Magnetic fieldsMagnetic fields

ObjectivesObjectives

• Describe the concept of a force field.

• Draw and interpret magnetic field diagrams.

• Describe basic properties of the Earth’s magnetic field, including its direction.

1. Which statements about force fields are true and which are false?

AssessmentAssessment

a. Force fields only exist in science fiction.

b. The Earth has a gravitational force field.

c. Fields can travel at the speed of light.

d. Force fields have no direction.

AssessmentAssessment

2. Draw or describe the direction of the magnetic field vectors at the locations on the diagram around a bar magnet.

3. Describe why the Earth’s north geographic pole is the south magnetic pole—not north!—.

4. Describe how the Earth’s magnetic field has changed historically.

Physics termsPhysics terms

• force field

• magnetic field

• magnetic field lines

• declination

What if . . .?What if . . .?

Would we know it instantly?

Would the Earth immediately stop orbiting?

What if the Sun suddenly ceased to exist?

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

on any receptive matter within its influence.

Examples:gravity fieldselectric fieldsmagnetic fields

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

The Sun doesn’t act on the Earth directly. Instead, the Sun creates a gravitational field in space.

The field acts on the Earth to hold it in its orbit.

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

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

Changes in the gravitational field travel at the speed of light.

So if the Sun suddenly ceased to exist . . .

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 involvedirect interaction ofmatter.

Contact forcesContact forces

Field forces can act through empty space without ANY DIRECT contact between objects.

Gravity forces, electric forces and magnetic forces are non-contact forces.

In the example shown: 1) Earth creates a gravitational field.2) The field interacts with the satellite.

Non-contact forcesNon-contact forces

Visualizing magnetic fieldsVisualizing magnetic fields

What does a magnetic field look like?

Fields are invisible, and exist at all points in space. They can be represented with field diagrams. These diagrams show the magnetic field lines or vectors.

What does a magnetic field look like?

Fields are invisible, and exist at all points in space. They can be represented with field diagrams. These diagrams show the magnetic field lines or vectors.

In this investigation, you will explore the magnetic fields around a bar magnet and around the Earth.

Visualizing magnetic fieldsVisualizing magnetic fields

Click to open the Interactive simulation on page 514

InvestigationInvestigation

Part 1: Trace a bar magnet’s field

1. Launch the interactive simulation; click to select the bar magnet option.

1. Pass your mouse over the diagram to reveal the magnetic field line vectors.

1. Find a few positions near to the north pole and follow the field arrows all the way to the south pole. Sketch the paths you followed.

InvestigationInvestigation

a. In the simulation, what does the direction and length of each arrow indicate?

b. What is the strength and direction of the magnetic field at a location

InvestigationInvestigationQuestions

o 5 cm to the right of the bar magnet’s middle?

o 5 cm to the left of middle?o 5 cm above the middle?

InvestigationInvestigationQuestions

c. If you increase the strength of the bar magnet, what happens to the magnetic field lines?

d. What is the meaning of the paths, from north to south magnetic pole, that you sketched out?

1. In the interactive simulation; click to select the Earth’s magnetic field option.

1. Pass your mouse over the diagram to reveal the magnetic field line vectors.

1. Use the magnetic field lines to find Earth’s north and south magnetic poles.

InvestigationInvestigationPart 2: Earth’s magnetic field

a. What is the strength of the Earth's magnetic field near one pole? the other pole? the equator?

b. Where are Earth's north and south magnetic poles?

c. How does the strength of Earth's magnetic field compare to the strength of the magnetic field 5 cm away from the bar magnet?

InvestigationInvestigationQuestions

Part 3: Tracing a bar magnet’s lines

InvestigationInvestigation

1. Place a bar magnet underneath a piece of poster board.

2. Use iron filings on top of the poster board to trace out the field lines.

3. Can you identify the location of the two poles of the magnet?

4. Can you distinguish north from south?

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

Magnets and a compassMagnets and a compass

Magnetic force fieldMagnetic force field

Field lines represent the force exerted on the north magnetic pole of a test magnet (the compass needle)

Magnetic force fieldMagnetic force field

1. Magnetic field lines point into south poles and out of north poles.

1. The closer the field lines, the stronger the magnetic force.

1. Field lines never cross.

1. Field lines make closed loops. ( lines passing through the magnet are not shown)

About magnetic field linesAbout magnetic field lines

Where is the magnetic field the strongest?

