7.1 Permanent and induced magnetism Flashcards
Poles of a magnet
The ends of a magnet are called poles.
Magnets have two poles: a north and a south.
When two magnets are held close together, there will be an attractive or repulsive force between the magnets depending on how they are arranged:
The Law of Magnetism states that:
Two like poles repel each other.
Two unlike poles attract each other.
The attraction or repulsion between two magnetic poles is an example of a non-contact force.
Permanent magnets
Permanent magnets are made out of permanent magnetic materials, for example steel.
A permanent magnet will produce its own magnetic field.
It will not lose its magnetism.
Induced magnets
When a magnetic material is placed in a magnetic field, the material can temporarily be turned into a magnet.
This is called induced magnetism.
When magnetism is induced on a material:
One end of the material will become a north pole.
The other end will become a south pole.
Magnetic materials will always be attracted to a permanent magnet.
This means that the end of the material closest to the magnet will have the opposite pole to magnets pole closest to the material.
Magnetic fields
The region around a magnet where a force acts on another magnet or on a magnetic material (such as iron, steel, cobalt and nickel).
All magnets are surrounded by a magnetic field.
Magnetic field lines
Magnetic field lines are used to represent the strength and direction of a magnetic field.
The direction of the magnetic field is shown using arrows.
The strength of the magnetic field is shown by the spacing of the magnetic field lines.
If the magnetic field lines are close together then the magnetic field will be strong.
If the magnetic field lines are far apart then the magnetic field will be weak
There are some rules which must be followed when drawing magnetic field lines. Magnetic field lines:
Always go from north to south (indicated by an arrow midway along the line)
Must never touch or cross other field lines.
Uniform magnetic field
A uniform magnetic field is one that has the same strength and direction at all points.
To show that the magnetic field has the same strength at all points there must be equal spacing between all magnetic field lines.
To show that the magnetic field is acting in the same direction at all points there must be an arrow on each magnetic field line going from the north pole to the south pole.
Earth’s magnetic field
On Earth, in the absence of any magnet or magnetic materials, a magnetic compass will always point north.
This is evidence that the core of the Earth is magnetic and creates its own magnetic field.
On Earth, the north arrow on a magnetic compass will point towards the geographic North Pole (in the Arctic Ocean).
This is because the geographic North Pole is a magnetic south pole (the magnetic field lines point into the pole).
The geographic South Pole (in Antarctica) is a magnetic north pole (the magnetic field lines point out of the pole).
Magnetic field around a wire
When a current flows through a conducting wire a magnetic field is produced around the wire
A conducting wire is any wire that has current flowing through it
The shape and direction of the magnetic field can be investigated using plotting compasses.
The magnetic field is made up of concentric circles
A circular field pattern indicates that the magnetic field around a current-carrying wire has no poles.
If there is no current flowing through the conductor there will be no magnetic field.
Increasing the amount of current flowing through the wire will increase the strength of the magnetic field.
This means the field lines will become closer together.
Magnetic field around a solenoid
When a wire is looped into a coil, the magnetic field lines circle around each part of the coil, passing through the centre of it.
To increase the strength of the magnetic field around the wire it should be coiled to form a solenoid.
The magnetic field around the solenoid is similar to that of a bar magnet.
The magnetic field inside the solenoid is strong and uniform.
One end of the solenoid behaves like the north pole of a magnet; the other side behaves like the south pole.
To work out the polarity of each end of the solenoid it needs to be viewed from the end.
If the current is travelling around in a clockwise direction then it is the south pole.
If the current is travelling around in an anticlockwise direction then it is the north pole.
If the current changes direction then the north and south poles will be reversed.
If there is no current flowing through the wire then there will be no magnetic field produced around or through the solenoid.
Magnetic field strength around a solenoid
The strength of the magnetic field produced around a solenoid can be increased by:
Increasing the size of the current which is flowing through the wire.
Increasing the number of turns in the coil.
Adding an iron core through the centre of the coils.
The iron core will become an induced magnet when current is flowing through the coils.
The magnetic field produced from the solenoid and the iron core will create a much stronger magnet overall.
