Magnetism Section 1

Section 1: Magnets and Magnetic Fields

Preview

• Key Ideas

• Bellringer

• Magnets

• Magnetic Fields

• Earth’s Magnetic Field

Magnetism Section 1

Key Ideas

〉What happens when the poles of two magnets

are brought close together?

〉What causes a magnet to attract or repel

another magnet?

〉How is Earth’s magnetic field oriented?

Magnetism Section 1

Bellringer

1. Most of us use magnets in our daily life without

thinking about it. Name five places that you use

magnets.

2. If you had two bar magnets and you touched the end

of one bar magnet to the end of the other bar magnet,

what are the two possible outcomes? If you rotated

one of the bar magnets 180°, what are the two

possible outcomes?

3. Explain why a compass is a useful tool for navigation.

Magnetism Section 1

Bellringer, continued

4. On Earth, there is the magnetic N pole and the geographic North Pole. According to the picture above, are they located at the same geographic location? Where do you think the magnetic S pole is located?

Magnetism Section 1

Magnets

〉What happens when the poles of two magnets are brought close together?

〉Two like poles repel each other. Two unlike poles attract each other.

• magnetic pole: one of two points, such as the ends of a magnet, that have opposing magnetic qualities

– All magnets have at least one pair of poles, a north pole and a south pole.

– It is impossible to isolate a magnet’s south pole from the magnet’s north pole.

Magnetism Section 1

Magnets, continued

• Some materials can be made into permanent magnets.

• A magnetized piece of iron is called a “permanent”

magnet, but its magnetism can be weakened or

even removed.• Iron is a soft magnetic material.

• It is easily magnetized.• It tends to lose its magnetic properties easily.

• Cobalt is a hard magnetic material.• It more difficult to magnetize. • Once magnetized, it doesn’t lose its magnetism

easily.

Magnetism Section 1

Magnetic Fields

〉What causes a magnet to attract or repel

another magnet?

〉Magnets repel or attract each other because

of the interaction of their magnetic fields.

• magnetic field: a region where a magnetic force can

be detected

Magnetism Section 1

Magnetic Fields, continued

• Magnets are sources of magnetic fields.

• Moving charges create magnetic fields.

• magnetic domains: groups of atoms that all line up

the same way and form small, magnetized regions

within a material

• Magnetic field lines are used to represent a magnetic

field.

– Field lines always form closed loops.

• The magnetic field gets weaker with distance from

the magnet.

Magnetism Section 1

Magnetic Fields, continued

• Magnetic field lines that are

close together indicate a

strong magnetic field.

• Field lines that are farther

apart indicate a weaker field.

• Magnetic field lines begin at

the north pole of a magnet and

end at the south pole of the

magnet.

Magnetism Section 1

Magnetic Fields, continued

• Compasses align with Earth’s magnetic field.

– The compass points in the direction that lies along

the magnetic field line at a given point.

– By convention, the pole of a magnet that points

north is painted red.

Magnetism Section 1

Earth’s Magnetic Field

〉How is Earth’s magnetic field oriented?

〉Earth’s magnetic field lines run from

geographic south to geographic north.

• The pole in Antarctica is a magnetic north pole.

• The pole in northern Canada is a magnetic south pole.

Magnetism Section 1

Earth’s Magnetic Field, continued

• Earth’s magnetic field has both direction and strength.

• The source of Earth’s magnetism is not yet fully

understood.

– The iron in the core is too hot to retain any

magnetic properties.

– The circulation of ions or electrons in the liquid

layer of Earth’s core may be the source of the

magnetism.

• The sun also has a magnetic field and ejects charged

particles into space.

Magnetism Section 2

Section 2: Magnetism from Electric Currents

Preview

• Key Ideas

• Bellringer

• Electromagnetism

• Electromagnetic Devices

Magnetism Section 2

Key Ideas

〉What happens to a compass near a wire that

is carrying a current?

〉Why are electric motors useful?

Magnetism Section 2

Bellringer

The temporary magnetic field created by current flowing

through a wire is used in many small appliances.

1. List as many things as possible that contain a small

electric motor.

2. What keeps the coil in an electric motor spinning?

3. A sewing machine needle can be magnetized by

gently stroking it with a magnet in one direction.

What is a possible explanation for this?

Magnetism Section 2

Electromagnetism

〉What happens to a compass near a wire that is carrying a current?

〉When the wire carries a strong, steady current, all of the compass needles move to align with the magnetic field created by the electric current.

• Hans Christian Oersted found that magnetism is produced by moving electric charges.

– Electric currents produce magnetic fields.

Magnetism Section 2

Electromagnetism, continued

• Use the right-hand rule to find the direction of the magnetic field produced by a current.

• right-hand rule:

– If you hold a wire in your right hand and point your

thumb in the direction of the positive current, the

direction that your fingers curl is the direction of the

magnetic field.

Magnetism Section 2

The Right-Hand Rule

• The thumb points in the

direction of the current.

• The fingertips point in

the direction of the

magnetic field.

• The magnetic field runs

counterclockwise.

Magnetism Section 2

Magnetic Field of a Current-Carrying

Wire

Magnetism Section 2

Electromagnetism, continued

• Solenoids and bar magnets have similar magnetic fields.

• solenoid: a coil of wire with an electric current in it

– In a solenoid, the magnetic field of each loop of wire adds to the strength of the magnetic field of any neighboring loops.

• The strength of a solenoid can be increased.

