Concept Summary

Magnetic PolesMagnetic forces are produced by

magnetic poles.Every magnet has both a North and

South pole.Like poles repel, unlike poles attract.

MonopolesMonopole: piece of a magnet that is

simply a north pole or a south pole

No matter how we break a magnet, the pieces are always dipoles!

A monopole cannot be isolated.

Magnetic Fields Magnetic fields transmit magnetic forces. Direction of the field is from N to S. Field is stronger where field lines are closer. Unit of magnetic field strength is the Tesla.

The older unit Gauss is sometimes used. Earth’s magnetic field strength is about 10-4 Tesla

or about 1 Gauss

What Causes a Magnetic Field?

Magnetic fields are produced by moving electric charges.

Electrons in atoms both orbit and “spin”. In most materials, electron spin

contributes more to magnetism than electron orbital motion.

Electrons are (very) tiny magnets.

What Causes a Magnetic Field?

Electrons with opposite spins cancel each other’s magnetic fields.

Electrons with spins aligned strengthen each other’s magnetic fields.

Iron, nickel, cobalt (and a few other elements) commonly have some aligned electrons.

An iron atom is a (very) tiny magnet.

Magnetic DomainsA region in which many atoms have

their magnetic fields aligned is called a magnetic domain.

How Magnets AttractA magnet near an unmagnetized piece

of iron causes: Growth of aligned domains in the iron Rotation of domains to align with the

magnetic field Attractive magnetic force on the iron

This causes the iron to become temporarily magnetized

Types of MagnetsPermanent Magnets

Once a magnet, always a magnet

Temporary Magnets Only temporary!!! Not always a magnet

Making a MagnetYou can make a magnet by:

Placing a magnetic material like iron in a strong magnetic field

Rubbing a magnetic material like iron with a strong magnet

Electric Currents & MagnetismSince moving charges create magnetic

fields, an electric current creates a magnetic field.

A coil of wire can concentrate the magnetic field and create an electromagnet.

Magnetic Forces on ChargesA static electric charge does not “feel” a

magnetic field. No magnetic force is exerted on it.

If an electric charge moves, it generates its own magnetic field, which interacts with the original magnetic field, so:

A magnetic field exerts a force on a moving electric charge.

MetersAn electric meter is a device that uses

an electric current to exert a force on a magnet.

MotorsAn electric motor uses a magnet to

exert a force on a current-carrying coil of wire.

An electric motor uses brushes and an armature to reverse the flow of current so that the coil of wire can rotate 360o.

The Earth as a MagnetEarth itself is a magnet.N and S poles do not correspond

exactly to the geographic poles. The discrepancy is called magnetic declination.

Strength of Earth’s field varies with time.N/S Poles have switched in the past.

The End