Magnetism and Electromagnetism

• Magnets produce magnetic fields
• Like poles repel, unlike poles attract
• The strength of the force between magnets depends on their distance apart
• All magnets have 2 poles - north and south
• You can show a magnetic field by drawing magnetic field lines
• The magnetic field lines would 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 fiels
• 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
• Compasses show the direction of magnetic fields
• Compasses points to the magnetic north and is used to determine direction.
• When they're not near a magnet, compasses always point north, this is because Earth generates its own magnetic field, which shows the inside (core) of the earth must be magnetic
• Magnets can be permanent or induced
• 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 attracctive
• When you take away the magnetic field, induced magnets quickly lose their magnetism (or most of it) and stop producing a magnetic field
• A moving charge creates a magnetic field
• 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
• Changing the direction of the current changed the direction of the magnetic field - use the right-hand thumb rule to work out which way it goes
• Right hand thumb rule:

  
• If your fingers point along the wire, then your thumb points in the direction of the magnetic field
• A solenoid is a coil of wire that produces a magnetic field around it.
• Solenoids are used as electromagnets because they can be switched on or off by turning the current on or off
• The magnetic field inside a solenoid is strong and uniform (it has the same strength and direction at every point in that region
• Outside the coil, the magnetic field is just like the one round a bar magnet
• You can increase the field strength of the solenoid by increasing the number of turns of the coil or by putting a block of iron in the centre of the coil. The iron core becomes an induced magnet whenever current is flowing
• If you stop the current, the magnetic field disappears. A solenoid with an iron core (a magnet whose magnetic field can be turned on and off with an electric current) is called an electromagnet
• Electromagnets ae used in some cranes to attract and pick up things made from magnetic materials like iron or steel, e.g. in scrap yards. Using an electromagnet means the magnet can be switched on when you want to pick stuff up, then switched off when you want to drop it.
• Electromagnets can also be used within other circuits to act as switches (e.g. in the electric starters of motors)
• A current in the magnetic field experiences a force