# Concept Summary Batesville High School Physics. Magnetic Poles  Magnetic forces are produced by magnetic poles.  Every magnet has both a North and South.

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Concept Summary Batesville High School Physics

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

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 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

Making a Magnet  You can make a magnet by:  Placing a magnetic material like iron in a strong magnetic field  Stroking 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.

Direction of the Force  The direction of the magnetic force on a moving charge is perpendicular to the magnetic field, and also perpendicular to the velocity of the particle!

Magnetic Force on a Moving Charge  If:  F = Magnetic force (in Newtons)  q = amount of charge (in Coulombs)  v = Velocity of the charge  B = Strength of the magnetic field (in Tesla)   = Angle between B and v   Then:

Magnetic Force on a Moving Charge  F = qvBsin   and the direction of F is perpendicular to both v and B

Magnetic Forces on Electric Currents  If a magnetic field exerts a force on a moving charge, a magnetic field must exert a force on a current-carrying wire.

Magnetic Forces on Electric Currents  A current-carrying wire in a magnetic field will deflect in a direction perpendicular to the magnetic field and the direction of the charges.

Meters  An electric meter is a device that uses an electric current to exert a force on a magnet.  Alternatively, a magnet can be used to deflect a current-carrying wire.

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 

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