Chapter 8Magnets

BIG IDEA: A magnet is surrounded by a magnetic field that exerts a force on other

magnets.

Section 1: SCSh 1a-b, 3c, 3e-f, 4a, 9c, SPS 10c

Section 2: SCSh 9c, SPS 10c

Section 3: SCSh 9c, SPS 10b

GPS Standards: 17.6 = Illustrates the interactions of electricity and magnetism by using electricity to create a magnetic field and magnetic induction to create an electric field.17.7 = Describe the interplay of electric and magnetic forces as the basis for electricity.

All magnets have the following common properties:

Magnets always have two opposite “poles,” called north and south.

If divided, each part of a magnet has both north and south poles; we never see an unpaired north or south pole.

When near each other, magnets exert magnetic forces on each other.

The forces between magnets depend on the alignment of the poles; two unlike poles will attract each other and two like poles will repel each other.

Horse Shoe Magnet

BarMagnet Electromagnet

MAIN IDEA: Like magnetic poles repel each other and

unlike poles attract each other.

In iron, cobalt, nickel and a few other metals the atoms exert a force on the other atoms around them.Because of these forces, large groups of atoms align their magnetic poles so that almost all like poles point in the same direction.

The groups of atoms with aligned magnetic poles are called magnetic domains.

QUESTION? How do magnetic poles interact with each other?

ANSWER: (Page 226) Like magnetic poles repel each other and unlike poles attract each other.

NOTE CHECKTAKE 2 MINUTES:

With a partner, check each other’s notes to be sure both

filled in all blanks

A compass needle is a magnet that is free to spin until it lines up in the north-south direction. The origin of the terms “north pole” and “south pole” of a magnet come from the direction that a magnetized compass needle points. The end of the magnet that pointed north was called the north pole of the magnet and the end that pointed south was called the south pole.

History of Magnetism

First magnetic metal found:GREECE

(2500 yrs ago in current Turkey)Greeks called it “Lodestone” or

“Magnesia” = magnetite

First compass used:CHINA

(in the twelfth century)

Chinese sailors used a compass made of

magnetite

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21

Name three types of magnets:

1. _______________________________________

2. _______________________________________

3. _______________________________________

Two historic discovery places regarding magnets and their importance:

1. _______________________________________

2. _______________________________________

If a magnetic material were melted, would it continue to be magnetic, why? (hint: magnetic domain)

1. _______________________________________

The magnetic force depended on the direction and orientation of the two magnets and also on the distance between them. The model of a magnetic field was developed to describe how a magnet exerts magnetic force.

First, every magnet creates an energy field, called the magnetic field, in the space around it.

Second, the field exerts forces on any other magnet that is within its range.

QUESTION? What does the force between two magnets depend on?

ANSWER: (Page 224) The strength of the force between two magnets increases as magnets move closer together and decreases as the magnets move farther apart. DISTANCE!

Electromagnets = A magnet that is created by electric current. This type of magnet is called an electromagnet.

A single wire wrapped into a coil around a core is called a solenoid.

apply more voltage by adding a second battery. add more turns of wire around the nail.

The second method works because the magnetism in the electromagnet comes from the total amount of current flowing around the nail. If there is 1 amp of current in the wire, each loop of wire adds 1 amp to the total amount that flows around the nail. Ten loops of 1 amp each make 10 total amps flowing around. By adding more turns, the same current is used over & over to get stronger magnetism.

KNOW: The more current would make an electromagnet stronger. 2 ways to increase the current.

NOTE CHECKTAKE 5 MINUTES :

With a partner, check each other’s notes to be sure both

filled in all blanks AND compare your 3-2-1 answers.

Wrap the wire in many turns around the nail and connect a battery. When current flows in the wire, the nail becomes a magnet To reverse north and south, reverse the connection to the battery, making the current flow the opposite way.

By adding more turns the resistance of the coil is also increased. Increasing the resistance makes the current a little lower & generates more heat. A good electromagnet is a balance between too much resistance and having enough turns to get a strong enough magnet.

Of course, nothing comes for free.

The magnetic force exerted by an electromagnet depends on three factors:The amount of electric current in the wire.

CURRENT ↑ then MAGNETIC FORCE ↑The amount of iron or steel in the

electromagnet’s core. METAL ↑ then MAGNETIC FORCE ↑

The number of coils in the solenoid.COILS ↑ then MAGNETIC FORCE ↑

Electric motors convert electrical energy into mechanical energy.

Permanent magnets and electromagnets can work together to make electric motors and generators. The secret is in the ability of an electromagnet to reverse from north to south. By changing the direction of electric current, the electromagnet changes from attract to repel, and spins the motor.

QUESTION? Why is it necessary to continually reverse the direction of the current flow in the coil of an electric motor?

ANSWER: (page 236-237) When the end of the coil move past a pole of the permanent magnet, reversing the current causes the end of the coil to be attracted to the other (next) pole of the permanent magnet.

All electric motors must have three things to work:

1. A rotating element (rotor) with magnets.2. A stationary magnet that surrounds the rotor.3. A commutator that switches the

electromagnets from north to south at the right place to keep the rotor spinning.

The electromagnet must switch from north to south as each rotor magnet passes by to keep the rotor turning. The switch that makes this happen is called a commutator.

Both electrical force and magnetic force exist between electric charges. Scientists now believe both forces are two aspects of one force, the electromagnetic force.

A current through a wire creates a magnet. The reverse is also true: If a magnet is moved through a coil of wire, then electric current is created.

This process is called electromagnetic induction because a moving magnet induces electric current to flow.

MAIN IDEA: A changing magnetic

force field can produce and electric current in a

wire loop.

NOTE CHECKTAKE 2 MINUTES :

With a partner, check each other’s notes to be sure both

filled in all blanks

When a magnet moves into a coil of wire, it induces electric current to flow in the coil. The current stops if the magnet stops moving. If the magnet is pulled back out again, the current flows in the opposite direction. A changing magnetic field is what makes

the electricity flow.

Electromagnetic induction enables us to transform mechanical energy (moving magnets) into electrical energy. Any machine that causes magnets to move past wire coils generates electric currents. These machines include giant electric power plants and computer disk drives.

Power plants use electromagnetic induction to create electricity. A generator is a combination of mechanical and electrical systems that converts kinetic energy into electrical energy.

A power plant generator contains a turbine that turns magnets inside loops of wire, generating electricity.

In the top sketch the north pole on the disk induces a south pole in the electromagnet, causing current to flow one way. When the disk rotates, the magnetism in the coil is reversed, and the electric current generated also reverses.

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21

Name three things a motor requires:

1. _______________________________________

2. _______________________________________

3. _______________________________________

Two forces combine to produce electromagnetic forces:

1. _______________________________________

2. _______________________________________

How does electromagnetic induction generate electricity (AC)

1. _______________________________________

NOTE CHECKTAKE 5 MINUTES :

With a partner, check each other’s notes to be sure both

filled in all blanks AND compare your 3-2-1 answers.

Explain how a magnet exerts a force.

Describe the magnetic field produced by an

electric current.

Explain how a electromagnet produced a

magnetic field.

Explain how an electric motor operates..

Describe how a generator produces an electric

current.