Notes on the electromagnetism part of physics 3 for AQA =)

  • Created by: Joanne
  • Created on: 26-05-12 21:19

Motor effect

  • when a wire carrying a current is in a magnetic field, a force may be exerted on the wire = the motor effect
  • no force when wire parallel to magnetic fields, biggest force when wire perpendicular to magnetic field (shown in diagram)
  • increase by: stronger magnetic fields or increasing size of current

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Electric motor

Electric Motor (http://gcserevision101.files.wordpress.com/2009/02/0021.jpg?w=313&h=204)

  • coil is connected to battery by two metal brushes
  • brushes are fixed to a split-ring commutator
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Electromagnetism continued

the coil spins when a current is passed through it because:

  • a force acts on either side due to the motor effect
  • the force on one side is opposite to the force on the other side (one goes up and the other down)

Split-ring commutator

  • split-ring commutator reverses the current (changes the direction) every half turn of the coil
  • since it is swapped every half turn, it is pushed continuously in the same direction
  • i.e. the force is up on one side, and down on the other - when the current is reversed, the up force switches to down and the down force switches to up so it moves continuously
  • imagine it turning clockwise - as it approaches down, that down force is changed to up so it turns up - it rotates in a clockwise direction and keeps going due to the split-ring commutator
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Electromagnetic induction

  • coils of wire spin (move) in a magnetic field
  • a potential difference (voltage) is induced (made) when the wire cuts across magnetic field lines
  • this is why more voltage is induced when the wire is perpendicular to the magnetic field lines
  • if you connect the wire to an ammeter and move it between the two poles of a U-shaped magnet, the ammeter pointer will deflect as a current is generated when the wire cuts the magnetic field lines
  • this is known as the dynamo effect

to increase the voltage induced:

  • increase the speed of the movement
  • increase the strength of the magnet
  • increase the number of coils on the wire
  • increase the area of the coil

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  • brushes attached to slip rings
  • alternating current
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Generator continued

a.c Generator Graph (http://gcserevision101.files.wordpress.com/2009/02/006-2.jpg?w=320&h=199)

  • faster the coil rotates, larger the peak value of the alternating current and greater the frequency
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Slip rings

  • they prevent the wires in the generator getting tangled
  • one slip ring connects to the end of the wire carrying the current, and the other is connected to the other end
  • the brushes connect the slip rings to the external circuit 
  • (which connects to something else e.g. a lamp and it will light up as the circuit supplies it with voltage induced in the generator)
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Wow great notes! Its good you put in about the splitting commuter and stuff cos I didn't really get that before sothankksss :)



Wow great notes! Its good you put in about the splitting commuter and stuff cos I didn't really get that before sothankksss :)



Glad they help =)

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