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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
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.
 A static
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 360o.
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
