Magnets and Magnetic Fields

Objectives Identify examples of magnetic forces in everyday life. 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.

Physics terms permanent magnet force field magnetize magnetic field magnetic poles magnetic field lines polarity declination magnetic force magnetic domains ferromagnetic material

Magnetism in everyday life In technologies: In financial settings: In industry: Magnets are used in hard drives, speakers, stereos or headphones. Credit and ATM cards store data on magnetized strips. Electric generators, junkyard lifters, and switches for alarm systems use magnets.

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.

The force decreases quickly when magnets are moved apart. 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.

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. Heating or melting a magnet destroys the magnetic effects. How else can you weaken a magnet? Dropping or striking it will also weaken a permanent magnet. Handle them with care. After students learn about magnetic fields, they can also understand that placing a magnet in an oppositely-directed magnetic field will also gradually weaken the magnet.

Are all materials magnetic? Most materials do NOT have noticeable magnetic properties. Paper is not magnetic. You need to use a magnet to hang a note on your refrigerator. The magnetic force between the magnet and the steel refrigerator passes right through the paper.

Are all materials magnetic? Magnetic materials: Iron, steel, nickel, and cobalt can be magnetized. Non-magnetic materials: Copper, wood, aluminum, paper and glass are examples of materials that cannot become magnetized.

Assessment Identify three objects that depend on magnetic forces for their operation.

Assessment Describe a fundamental difference between electricity and magnetism.

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 . . . 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 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.

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. A field diagram shows the magnetic field lines—or vectors.

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.

Magnetic field lines Magnetic field lines point in the direction of the force exerted on the north magnetic pole of a test magnet – such as a compass needle

About magnetic field lines Magnetic field lines point out of north poles and into south poles. The closer the field lines, the stronger the magnetic force. Field lines never cross. Magnetic 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?

Creating magnetic fields How are magnetic fields created? All magnetism is due to the motion of charged particles. In magnetic materials the field is generated by the spin and orbital motion of the electrons in the atoms.

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.

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 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.

Magnetic fields cancel for these randomly-oriented domains. 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. The size of a domain is a tenth of a millimeter or less in extent.

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.

Earth’s magnetic field The Earth has a magnetic field that has been vital to navigation for thousands of years. There isn’t a giant bar magnet buried inside the Earth, though. Earth’s magnetic field is probably created by electric currents within the Earth’s core.

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?

Do compasses point due north? No. There is a slight deviation between geographic north and magnetic north called the magnetic declination. A correction of a few degrees must be made to compass headings to align them to true geographic north. This correction differs in different parts of the world.

Earth’s changing magnetic field The Earth’s magnetic field also shifts over time. A century ago, magnetic north was around 70o N latitude. It is now at about 83o N latitude. Earth’s magnetic field also 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.

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.

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

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