
     • more turns on secondary coil than primary
       • secondary voltage is stepped up
   • step down
     • fewer turns on secondary coiil
       • secondary voltage stepped down
   • efficiency
     • almost 100% efficient
       • low resistance windings
         • reduce power wasted due to heating effect of current
       • laminated core
         • consists of layers of iron separated by layers of insulator
           • reduces eddy currents
             • induced currents in core itself
             • magnetic flux is as high as possible
             • heating effect of induced currents in core is reduced
       • core of soft iron
         • easily magnetised and demagnetised
           • reduces power wasted through repeated magnetisation and demagnetisation

4. Laws of electromagnetic induction

   • Faraday's law
     • magnetic flux
       • product of magnetic flux density and area
       • useful for calculating induced emfs
       • unit
         • weber
           • Wb
       • flux density
         • flux per unit area passing at right angles through the area
       • when flux linkage through a circuit changes
         • an emf is induced in the circuit
         • flux due to
           • permanent magnet
             • motion of magnet relative to circuit is necessary to cause induced emf
               • ac generator
               • dynamo
           • current-carrying wire
             • changing current in wire causes induced emf
               • transformer
               • induction coil
     • the induced emf in a circuit is equal to the rate of change of flux linkage through the circuit
   • Lenz's Law
     • the direction of the induced current is always such as to oppose the change that causes the current
     • bar magnet pushed into a coil connected to a meter
       • meter deflects
         • induced current creates a magnetic field in the coil which opposes the incoming N-pole
       • bar magnet pulled out of the coil
         • meter deflects in opposite direction
           • current passes round one end in clockwise direction so induces a south pole which opposes magnet moving away
     • explanation
       • energy is never created or destroyed
         • induced current could never be in a direction to help the change that causes it
           • this would mean producing electrical energy from nowhere
   • solenoid
     • long coil of wire
     • magnetic field lines are like pattern for bar magnets

5. back emf

   • an emf is induced in the spinning coil of an electric motor
     • flux linkage through coil changes
     • induced emf is back emf
       • acts against the pd
   • induced emf is proportional to speed of rotation of motor
     • low speed
       • current is high
         • induced emf is small
     • high speed
       • current is low
         • induced emf is high