Download Magnetism

What is a Magnet?
 Magnets
are made from a very small
handful of chemical elements who have
special magnetic properties
 Iron, Cobalt and Nickel are the three
elements that are normally used to
make magnets- all are metals close to
each other on Periodic Table
 Some “rare earth” elements can make
very powerful magnets- Neodymium is
one commonly used
Magnets in History
 Magnets
named after area of Turkey
called Magnesia in ancient times
 Strange “magic” stone that stuck to iron;
regarded as curiosity by Greeks
 Used by Chinese as first compasses for
ship navigation (shaped piece of
magnetite floated in bowl of water) or on
specially marked boards
 Suspended magnetized needle used in
modern compasses
What is a Magnet ctd…
A
magnet creates a magnetic field
around it that can exert a force
 Magnetic forces take the form of loops
that curve from one side of a magnet to
the other, like a closed electric circuit
 No-one really knows what a magnetic
field actually is- proposed to be
distortions in space-time itself
Magnetic Poles
 The
loops of magnetic forces are
produced by magnetic poles, one on
each side of a magnet.
 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.
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 rare
earth 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.
 A typical magnet that has many
domains with their atoms all aligned in
the same direction creates a strong
magnetic field; if there are fewer
domains, the magnet is weaker
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
 In
both cases, the magnetic field physically
“turns around” the iron atoms so they all
spin in the same direction, including the
spin of their electrons
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.
 Magnetism and electricity are really just
different aspects of the same force- you
cannot have one without the other!!
Making an Electromagnet

A battery and coil of wire creates a magnetic
field- as the electrons move they create a
“sleeve” of magnetic force around the wire
 You can make the electromagnet stronger in
3 ways:
 Increase the current (moving charges)
 Increase the nmber of loops in the coil
 Add an iron, cobalt or nickel core or else
increase its size if one is already present
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. This means you can use magnetic fields to
“steer” electrons in different directions

This is how an old TV works- a shaped, modulated
electron beam is aimed at a screen to make a
desired pattern that changes and moves. This
produces the picture that you see
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.
 One magnet repels the armature half a turn,
the other attracts half a turn to make it move.
This happens fast- armature spins very
quickly to create mechanical motion
Generators
 If
you rotate a magnet in a coil of wire, it
induces a current to flow through the
wire- electromagnetic induction
 Used for generators, which turn
mechanical movement into electric
power
 Used in car engines to recharge a
battery by reversing the chemical
reaction inside the battery
The Earth as a Magnet
 Earth
itself is a huge magnet. The outer
core is molten iron and nickel; spins
inside the Earth and produces
magnetism
 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 Earth as a Magnet
The Earth’s magnetic field is powerful
enough to deflect high energy particles
streaming out from the Sun towards us
(solar wind)
 Interaction of solar wind with magnetic
field produces light in a hazy, beautifully
colored display
 Called Aurora Borealis- Northern Lights
