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Physics – Key Topic 7
4.7 Magnetism and electromagnetism
4.7.1 Permanent and induced magnetism, magnetic forces and fields
4.7.1.1 Poles of a magnet
The poles of a magnet are the places where the magnetic forces are strongest. When
two magnets are brought close together they exert a force on each other. Two like
poles repel each other. Two unlike poles attract each other. Attraction and repulsion
between two magnetic poles are examples of non-contact force
A permanent magnet produces its own magnetic field - a region where magnets and
other magnetic materials experience a force. An induced magnet is a material that
becomes a magnet when it is placed in a magnetic field. Induced magnetism always
causes a force of attraction. When removed from the magnetic field an induced
magnet loses most/all of its magnetism quickly
4.7.1.2Magnetic fields
The region around a magnet where a force acts on another magnet or on a magnetic
material (iron, steel, cobalt and nickel) is called the magnetic field.
- The force between a magnet and a magnetic material is always one of attraction.
- The strength of the magnetic field depends on the distance from the magnet. The
field is strongest at the poles of the magnet.
- The closer the lines are together in a field, the stronger the field. This means that
the strength of the field is strongest at the poles
The direction of the magnetic field at any point is given by the direction of the force
that would act on another north pole placed at that point. The direction of a
magnetic field line is from the north (seeking) pole of a magnet to the south(seeking)
pole of the magnet
When a current flows through a wire a magnetic field is created around the wire. The
field is made up of concentric circles perpendicular to the wire, with the wire in the
centre.
- You can see this by placing a compass near a wire that is carrying current.
- As you move the compass, it will trace the direction of the magnetic field.
Changing the direction of the current changes the direction of the magnetic field
, - The strength of the magnetic field produced changes with the current and the
distance from the wire. The larger the current through the wire or the closer to
the wire you are, the stronger the field is
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 needle points in the
direction of the magnetic field it is in. You can move a compass around a magnet and
trace the needle’s position on some paper to build up a picture of what the magnetic
field looks like
When they are not near a magnet, compass needles point north because the earth
generates its own magnetic field, which shows that the inside of the earth must be
magnetic The closer the lines are together in a field, the stronger the field. This
means that the strength of the field is strongest at the poles
You can increase the strength of the magnetic field by:
- using a soft iron core
- increasing the current through the coil
- increasing the number of turns on the coil
4.7.1 The motor effect
4.7.2.1 Electromagnetism
When a current flows through a conducting wire a magnetic field is produced around
the wire. The strength of the magnetic field depends on the current through the wire
and the distance from the wire.
Shaping a wire to form a solenoid increases the strength of the magnetic field created
by a current through the wire. The magnetic field inside a solenoid is strong and
uniform
- This happens because the field lines around each loop of wire line up with each
other. This results in lots of field lines pointing in the same direction that are very
close to each other. This means the field is stronger
The magnetic field around a solenoid has a similar shape to that of a bar magnet.
Adding an iron core increases the strength of the magnetic field of a solenoid. An
electromagnet is a solenoid with an iron core. They are temporary magnets, meaning
they can be switched on and off – the iron core becomes an induced magnet
whenever current is flowing
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