Magnetic fields

Objectives 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.

Assessment Which statements about force fields are true and which are false? Force fields only exist in science fiction. The Earth has a gravitational force field. Fields can travel at the speed of light. Force fields have no direction.

Assessment Draw or describe the direction of the magnetic field vectors at the locations on the diagram around a bar magnet. Describe why the Earth’s north geographic pole is the south magnetic pole—not north!—. Describe how the Earth’s magnetic field has changed historically.

Physics terms force field magnetic field magnetic field lines declination

What if . . .? What if the Sun suddenly ceased to exist? Would we know it instantly? Would the Earth immediately stop orbiting?

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? 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? Changes in the gravitational field travel at the speed of light. So if the Sun suddenly ceased to exist . . .

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. Point out that the Earth would continue to move, but in a straight line tangent to the orbit.

Contact forces When you push something uphill you can feel it pushing back on you. These forces are contact forces. Contact forces involve direct interaction of matter.

Non-contact 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.

Visualizing 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.

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

Investigation Click to open the Interactive simulation on page 514

Investigation Part 1: Trace a bar magnet’s field Launch the interactive simulation; click to select the bar magnet option. Pass your mouse over the diagram to reveal the magnetic field line vectors. 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.

Investigation Questions In the simulation, what does the direction and length of each arrow indicate? What is the strength and direction of the magnetic field at a location 5 cm to the right of the bar magnet’s middle? 5 cm to the left of middle? 5 cm above the middle?

Investigation Questions If you increase the strength of the bar magnet, what happens to the magnetic field lines? What is the meaning of the paths, from north to south magnetic pole, that you sketched out?

Investigation Part 2: Earth’s magnetic field In the interactive simulation; click to select the Earth’s magnetic field option. Pass your mouse over the diagram to reveal the magnetic field line vectors. Use the magnetic field lines to find Earth’s north and south magnetic poles.

Investigation Questions What is the strength of the Earth's magnetic field near one pole? the other pole? the equator? Where are Earth's north and south magnetic poles? How does the strength of Earth's magnetic field compare to the strength of the magnetic field 5 cm away from the bar magnet?

Investigation Part 3: Tracing a bar magnet’s lines Place a bar magnet underneath a piece of poster board. Use iron filings on top of the poster board to trace out the field lines. Can you identify the location of the two poles of the magnet? Can you distinguish north from south?

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

Magnetic force field

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

About magnetic field lines Magnetic field lines point into south poles and out of north poles. The closer the field lines, the stronger the magnetic force. Field lines never cross. Field lines make closed loops. ( lines passing through the magnet are not shown)

Test your knowledge Where is the magnetic field the strongest? Where is it weakest?

Test 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.

Earth’s magnetic field 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.

Which pole is “north”? 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.

Do compasses point due 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.

Earth’s changing magnetic field 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. Page 513 of the e-Book contains a video of the Earth’s magnetic field changing over time.

Creating magnetic fields 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.

Magnetic properties of atoms 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 domains Magnetic domains are very small (1μm to 100 μm). Domains in unmagnetized materials are randomly aligned and cancel each other. The size of a domain is a tenth of a millimeter or less in extent.

Domains in a magnet In magnets, the domains are slightly more aligned in one direction than in others. That alignment creates a net magnetic field.

Ferromagnetic materials 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.

Assessment Which statements about force fields are true and which are false? Force fields only exist in science fiction. The Earth has a gravitational force field. Fields can travel at the speed of light. Force fields have no direction.

Assessment Which statements about force fields are true and which are false? Force fields only exist in science fiction. False The Earth has a gravitational force field. True Fields can travel at the speed of light. True Force fields have no direction. False

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

Assessment 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!

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

Assessment 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.

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

Assessment 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.