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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