MAGNETISM
SPH3U
Permanent Magnets
• A permanent magnet has two poles: North and South.
• Like poles repel. Unlike poles attract.
• These repulsive or attractive forces can act at a distance
(no contact is required). The region in space over which
these forces can act is called a magnetic field.
No Monopoles
• Every magnet is a dipole: it must have two poles. If a
dipole magnet is broken in two, it becomes two dipoles:
Many Dipoles
Why does this happen?
A bar magnet is made up of
many smaller dipoles, each
with North and South poles,
all aligned.
The dipoles may be knocked
out of alignment by heating
or otherwise abusing the
material.
Repairing Magnets
• A bar magnet may be re-magnetized by placing it in a
magnetic field. This is induced magnetism.
Magnetic Fields
• A magnet that is moved in space near a second magnet
experiences a magnetic field.
• A magnetic field can be represented by field lines.
• The iron filings in the picture below show the magnetic
field lines.
• The strength of the
magnetic field is greater
where the lines are
close together and
weaker where they are
farther apart.
Magnetic Fields
• Magnetic lines are drawn out of the North Pole and into
the South Pole but they don’t stop and start there: the
magnetic field lines are drawn through the poles.
• By convention, the lines proceed from S to N inside a
magnet and from N to S outside a magnet, forming closed
loops
• The lines do not cross one another.
Magnetic Poles
• At the poles, the magnetic fields lines are closer together.
Direction of Magnetic Field Lines
• These lines are a map of the magnetic field around a bar
magnet. The needle of a magnetic compass will follow the
lines, with the north end showing the direction of the field.
The Source of Magnetic Fields
• Permanent Magnets:
• Moving electrons produce magnetic fields.
• In most materials these magnetic fields cancel one another and
neutralize the overall magnetic effect.
• In other materials such as iron, cobalt, and nickel, the atoms
behave as tiny magnets because of certain orientations of the
electrons inside the atom.
• Atoms themselves have magnetic properties due to the
spin of the atom’s electrons.
• Groups of atoms join so that their magnetic fields are all
going in the same direction
• These areas of atoms are called “domains”.
What are Magnetic Domains?
• Magnetic substances like iron, cobalt, and nickel are composed
of small areas where the groups of atoms are aligned like the
poles of a magnet. These regions are called domains. All of the
domains of a magnetic substance tend to align themselves in
the same direction when placed in a magnetic field. These
domains are typically composed of billions of atoms.
Induced Magnetism
• Each “domain” in this first image has randomly distributed
magnetic fields that cancel each other out.
• When an unmagnetized substance is placed in a
magnetic field, the substance can become magnetized.
• This happens when the spinning electrons line up in the
same direction.
The Earth is a Big Magnet
• The Earth’s magnetic field is thought to originate with
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moving charges.
The core is probably composed of iron and nickel, which
flows as the Earth rotates, creating electrical currents that
result in the Earth’s magnetic field.
Note that the magnetic north pole
and the geographic North Pole are
not in the same place.
The magnetic north pole acts as
if the south pole of a huge bar
magnet were inside the earth.
You know that it must be a magnetic
south pole since the north end of a
magnetic compass is attracted to it
and opposite poles attract.
When a charged
particle enters a
magnetic field, an
electric force is
exerted on it. If a
charged particle
moves at an angle to
a magnetic field, the
magnetic force
acting on it will
cause it to move in a
spiral around the
magnetic field lines.
The solar wind is constantly bombarding
the Earth’s magnetic field. Sometimes
these charged particles penetrate that
field. These particles are found in two
large regions known as the Van Allen
Belts.
The Earth’s magnetic field extends far into
space. It is called the “magnetosphere.”
When the magnetic particles from the sun, called
“solar wind”, strike this magnetosphere, we see a
phenomenon called…
MAGNETIC FIELDS
PRODUCED BY
CONDUCTORS
SPH3U/SPH4C
Oersted’s Discovery
• In 1819, the Danish
physicist Hans Christian
Oersted (1777-1851)
discovered the connection
between electricity and
magnetism by accident
while lecturing at the
University of Copenhagen.
He noticed that a compass
needle placed closely to a
current carrying wire would
take up a position nearly
perpendicular to the
direction of the current.
Showing Directions
To show that a current, field line, or force is directed out of
the page (towards us), we draw:
To show that a current or field line is directed into the page,
we draw:
Principle of Electromagnetism
• Whenever an electric current moves through a conductor,
a magnetic field is created in the region around the
conductor.
• The magnetic field lines for a straight
conductor are concentric circles around the
conductor.
Right-Hand Rule #1
• When the thumb is pointed in the direction of conventional
current flow, the fingers curl in the direction of the
magnetic field.
Parallel Wires
Electromagnets
• A device that exerts a magnetic force using electricity.
• The magnetic field around a straight conductor can be
intensified by bending the wire into a loop.
Coil or Solenoid
• The magnetic field can be further intensified by combining
the effects of a large number of loops would close
together to form a coil, or solenoid.
How is the scrap metal held up by the
crane?
Right-Hand Rule #2
• If a coil is grasped in the right hand with the curled fingers
representing the direction of electric current, the thumb
points in the direction of the magnetic field inside the coil.
Parallel Coils
Factors Affecting the Magnetic Field of a
Coil
• Current in the Coil
• The more current, the greater the concentration of magnetic field
lines in the core.
• Number of Coils
• The more loops, the stronger the magnetic field since the magnetic
field is the sum of the field of each loop.
• Type of Core Material
• The core of a coil can greatly affect the coil’s magnetic field
strength.
• A core of iron will increase the strength compared to that of air.
Type of Core Material
• The core material becomes an induced magnet, as its
atomic dipoles align with the magnetic field of the coil.
The core itself becomes an induced magnet.
• Not all materials may be easily magnetized. Those that
can are called ferromagnetic. They include iron ore
(lodestone), cobalt, and nickel.
• The strongest permanent magnets are made from rare
earth (lanthanoid) elements, the strongest of these being
neodymium-iron-boron (NIB) magnets, now greatly
reduced in price and used in children’s toys.
Type of Core Material
• Ferromagnetism
• Materials that become strong induced magnets when placed in a
coil.
• Iron, nickel, cobalt, and their alloys.
• Paramagnetism
• Materials that magnetize slightly when placed in a coil and increase
the field strength by a barely measurable amount.
• Oxygen and aluminum.
• Diamagnetism
• Materials that cause a very slight decrease in the magnetic field of
a coil.
• Copper, silver, and water.
Applications of Electromagnetism
Magnetic Forces
• Since current will produce a magnetic field, the interaction
of this field with an external magnetic field will result in a
force acting on the moving charge. This is the Motor
Principle.