Transcript Magnets

Magnets
Magnetic poles
Magnets have two poles with opposite
polarity (north and south poles).
Magnetic poles
Magnets have two poles with opposite
polarity (north and south poles).
This is a basic difference between
electricity and magnetism:
•You CAN separate positive and
negative electric charges.
•You CAN’T separate a magnetic north
pole from its south pole.
Attraction and repulsion
Opposite poles attract:
Like poles repel:
The force decreases quickly when magnets are moved apart.
Torques due to magnetic forces
Attractive and repulsive magnetic
forces can also exert torques that
cause a magnet to twist or rotate.
This concept is used in electric
motors.
Permanent magnets
A permanent magnet retains its
magnetic properties at all times.
Temporarily magnetized objects
A permanent magnet retains its
magnetic properties at all times.
Some objects are only magnetized
temporarily when magnets are close by.
Are there liquid magnets?
No. All permanent magnets are solid.
• Magnetic effects come from the
small-scale organization of atoms
within the material.
• Destroy or damage a magnet by:
Heating or melting
Dropping or striking
Investigation
In Investigation 18A
you will experiment
with magnetic forces.
Can anything block a
magnetic force?
Investigation
Part 1: Magnetic force between bar magnets
1. Place a free magnet and a test magnet on a ruler.
2. Slowly move the test magnet closer until the
free magnet begins to move. Measure this
distance.
3. Repeat for different combinations of poles:
north-south; south-south; and north-north.
4. Connect five small magnets together. Repeat
these tests using this test magnet.
Investigation
Questions
a. What is the distance over which the magnetic
force between these magnets acts?
b. Does the combination of five magnets
create a stronger magnetic force?
c. Why does the free magnet only start to move
suddenly? Hint: does the free magnet have
to overcome a different force?
Magnetic fields
What is a force field?
A force field is an
organization of energy in
space that creates a force
on any receptive matter
within its influence.
Examples:
• gravity fields
• electric fields
• magnetic fields
What is a force field?
Changes in the gravitational
field travel at the speed of
light.
So if the Sun suddenly
ceased to exist . . .
it would take 8.3 minutes for
us to see it disappear AND
for the Earth to stop orbiting.
Contact forces
When you push something
uphill you can feel it pushing
back on you.
These forces are contact
forces.
Contact forces involve
the direct interaction of
matter.
Non-contact forces
Field forces can act through empty
space with NO DIRECT CONTACT
between objects.
Gravity forces, electric forces, and
magnetic forces are all field forces.
In the example shown:
1) Earth creates a gravitational field.
2) The field interacts with the satellite.
Magnets and a compass
Compasses provide a way to detect the presence of a magnetic field.
Magnetic field lines
Compasses provide a way to detect the presence of a magnetic field.
About magnetic field lines
1. Magnetic field lines point out of
north poles and into south poles.
2. The closer the field lines, the
stronger the magnetic force.
3. Field lines never cross.
4. Magnetic field lines make closed
loops ( lines passing through the
magnet are not shown ).
Magnetic properties of atoms
Each atom in the material acts like a little
magnet, creating its own magnetic field.
The atoms self-organize into domains—
regions in which the magnetic fields of
the atoms are aligned.
Domains in magnetic materials
Magnetic domains are very
small (1 μm to 100 μm).
• In magnetized materials, the
domains are slightly more
aligned in one direction than
in others–creating a net
magnetic field.
Magnetic fields of these domains
combine to produce a net field.
Domains in magnetic materials
Magnetic domains are very
small (1 μm to 100 μm).
• In unmagnetized materials,
the domains are randomly
aligned – so there is no net
magnetic field.
Magnetic fields cancel for these
randomly-oriented domains.
Ferromagnetic materials
Iron, nickel and magnetite are
ferromagnetic.
When a magnet is nearby, many
of the magnetic domains align
with the external field.
Domains aligned with the magnet
increase in size; those that point
the other way shrink, creating a
strong magnet.
Which pole is “north”?
The north magnetic pole of a
compass is attracted to Earth’s
north geographic pole.
•The geographic north pole must
be a magnetic south pole!
The north magnetic pole of a
compass is repelled by Earth’s
geographic south pole.
•The geographic south pole must
be a magnetic north pole!
Do compasses point due north?
Earth’s geographic north pole
aligns with the planet’s axis of
rotation.
Earth’s magnetic “north” pole
is the place that attracts the
north end of a compass needle.
Are these two places exactly
the same?
Homework/Classwork
P 538
#1-11, 38, 44, 47, 93