MagnetsMagnets

Magnetic polesMagnetic polesMagnets have two poles with opposite polarity (north and south 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.

Magnetic polesMagnetic poles

Opposite poles attract:

Like poles repel:

The force decreases quickly when magnets are moved apart.

Attraction and repulsionAttraction and repulsion

Attractive and repulsive magnetic forces can also exert torques that cause a magnet to twist or rotate.

This concept is used in electric motors.

Torques due to magnetic forcesTorques due to magnetic forces

Permanent magnetsPermanent magnetsA permanent magnet retains its magnetic properties at all times.

A permanent magnet retains its magnetic properties at all times.

Some objects are only magnetized temporarily when magnets are close by.

Temporarily magnetized objectsTemporarily magnetized objects

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

Are there liquid magnets?Are there liquid magnets?

In Investigation 18A you will experiment with magnetic forces.

Can anything block a magnetic force?

InvestigationInvestigation

1. Place a free magnet and a test magnet on a ruler.

Part 1: Magnetic force between bar magnets

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.

InvestigationInvestigation

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?

Questions

InvestigationInvestigation

Magnetic fieldsMagnetic fields

What is a force field?What is a force field?

A force field is anorganization of energy inspace that creates a force

on any receptive matter within its influence.

Examples:• gravity fields• electric fields• magnetic fields

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.

What is a force field?What is a force field?

When you push somethinguphill you can feel it pushingback on you.

These forces are contactforces.

Contact forces involvethe direct interaction ofmatter.

Contact forcesContact forces

Non-contact forcesNon-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.

Compasses provide a way to detect the presence of a magnetic field.

Magnets and a compassMagnets and a compass

Magnetic field linesMagnetic field lines

Compasses provide a way to detect the presence of a magnetic field.

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 ).

About magnetic field linesAbout magnetic field lines

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.

Magnetic properties of atomsMagnetic properties of atoms

Domains in magnetic materialsDomains 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.

• In unmagnetized materials, the domains are randomly aligned – so there is no net magnetic field.

Magnetic domains are very small (1 μm to 100 μm).

Magnetic fields cancel for these randomly-oriented domains.

Domains in magnetic materialsDomains in magnetic 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.

Ferromagnetic materialsFerromagnetic materials

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!

Which pole is “north”?Which pole is “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?

Do compasses point due north?Do compasses point due north?

P 538#1-11, 38, 44, 47, 93

Homework/ClassworkHomework/Classwork