Download Chapter 10 Magnets Notes

Chapter 10
Magnets
All magnets have the following
common properties:
Magnets always have two opposite “poles,”
called north and south.
If divided, each part of a magnet has both
north and south poles; we never see an
unpaired north or south pole.
When near each other, magnets exert
magnetic forces on each other.
The forces between magnets depend on the
alignment of the poles; two unlike poles will
attract each other and two like poles will repel
each other.
A compass needle is a magnet that
is free to spin until it lines up in the
north-south direction. The origin of
the terms “north pole” and “south
pole” of a magnet come from the
direction that a magnetized compass
needle points. The end of the
magnet that pointed north was called
the north pole of the magnet and the
end that pointed south was called the
south pole.
History of
Magnetism
The magnetic force depended on the direction and
orientation of the two magnets and also on the
distance between them. The model of a magnetic
field was developed to describe how a magnet
exerts magnetic force.
 First, every magnet creates an energy
field, called the magnetic field, in the
space around it.
 Second, the field exerts forces on any
other magnet that is within its range.
Electromagnets = A magnet that is
created by electric current. This type of
magnet is called an electromagnet.
The right hand rule: When your
fingers curl in the direction of current,
your thumb points toward the magnet’s
north pole.
Wrap the wire in many turns
around the nail and connect
a battery. When current
flows in the wire, the nail
becomes a magnet. Use the
right hand rule to figure out
which end of the nail is the
north pole and which is the
south pole. To reverse north
and south, reverse the
connection to the battery,
making the current flow the
opposite way.
As expected the more current would make
an electromagnet stronger.
There are two ways to increase the current.
 apply more voltage by adding a second battery.
 add more turns of wire around the nail.
The second method works because the magnetism in
the electromagnet comes from the total amount of
current flowing around the nail. If there is 1 amp of
current in the wire, each loop of wire adds 1 amp to
the total amount that flows around the nail. Ten
loops of 1 amp each make 10 total amps flowing
around. By adding more turns, the same current is
used over & over to get stronger magnetism.
Of course, nothing comes for free. By adding
more turns the resistance of the coil is also
increased. Increasing the resistance makes
the current a little lower & generates more
heat.
A good electromagnet is a balance between too
much resistance and having enough turns to get a
strong enough magnet.
The magnetic force exerted by an
electromagnet depends on three factors:
 The amount of electric current in the
wire.
 The amount of iron or steel in the
electromagnet’s core.
 The number of turns in the coil.
Electric motors convert electrical
energy into mechanical energy.
Permanent magnets and
electromagnets can
work together to make
electric motors and
generators. The secret
is in the ability of an
electromagnet to
reverse from north to
south. By changing the
direction of electric
current, the
electromagnet changes
from attract to repel,
and spins the motor.
All electric motors must have three
things to work:
A rotating element (rotor) with magnets.
2. A stationary magnet that surrounds the rotor.
3. A commutator that switches the
electromagnets from north to south at the right
place to keep the rotor spinning.
1.
The electromagnet must switch from north to
south as each rotor magnet passes by to keep
the rotor turning. The switch that makes this
happen is called a commutator.
Both electrical force and magnetic force
exist between electric charges. Scientists
now believe both forces are two aspects of
one force, the electromagnetic force.
A current through a wire creates a magnet. The
reverse is also true: If a magnet is moved
through a coil of wire, then electric current is
created.
This process is called electromagnetic induction
because a moving magnet induces electric
current to flow.
When a magnet moves into a coil of wire, it induces
electric current to flow in the coil. The current stops if
the magnet stops moving. If the magnet is pulled
back out again, the current flows in the opposite
direction. A changing magnetic field is what makes
the electricity flow.
Electromagnetic induction enables us to
transform mechanical energy (moving magnets)
into electrical energy. Any machine that causes
magnets to move past wire coils generates electric
currents. These machines include giant electric
power plants and computer disk drives.
Power plants use electromagnetic
induction to create electricity. A generator
is a combination of mechanical and electrical
systems that converts kinetic energy into
electrical energy.
A power plant generator
contains a turbine that
turns magnets inside
loops of wire, generating
electricity. In the top
sketch the north pole on
the disk induces a south
pole in the
electromagnet, causing
current to flow one way.
When the disk rotates,
the magnetism in the coil
is reversed, and the
electric current
generated also reverses.