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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