Magnetism

 Loadstones are:

·        natural magnets found in ground

·        contain iron ore (magnetite)

·        earliest use as simple compass because it will align north-south

 Man-made magnets: most are made from iron, steel, nickel, cobalt

 Magnetic substances are materials that are attracted to a magnet

 Theory of magnetism à most elements electrons spin opposed to one another and magnetic forces cancel each other out but in magnetic substances this spinning motion reinforces each other  (domain theory)

 Law of magnetic poles

·        a magnet has the greatest strength at the poles (little or no strength in middle)

·        ends are called poles (free ends)

·        hang bar magnet from ring stand – the bar end pointing north = north pole

 Law of magnetic pole – like poles repel, unlike poles attract

Force of attraction or repulsion depends on strength of magnets and distance between poles

 Magnetic field – region around a magnet in which magnetic effects are observed

 Lines of force – closed arc from north to south pole, never cross, most concentrated at poles

 Making a magnet

ü      stroke bar of magnetic material in one direction with a magnet (re-aligns domains)

ü      use electricity

ü      bring magnet nearby (if soft iron)

 How to demagnetize a magnet

v     heat

v     contact (pass magnet alternating direction)

v     hammering

Electromagnets

When electric current passes through a wire – a magnetic field is produced in the space surrounding the wire

(electromagnet = placing soft iron bar in center of coil of wire with electricity passing through it)

 3 things needed to make electromagnet:

v     soft iron bar (core)

v     coil of insulated wire

v     source of electric current

 How to change strength of electromagnet

v     varying the number of turns of wire around core

v     increasing the current

v     varying the thickness of core

v     change core material (if use wood = no electromagnet, use cobalt = strength increased 170 times, but if you use iron = strength increased 1000 times

 Electromagnet summary

Ø      magnetic field around straight wire that carries current

Ø      bending wire into loops causes each loop to act as a separate magnet

Ø      adding more coils increased strength of electromagnet

Ø      increasing number of electrons through wire increases strength of electromagnet

Ø      larger and thicker iron core provides more magnetic domains = increases strength of electromagnet