Where is it weakest?

Test your knowledgeTest your knowledge

Where is the magnetic field the strongest?

It is strongest at the poles, and inside the magnet.

Where is it weakest?

It is weakest far from the magnet where the field lines appear widely spaced.

Test your knowledgeTest your knowledge

Earth's magnetic field is why compasses point north.

Earths magnetic field has been vital to navigation for thousands of years.

Humans made use of Earth’s magnetic field long before scientists could explain the forces between the compass and the Earth.

Earth’s magnetic fieldEarth’s magnetic field

The north magnetic pole of a compass points toward Earth’s north geographic pole.

•The geographic north pole must be a magnetic south pole.

The north magnetic pole of a compass points away from Earth’s geographic south pole.

•The geographic south pole must be a magnetic north pole.

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

Magnetic north is a few degrees off axis. The deviation between geographic north and magnetic north is called magnetic declination.

A correction must be made to compass headings to align them to true north.

The correction differs in different parts of the world.

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

A century ago, magnetic north was around 70o N latitude. It is now at about 83o N latitude.

Earth’s magnetic field completely reverses itself every million years or so.

Earth’s changing magnetic fieldEarth’s changing magnetic field

How are magnetic fields created?

•All magnetism is due to the motion of charged particles.

•In magnetic materials, the magnetic field is generated by the spin and orbital motion of the electrons in the atoms.

•Each atom in the material acts like a little magnet, creating its own magnetic field.

Creating magnetic fieldsCreating magnetic fields

Each atom in the material acts like a little magnet, creating its own magnetic field.

These atoms self-organize into domains—regions in which the magnetic fields of the atoms are aligned.

Magnetic properties of atomsMagnetic properties of atoms

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

Domains in unmagnetized materials are randomly aligned and cancel each other.

Magnetic domainsMagnetic domains

In magnets, the domains are slightlymore aligned in one direction than in others.

That alignment creates a net magnetic field.

Domains in a magnetDomains in a magnet

Iron, nickel and magnetite are ferromagnetic.

With a magnet near, 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

1. Which statements about force fields are true and which are false?

AssessmentAssessment

a. Force fields only exist in science fiction.

b. The Earth has a gravitational force field.

c. Fields can travel at the speed of light.

d. Force fields have no direction.

1. Which statements about force fields are true and which are false?

AssessmentAssessment

a. Force fields only exist in science fiction. False

b. The Earth has a gravitational force field. True

c. Fields can travel at the speed of light. True

d. Force fields have no direction. False

AssessmentAssessment

2. Draw or describe the direction of the magnetic field vectors at the locations on the diagram around a bar magnet.

AssessmentAssessment

2. Draw or describe the direction of the magnetic field vectors at the locations on the diagram around a bar magnet.

Magnetic force vectors point towards the magnet’s south magnetic pole and away from the north magnetic pole.

The curves traced by the filings can help you orient the field vectors!

AssessmentAssessment

3. Describe why the Earth’s north geographic pole is the south magnetic pole—not north!—.

3. Describe why the Earth’s north geographic pole is the south magnetic pole—not north!—.

The direction of the magnetic field is the direction of the force on the north pole of a magnetic compass needle.

The Earth’s north geographic pole attracts the north pole of a compass needle. This means it must actually be a south magnetic pole.

AssessmentAssessment

4. Describe how the Earth’s magnetic field has changed historically.

AssessmentAssessment

4. Describe how the Earth’s magnetic field has changed historically.

The Earth’s magnetic field is slowly changing. A century ago magnetic north was at about 70o N latitude and it is now around 83o N latitude.

From the geographical record we also know that Earth’s magnetic field completely reverses itself every million years or so.

AssessmentAssessment