– More loops or more current can create a stronger magnetic field.

• electromagnet: a coil that has a soft iron core and that acts as a magnet when an electric current is in the coil

– The magnetic field of the rod adds to the coil’s field.

Magnetism Section 2

Electromagnetism, continued

• Moving charges cause magnetism.

– Negatively charged electrons moving around the nuclei of atoms make magnetic fields.

– Atomic nuclei also have magnetic fields because protons move within the nucleus.

– Each electron has a property called electron spin,which also produces a tiny magnetic field.

• The magnetism of the uncanceled fields in certain materials combines to make the materials magnetic overall.

Magnetism Section 2

Electromagnetic Devices

〉Why are electric motors useful?

〉A motor can perform mechanical work when it is attached to an external device.

• electric motor: a device that converts electrical energy to mechanical energy

• Galvanometers detect current.

– galvanometer: an instrument that detects, measures, and determines the direction of a small electric current

Magnetism Section 2

Electromagnetic Devices, continued

• ammeter: measures current

• voltmeter: measures voltage.

• Motors use a commutator to spin in one direction.

– commutator: a device used to make the current

change direction every time the flat coil makes a half

revolution.

– brushes: devices that connect the wires to the

commutator

Magnetism Section 2

Electric Motor

Magnetism Section 3

Section 3: Electric Currents from Magnetism

Preview

• Key Ideas

• Bellringer

• Electromagnetic Induction

• The Electromagnetic Force

• Transformers

Magnetism Section 3

Key Ideas

〉What happens when a magnet is moved into or

out of a coil of wire?

〉How are electricity and magnetism related?

〉What are the basic components of a

transformer?

Magnetism Section 3

Bellringer

1. List as many items as you can that use DC current.

(Hint: Batteries supply DC current.)

2. List as many items as you can that use AC current.

(Hint: Standard wall outlets supply AC current.)

3. If you want to plug a CD player that normally uses

batteries into a wall socket, an AC adapter is

required. What is the function of the AC adapter?

4. Electric power that goes into a neighborhood must be

stepped down, or decreased, in voltage before it

goes into a home. Explain why this is necessary.

Magnetism Section 3

Electromagnetic Induction

〉What happens when a magnet is moved into or out of a coil of wire?

〉Moving a magnet into and out of a coil of wire causes charges in the wire to move.

• electromagnetic induction: the process of creating a current in a circuit by changing a magnetic field

• Faraday’s law states:

An electric current can be produced in a circuit by a changing magnetic field crossing the circuit.

Magnetism Section 3

Electromagnetic Induction, continued

• As the loop moves in and out of the magnetic field of

the magnet, a current is induced in the circuit.

• Rotating the circuit or changing the strength of the

magnetic field will also induce a current in the circuit.

• Electromagnetic induction obeys conservation of

energy.

– Pushing a loop through a magnetic field requires

work.

Magnetism Section 3

Electromagnetic Induction, continued

Magnetism Section 3

Electromagnetic Induction, continued

• The magnetic force acts on moving electric charges.

– The force is at its maximum value when the

charge moves perpendicularly to the field.

– As the angle between the charge’s direction and

the direction of the magnetic field decreases, the

force on the charge decreases.

• The magnetic force acts on wires carrying a current.

Magnetism Section 3

Electromagnetic Induction, continued

Magnetism Section 3

Electromagnetic Induction, continued

• Generators convert mechanical energy into electrical

energy.

• generator: a machine that converts mechanical

energy to electrical energy

• alternating current (AC): an electric current that

changes direction at regular intervals

– For each half rotation of the loop, the current

produced by the generator reverses direction.

• AC generators produce the electrical energy you use

in your home.

Magnetism Section 3

AC Generator

Magnetism Section 3

Induced Current

Magnetism Section 3

The Electromagnetic Force

〉How are electricity and magnetism related?

〉Electricity and magnetism are two aspects of a

single force, the electromagnetic force.

• The energy that results from these two forces is called

electromagnetic (EM) energy.

• Light is a form of electromagnetic energy.

• EM waves are made up of oscillating electric and

magnetic fields that are perpendicular to each other.

Magnetism Section 3

Transformers

〉What are the basic components of a transformer?

〉In its simplest form, a transformer consists of

two coils of wire wrapped around opposite

sides of a closed iron loop.

• transformer: a device that increases or decreases

the voltage of alternating current

• primary coil: wire attached to a source of alternating

current

• secondary circuit: wire attached to an appliance

Magnetism Section 3

Transformers, continued

• Current in the primary coil creates a changing

magnetic field that magnetizes the iron core.

• The changing magnetic field of the iron core then

induces a current in the secondary coil.

• Transformers can increase or decrease voltage.

– The voltage induced in the secondary coil of a

transformer depends on the number of loops, or

turns, in the coil.

Magnetism Section 3

Transformers, continued

• In a step-up transformer, the primary coil has fewer

turns than the secondary coil does.

– The voltage across the secondary coil is greater

than the voltage across the primary coil.

• In a step-down transformer, the secondary coil has

fewer loops than the primary coil does.

– The voltage across the secondary coil is lower than

the voltage across the primary coil.

Magnetism Section 3

Transformers, continued

• Transformers must obey the law of conservation of

energy.

– The current in the secondary coil of a step-up

transformer is always less than the current in the

primary coil.

• Transformers are used in the transfer of electrical

energy.

Magnetism Section 3

Transformers, continued