Concept Summary

Magnetic Poles  Magnetic forces are produced by magnetic poles.  Every magnet has both a North and South pole.  Like poles repel, unlike poles attract.

Monopoles  Monopole: 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 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 Domains  A region in which many atoms have their magnetic fields aligned is called a magnetic domain.

How Magnets Attract  A 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 Magnets  Permanent Magnets  Once a magnet, always a magnet  Temporary Magnets  Only temporary!!! Not always a magnet

Making a Magnet  You 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 & Magnetism  Since 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 Charges  A 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.

Meters  An electric meter is a device that uses an electric current to exert a force on a magnet.

Motors  An 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 360 o.

The Earth as a Magnet  Earth 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 