Magnetism and Electromagnetism

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Permanent and Induced Magnets

All magnets have two poles - north and south

All magnets produce magnetic field - region where other magnets or magnetic materials experience a non-contact force

You can show a magnetic field by drawing magnetic field lines

Lines always go from north to south and they show which way a force would act on a north pole if it was put that point in the field

Closer together lines are, stronger the magnetic field

Further away from magnet you get, weaker the field is

Magnetic field is strongest at the poles of a magnet - means magnetic forces are also strongest at the poles

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 will repel each other

Two unlike poles will attract each other

Inside a compass is a tiny bar magnet

North pole of this magnet is attracted to south pole of any other magnet it's near

Compass points in the direction of magnetic field it's in

Can move a compass around a magnet and trace its position on some paper to build picture of what magnetic field looks like

When they're not near a magnet, compasses always point north

Earth generates own magnetic field - inside must be magnetic

Two types of magnets: permanent and induced

Permanent magnets produce their own magnetic field

Induced magnets turn into magnets when put into magnetic field

Force between permanent and induced magnets is always attractive

When you take away magnetic field, induced magnets quickly lose their magnetism and stop producing magnetic field

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Electromagnetism

When current flows through a wire, a magnetic field is created around the wire

Field's made of concentric circles perpendicular to the wire - wire in centre

Can see this by placing a compass near a wire that's carrying a current

As you move compass, it'll trace direction of magnetic field

Changing direction of current, changes direction of magnetic field

Strength of magnetic field produced changes with the current and distance from the wire

Larger current through wire, or closer to wire you are, the stronger the field is

You can increase strength of magnetic field that a wire produces by wrapping the wire into a coil called a solenoid

Field lines around each loop of wire line up with each other

Results in lots of field lines pointing in same direction that are very close to each other - closer field lines are, stronger field is

Magnetic field inside is strong and uniform - same strength and direction at every point in that region

Outside the coil, magnetic field is just like the one round a bar magnet

Means ends of solenoid act like north and south poles of a bar magnet - work out which is which using right-hand rule

Can increase field strength of solenoid even more by putting a block of iron in centre of coil

Iron core becomes an induced magnet whenever current is flowing

If you stop current, magnetic field disappears

Solenoid with an iron core (magnet whose magnetic field can be turned on and off with an electric current) is an electromagnet

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The Motor Effect

When a current-carrying wire is put between magnetic poles

Magnetic field around the wire interacts with magnetic field it has been placed in

Causes magnet and conductor to exert a force on each other

Called motor effect and causes wire to move

To experience full force, wire must be at 90 degrees to magnetic field

If wire runs parallel to magnetic field, it won't experience any force

At angles in between, it'll feel some force

Force always act perpendicular to magnetic field of magnets and direction of current in the wire

Good way of showing direction of the force is to apply current to set of rails inside horseshoe magnet

Bar is placed on rails, which completes circuit

Generates a force that rolls bar along rails

Magnitude of the force increases with strength of magnetic field

Force increases with amount of current passing through conductor

Force acting on a conductor in a magnetic field depends on 3 things:

Magnetic flux density - how many field lines there are in a region

Shows strength of magnetic field

Size of current through conductor

Length of conductor that's in magnetic field

When current is perpendicular to magnetic field it's in, force acting on it can be found using the equation:

Force = magnetic flux density x current x length

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Electric Motors

A force is exerted on any current-carrying conductor in a magnetic field ]

Can find direction of this force using the left hand rule

Index finger = direction of magnetic field

Middle finger = direction of current

Thumb = direction of force

Left hand rule shows that if either current or magnetic field is reversed, then direction of force will also be reversed

Can be used for all things - like motors

Diagram shows a basic dc motor

Forces act on two side arms of a coil of wire that's carrying a current

Just the usual forces which act on any current in a magnetic field

Because coil is on a spindle and forces act one up and one down, it rotates

Split-ring commutator is a clever way of swapping contacts every half turn to keep motor rotating in the same direction

Direction can be reversed either by swapping polarity of dc supply (reversing current) or swapping magnetic poles over (reversing field)

Speed of motor can be increased by increasing current, adding more turns to the coil or increasing magnetic flux density

Can use left hand rule to work out which way coil will turn