P3- using magnetic fields to keep things moving

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  • the two ends of a magnet, south and north attract, they create a magentic field
  • a magnetic field line is the line along which a plotting compass points
  •  the magnetic field lines loop from one pole to the other
  • same= repel
  • different= attract
  • when a current flows through a wire a magentic field is produced around the wire
  • an electromagnet is made by wrapping insultated wire around a iron core
  • when a current flows through the wire the iron becomes strongly magnetised, when current switched off it looses its magnetism so it is a tempory electromagent
  • they are useful because they are used in devices like scrapyard cranes, circuit breakers, electric bells and relays

the motor effect

  • the motor effect is when a wire placed in a magnetic field experiences a force
  • force is at maximum if wire is at angle of 90' to magnetic field
  • force is zero is wire is parallel to magnetic field
  • flemings left hand rule determines direction of the force: thumb=thrust, first finger=field and second finger=current
  • size of force can be increased if: strength of magnetic field is increased or current is increased
  • direction of force on wire is reversed if direction of current or diretion of magnetic field is reversed
  • electric motors use the motor effect
  • speed of motor is increased by increased current and direction can be reversed  by changing directon of current
  • when a current passes through the coil it spins because force acts on each side of the coil due to the motor effect and because force on one side is opposite to the other
  • the split ring commutator reverses direction of current every half turn…


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