MAGNETISM

A Strangely Attractive Topic

HISTORY Term comes from the ancient Greek city of

Magnesia, at which many natural magnets were found.

We now refer to these natural magnets as lodestones (lode means to lead or to attract) which contain magnetite, a natural magnetic material Fe3O4.

HISTORY Pliny the Elder (23-79 AD Roman) wrote of a

hill near the river Indus that was made entirely of a stone that attracted iron.

HISTORY Chinese as early as 121 AD knew that an iron

rod which had been brought near one of these natural magnets would acquire and retain the magnetic property, and that such a rod when suspended from a string would align itself in a north-south direction.

Use of magnets to aid in navigation can be traced back to at least the eleventh century.

HISTORY

Basically, we knew the phenomenon existed

and we learned useful applications for it.

We did not understand it.

Not until its connection to electrical charges and currents was discovered.

What is magnetism? Magnetism is the force

of attraction or repulsion of a magnetic material due to the arrangement of its atoms, particularly its electrons.

What causes magnetism? Atoms themselves have magnetic properties

due to the spin of the atom’s electrons. Groups of atoms join so that their magnetic

fields are all going in the same direction. These areas of atoms are called “domains”.

What causes magnetism? When an unmagnetized substance is placed in

a magnetic field, the substance can become magnetized.

This happens when the spinning electrons line up in the same direction.

What causes magnetism? An unmagnetized substance looks like this…

While a magnetized substance looks like this…

How to “break” a magnet? Drop it.

Heat it.

This causes the domains to become random again!

A big natural magnet.. It exerts magnetic

forces and is surrounded by a magnetic field that is strongest near the North and South magnetic poles.

Magnetic South Pole

Magnetic North Pole

Geographic North Pole

Geographic South Pole

PROPERTIES OF MAGNETS There are north poles and south poles. Magnets set up a magnetic field around it. Like poles repel, unlike poles attract. Magnetic forces attract only magnetic

materials. Magnetic forces act at a distance.

1. North and South Poles Every magnet has at least one north pole and

one south pole. If you take a bar magnet and break it into two

pieces, each piece will again have a north pole and a south pole. 

No matter how small the pieces of the magnet become, each piece will have a north pole and a south pole. 

1. North and South Poles, cont. It has not been shown to be possible to end up

with a single North pole or a single South pole, which is a monopole ("mono" means one or single, thus one pole).

2. Magnetic Fields Michael Faraday realized that a magnet has

a ‘magnetic field’ distributed throughout the surrounding space.

This field exerts a force on any charge/magnetic material on it.

2. Magnetic Fields, cont. Field lines converge where the magnetic force

is strong, and spread out where it is weak. In a compact bar magnet or dipole, field lines spread out from one pole and converge towards the other.

3. Like poles, unlike poles LAW of MAGNETISM: Like poles repel, unlike

poles attract.

4. Attract only magnetic materials Magnets only attract certain types of metals, other

materials such as glass, plastic and wood aren't attracted.

Metals such as iron, nickel and cobalt are attracted to magnets.

Most metals however are not attracted to magnets, these include copper, silver, gold, magnesium, platinum, aluminium and more. They may however magnetize a small amount while placed in a magnetic field.

Magnetism can attract magnetic objects or push them away.

5. Action at a Distance Although two

magnets may not be touching, they still interact through their magnetic fields.

This explains the ‘action at a distance’, say of a compass.

Electricity to MagnetismHans Oersted

first observed that a current in a wire affects a nearby compass needle

Implication: an electric current creates a magnetic field

Electromagnets- Magnets created using a current

Magnetic strength increases with: more current in the wire more coils of wire bigger ferromagnetic core

Uses of Electromagnetism1. Electric motors Converts electrical energy to mechanical

energy Anytime electricity is converted into a motion

its through an electric motor

Uses of Electromagnetism2. Current meters Galvanometer – used to measure small currents Ammeter – used to measure currents Voltmeter – used to measure voltage

Magnetism to ElectricityMichael Faraday

if an electric current can create a magnetic field, then maybe a magnetic field can create a electric current

this led to his Law of Electromagnetic Induction

Law of Electromagnetic Induction Any change in the magnetic field of a coil of

wire will cause a voltage to be induced in the coil, called the induced voltage.

If the conductor circuit is closed, the current will also circulate through the circuit and this current is called induced current.

How to increase induced voltage/current Increase number of coils Increase magnetic field Increase speed of relative motion between coil

and magnetic field

Uses of Electromagnetic Induction1. Generator Converts mechanical energy to electrical energy Used in most power plants to create electricity In coal, oil, natural gas, nuclear or geothermal

power plants, the heat is used to boil water into steam which is then used to rotate the turbines

In wind, hydroelectric (dam/falls) and tidal power plants, the turbines are rotated by the wind/water

The only sources of electricity that does not use generators are solar, chemical (batteries), piezoelectric (crystals) and thermoelectric (heat)

Uses of Electromagnetic Induction2. Transformer Used to increase or

decrease voltage/current Step-up or Step-down High voltage electricity is

used in long distance transmission

Low voltage is used in homes