P7

• The Motor Effect

  When a current-carrying wire is put between magnetic poles, the magnetic field around the wire interacts with the magnetic field it has been placed in. This causes the magnet and the conductor to exert a force on each other. This is called the motor effect and can cause a wire to move

  • to experience the full force, the wire has to be at 90* to the magnetic field. If the wire runs parallel to the magnetic field, it won't experience any force at all
  • the force always acts at right angles to the magnetic field of the magnets and the direction of the current in the wire
• A Current In A Magnetic Field Experiences A Force

  • a way of showing the direction of the force is to apply a current to a set of rails inside a horseshoe magnet. A bar is placed on the rails, which completes the circuit. This generates a force that rolls the bar along the rails
  • the magnitude of the force increases with the strength of the magnetic field
  • the force also increases with the amount of current passing through the conductor
• Fleming's Left Hand Rule Shows The Direction Of The Force

  • Point first finger in direction of the field
  • Point second finger in direction of the current
  • Your thumb will point in the direction of the force
• DC Electric Motor

  • forces act on the two side arms of a coil of wire that's carrying a current
  • because the coil is on a spindle and the forces act one up one down, it rotates
  • the split-ring commutator swaps the contacts every half turn to keep the motor rotating in the same direction
  • the direction of the motor can be reversed either by swapping the polarity of the dc supply (reversing the current) or swapping the magnetic poles over (reversing the field)
• How A Loudspeaker Works

  • an alternating current is sent through a coil of wire attached to the base of a paper cone
  • the coil surrounds one pole of a permanent magnet, and is surrounded by the other pole, so the current causes a force on the coil, which causes the cone to move
  • when the current reverses, the force acts in the opposite direction, causing the cone to move in the opposite direction too
  • variations in the current make the cone vibrate, making air around the cone vibrate, creating variations in pressure that cause a sound wave
• Loudspeakers - Sound Wave Produced

  The frequency of a sound wave is the same as the frequency of the ac, so by controlling the frequency of the ac you can alter the sound wave produced
• The Generator Effect:

  The induction of a potential difference in a wire which is moving relative to a magnetic field, or experiencing an change in magnetic field
• Transformers

  • change the size of the potential difference of an alternating current
  • all have two coils of wire, the primary and the secondary, joined with an iron core
  • when an alternating potential difference is applied across the primary coil, the iron core magnetises and demagnetises quickly
  • if the second coil is part of a complete circuit, it causes a current to be induced
  • the ratio between the primary and secondary potential different is the same as the ratio between the number of turns on the primary and secondary coils
• Step-up transformers

  • increase the potential difference
  • more turns on the secondary coil than the primary coil
• Step-down transformers

  • decrease the potential difference
  • more turns on the primary coil than the secondary
• you can increase the strength of the magnetic field that a wire produces by wrapping the wire into a coil called a solenoid.

  This happens because the field lines around each wire line up with each other, resulting in lots of field lines pointing in the same direction that are very close to each other
• the magnetic field inside a solenoid is strong and uniform
• • you can increase the field strength of the solenoid even more by putting a block of iron in the centre of the coil.
  • The iron core becomes an induced magnet whenever a current is flowing
  • If you stop the current, the magnetic field disappears

  a solenoid with an iron core is called an electromagnet
• Transformers have an iron core because it's easily magnetised
• Electromagnets can be turned on and off