Chapter 7: Magnetism and Electromagnetism

7.1-Permanent and Induced Magnets

  • Magnets produce magnetic fields
  • All magnets have two poles-north and south
    • All magnets produce a magnetic field, a region where other magnets or magnetic materials experience a force.(This is a non contact force-similar to the force on charges in an electric field)
  • You can show a magnetic field by drawing magnetic field lines
    • The lines always go from north to south and they show which way a force would act on a north pole if it was put at that point in the field
    • The closer together the lines are. the stronger the magnetic field.
    • The further away from a magnet you get, the weaker the field is
    • The magnetic field is strongest at the poles of a magnet.
    • This means that the magnetic forces are also strongest at the poles
  • The force between a magnet and a magnetic material is always attractive, no matter the pole
  • If two poles of a magnet are put near each other, they will each exert a force on each other.
  • This force can be attractive or repulsive.
  • Two poles that are the same(these are called like poles) will repel each other.
  • Two unlike poles will attract each.
  • Compasses show the directions of magnetic fields
    • Inside a compass is a tiny bar magnet.
    • The north pole of this magnet is attracted to the south pole of any other magnet it is near.
    • So the compass points in the direction of the magnetic field it is in
    • You can move a compass around a magnet and trace its position on some paper to build up a picture of what the magnetic field looks like
  • When they’re not near a magnet, compasses always point north.
    • This is because the Earth generates its own magnetic field, which shows the inside(core) of the Earth must be magnetic
  • Magnets can be permanent or induced
    • There are two types of magnet-permanent magnets and induced magnets
    • Permanent magnets produce their own magnetic field
    • Induced magnets are magnetic materials that turn into a magnet when they’re put into a magnetic field
    • The force between permanent and induced magnets is always attractive
    • When you take away the magnetic field, induced magnets quickly lose their magnetism, or most of it, and stop producing a magnetic field

7.2-The Motor Effect

The motor effect can happen when you put a current-carrying wire in a magnetic field.

  • A current in a magnetic field experiences a force

  • When a a current-carrying wire, or any other conductor, 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 the 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.

  • At angles in between, it’ll feel some force.

    • The force always acts at right angles to the magnetic field of the magnets and the direction of the current in the wire
  • A good 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(strength) of the force increases with the strength of the magnetic field
    • The force also increases with the amount of current passing through the conductor

    You can find the size of the force

  • The force acting on a conductor in a magnetic field depends on three things

  • The magnetic flux density-how many field(flux) lines there are in a region.

    • This shows the strength of the magnetic field
  • The size of the current through the conductor

  • The length of the conductor that’s in the magnetic field

  • When the current is at 90 to the magnetic field it is in, the force acting on it can be found using the equation:

  • F=BI1 Force=Magnetic flux density x current x length

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And which way it’s acting

  • You can find the direction of the force with Fleming’s left hand rule
    • Using your left hand, point your First finger in the direction of the Field
    • Point your seCond finger in the direction of the Current
    • Your thuMb will then point in the direction of the force(Motion)
    • Fleming’s left hand rule shows that if either the current or the magnetic field is reversed, then the direction of the force will also be reversed.
    • This can be used for all sorts of things-like motors

7.3-Electric Motors and Loudspeakers

  • A current-carrying coil of wire rotates in a magnetic field

  • The diagram below shows a basic dc motor.

  • Forces act on the side arms of a coil of wire that’s carrying a current.

    • These forces are just the usual forces which act on any current in a magnetic field.
  • Because the coil is on a spindle and the forces act one up and one down, it rotates

  • The split-ring commutator is a clever way of swapping 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).

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You can use Fleming’s left-hand rule to work out which way the coil will turn

  • Draw in current arrows (from positive to negative)
  • Using Fleming’s left-hand rule on one branch
  • Point your first finger in the direction of the magnetic field (this is north to south)
  • Point your second finger in the direction of the current
  • Draw in the direction of motion (the direction your thumb in pointing in) \n

·Loudspeakers work because of the motor effect

  • Loudspeakers and headphones both use electromagnets:
    • An alternating current(ac) 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, which causes the cone to move in the opposite direction too.
    • So, variations in the current make the cone vibrate, which makes the air around the cone vibrate and creates the variations in pressure that cause a sound wave.
    • The frequency of the 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.

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