Electricity and Magnetism:Magnetic Fields

Mr D. Patterson

Outcomes• explain the attraction and repulsion effects for

magnets, the behaviour of freely suspended magnets and magnetic compasses, and describe the nature of the Earth's magnetic field

• describe, using diagrams, the magnetic field in various magnetic configurations

This train levitates above the track using magnets

Properties of Magnetism• Non contact force• North and south poles (dipoles)• Like poles repel while unlike poles attract

Magnetic Fields• A magnetic field shows the direction the north pole of a small

bar magnet will point.• Draw field lines away from north poles and toward south

poles. • Never cross the lines.

The Earth is a Bar Magnet• A magnet, in the absence of any other forces will

align itself with the magnetic field lines.

• This is the basis for the compass.

• Magnetic flux density (B) is a vector quantity measuring the strength and direction of a magnetic field

• It is indicated visually by the closeness of the field lines and measured using units of Teslas (T)

Magnetic flux Density

Larger magnetic flux density

Smaller magnetic flux density

Ferromagnetic materials• Magnetic effects are created because of:

– The motion of charged particles like electrons– A property of electrons called “spin”. Electrons can either be in a state of spin up or spin

down.

Ferromagnetic materials• Ferromagnetic materials such as iron, nickel, cobalt and some rare earth

metals will become permanent magnets when all their magnetic domains line up