MAGNETISM

Magnetism

• Lodestones: Natural magnets found in Magnesia, Greece.

• Often the subject of curiosity and eventually were used as devices for navigation.

Magnetism

• Coulomb:• Charles Coulomb studied lodestones and the

forces between them• They contain pieces of “iron ore” called

magnetite

• Hans Christian Oersted was the first man to discover the relationship between magnetism and electric current.

Poles

Magnetic Poles

• Every magnet has a north and a south pole• How are poles similar to electric charges? Like poles repel Unlike poles attract Magnetic strength

• How are they different? Poles MUST come in pairs Breaking a magnet?

Breaking a Magnet• Unlike charges, magnetic

poles ALWAYS come in pairs

Magnetic Poles

• North-Seeking

• South-Seeking

What have Natural Poles?

• Metals Like iron, nickel and cobalt– Due to a net number of electrons spinning in the

same direction, these metals have natural magnetic dipole moments.

– Iron is the strongest

AND…

Because of this we can also say that atoms are tiny tiny magnets!

Why does Magnetism Occur

• Movement of charges cause magnetism

• Atoms are like magnets due to opposite spin of electrons

• Most atoms are neutral because of an equal amount of opposing spin

Why does Magnetism Occur

• Some atoms have a net spin so they are magnets (iron, nickel, cobalt)

• The spin tends to dictate magnetism more than the orbital motion

Magnetic Domains

• Clusters of aligned atoms

• These are like tiny magnets

• When all of the domains line up… you get a magnet

Field lines

• Just like with electric charges, field lines can also be drawn for magnets.

• A great way to actually see field lines is to use iron filings.

•Also, just like with electric charge, strength of field depends inversely on the distance to the pole

Planets!

-Earth, as well as other planets, have natural magnetic fields.

-This can be noticed by the fact that for hundreds of years people have used compasses to navigate.

-People, however, might take the earth’s magnetic field for granted… what's the point?

Earths Magnetic FieldThe sun constantly bombards the earth with dangerous charged particles.

Without the Earth’s Magnetic field, we would constantly be exposed to these particles… they would kill us…

BUT the earth’s field spins these particles to our poles along it’s magnetic field lines… protecting us!

Not only that… from this we also get…

Earth’s Magnetic Field• Auroras!

Also happens at the south pole

(Aurora Australis)

Also called the northern lights

(Aurora Borealis)

Aurora• This occurs when the charged particles

make contact with the atmosphere.

• This happens on other planets as well!

Aurora on Saturn

Back to Earth’s Magnetic Field:

• Why does the magnetic field on earth occur?

• The most common theory as to why Earth has a magnetic field is the fact that Earth is not a solid rock, but it has flowing currents of magma underneath its surface.

Earth’s Magnetic Field

• Convection currents in the Earth make the magnetic field.

• Also, as the magma sloshes back and forth, the magnetic field switch direction over long periods of time.

• This has happened more than 20 times in the past 5,000,000 years

Evidence?• The Mid Atlantic Ridge:

– Stripes in the rock along the ridge show how magnetic domains were oriented when the magma rose form the Earth’s core.

Creating Magnets• If all the magnetic domains in a material line up to

form a magnet, there are a number of ways to create a permanent magnet.

1. By placing non-magnetized metals against or near magnets, the domains in the non-magnetized material will line up and a net magnetic moment

will form:

Creating Magnets

2. By rubbing a material with unaligned domains with a magnet, the domains will align themselves, forming a magnet.

3. One of the best ways is to heat the material (like the magma from the mid-atlantic ridge) and then pass it through a magnetic field. When the material cools, its domains will have settled in an aligned fashion.

Practical Uses

• Compass

• Motors/ Generators

• Meters

• Maglev Trains

Magnetism and Electricity• When charges flow through a wire they

also generate a magnetic field.

Changing the direction of the current changesthe direction of the field.

Electromagnets• An electromagnet can be

made by coiling a wire and then passing a current through the wire. When coiled around a bar of iron this works even better since it aligns the domains in the iron.

Uses for electromagnets

• Maglev Trains, MRI, Construction, Particle accelerators, etc.

Charges and Magnetic Fields• If a charge moves in

relation to a magnetic field, the field will exert a force on the charge (this is a perpendicular force).

Forces Continued

• A current-carrying wire is also deflected by a magnetic field.

•Direction of current determines direction of force.•Force is a maximum when current is perpendicular to field lines.

Galvanometers• simple meter

• Magnetic needle in a several loops of wire

• current causes needle to deflect

• magnitude of current can be measured

Other Meters

• A Galvanometer is a simple ammeter.

• These work in a similar way.

DC Motor

• Motors us these principles to work as well