Chapter 2

Electromagnetism

Chapter 2 Bellringers

Friday 9/11/09

What do you know about magnets?

North and south poles

North and south poles attract

Like poles repel each other

Bar magnets and horseshoe magnets

Section 2-1: Magnets and Magnetism (Pgs. 40-46)

Properties of Magnets

Magnet- any material that attracts iron or things made of iron.

Properties of magnets

1. All have a south and north pole

2. Exert forces on each other

3. Surrounded by magnetic field

Magnetic Poles

A magnet’s magnetic forces are NOT the same throughout the magnet.

Strongest forces are near the ends of a magnet.

Each end is a magnetic pole

Magnetic poles- points on a magnet that have opposite magnetic qualities.

North and South

End of the magnet that points North is the magnet’s north pole

End of the magnet that points South is the magnet’s south pole

Poles always occur in pairs

Never find a magnet with just a south pole or just a north pole

Magnetic Forces

Magnetic force- attraction or repulsion created by spinning electric charges in the magnet.

Forces are always present in a magnet

Like poles repel each other

Opposite poles attract each other

Magnetic Fields

Magnetic field- region around a magnet’s pole in which magnetic forces act.

Shape of field can be shown with lines

Magnetic field lines- map the shape and strength of magnetic force on a magnet

Closer the lines, stronger the force

Lines closest at magnet’s poles

Monday 9/14/09

Name one property that all magnets have.

All have a north and south poleAll exert forces on each otherAll have magnetic fields

What creates the magnetic forces in a magnet?

Spinning electric charges from the electrons in the magnet’s atoms

Cause of Magnetism?

Whether or not a material is magnetic, depends on the material’s atoms

An atom’s moving electrons cause magnetism

As electrons move, it creates a magnetic field around the atom

Each atom becomes like a tiny magnet

Materials that are magnetic have their atoms lined up in domains

Domains- groups of atoms in small areas

Arrangement of domains determines whether an object is magnetic

North and south poles of all the atoms in a domain line up

Material is then magnetized

If poles do not line up, magnetic fields of all the atoms cancel out, and material is not magnetized

Losing Alignment

Magnets domains can move, making the magnet demagnetized

Domains can move by:

1. Dropping a magnet

2. Putting magnet in an opposite magnetic field

3. Increasing temperature

Higher temps cause atoms to vibrate faster causing them not to be lined up

Making a magnet

Item must have domains that move

Examples: iron, nickel, and cobalt

Rub one pole of a magnet in one direction on the item

Item’s domains become aligned with magnetic field of the magnet

Since domains are aligned, the item is magnetized

What can be picked up by a magnet?

Anything that will allow it’s domains to move

Example: paper clip

Bring magnet close to paper clip

Magnet’s magnetic field causes paper clip’s domains to align – causes paper clip to become temporary magnet

Paper clip’s north pole will attract to magnet’s south pole and vice versa

Tuesday 9/15/09

What must atoms do in order for an item to be magnetic?

Atoms must line up their north and south poles

Name three things that can demagnetize a magnet?

Drop itPut it in an opposite magnetic field

Increase the temperature

Cutting a magnet

End up with two separate magnets

Each piece has a north and south pole

REMEMBER Properties of Magnets

1. All have a south and north pole

Kinds of magnets

1. Ferromagnets - are made of iron, nickel, or cobalt

2. Electromagnets - are made by electric current

3. Temporary magnets – easy to magnetize, but lose magnetism quickly

4. Permanent magnets – difficult to magnetize, but keep magnetism longer

Earth as a Magnet

William Gilbert suggested Earth was a giant magnet.

Earth’s magnetic poles are near the geological poles

Magnetic south is in geological north

In order for a magnet’s north pole to point geological north, it must be attracted to a magnetic south

Magnetic north is in geological south

Why Earth is magnetic?

There is NO bar magnet in the Earth

Earth’s magnetic field is created by electric charges that exist in the Earth’s core

Outer core is liquid iron and cobalt and moves as Earth rotates.

As a result, electric charges also rotate creating a magnetic field

Magnetic Light Show

Aurora – beautiful display of lights that appear in the sky above the poles

Formed when charged particles from the sun hit oxygen & nitrogen atoms in the air

Earth’s magnetic field blocks charged particles from the sun

Earth’s Magnetic Field

So why are auroras seen at poles?

Earth’s magnetic field bends in at the poles

Charged particles able to enter the atmosphere near the poles

Northern lights (aurora borealis) appear at north pole

Southern lights (aurora australis) appear at south pole

Section 2-2 Magnetism from Electricity

Discovery of Electromagnetism

Discovered by Hans Christian Oersted

Held a compass near a wire carrying an electric current

Compass needle did not point north

Only thing that can cause this is another magnetic field

Oersted concluded that electric current produces a magnetic field

Also found that the direction of the magnetic field depended on direction of the current.

Run current one direction, compass needles turn in a clockwise direction

Run current the other direction, compass needles turn counterclockwise direction

Electromagnetism – the interaction between electricity and magnetism

Thursday 9/17/09How did Oersted discover that electric current produces a magnetic field?

Held a compass near a current carrying wire and compass did not point North

What determines the direction of a magnetic field in a current carrying wire?

The direction of the current

Using Electromagnetism

Magnetic field generated by an electric current is weak.

Solenoids and electromagnets strengthen this magnetic field

Maglev (magnetic levitation) trains are pulled down the track by an electromagnet

Solenoids

Solenoid- coil of wire that produces a magnetic field when carrying electric current.

Magnetic field produced by a solenoid can be increased two ways

1. Increase the numbers of loops in the solenoid

2. Increase the current running through the wire

Electromagnets

Electromagnet- a solenoid wrapped around an iron rod.

Magnetic field of the solenoid makes the atoms in the iron line up.

Both the solenoid and the iron now have a magnetic field

The two magnetic fields combine to create a strong magnetic field for the electromagnet

Magnetic field of an electromagnet can be made even stronger

Same ways as a solenoid

1. Increase the number of loops in the solenoid

2. Increase the current running through the solenoid

Turning Electromagnets On and Off

Solenoid only has magnetic field if electric current is in the wire

Turn on current, solenoid has a magnetic field and the iron also has a magnetic field

Turn off current, magnetic field is no longer present and iron is not magnetic

What is a coil of wire with an electric current?

Solenoid

What do we add to a solenoid to make it an electromagnet?

Iron

What two things increase the magnetic fields of solenoids and electromagnets?

Increase number of loops

Increase current

Applications of Electromagnetism

A current carrying wire can cause a compass needle to move

If a current carrying wire can cause a magnet to move, can a magnet cause a current carrying wire to move?

Yes, and this is useful in electric motors

Electric Motors

Electric Motors - device that changes electrical energy into mechanical energy.

All have a loop of wire that can rotate

Located between the poles of a permanent magnet

Loop of wire is attached to a battery

Wire now has a current running through it so it also has a magnetic field

Since it is in between the poles of a permanent magnet, the permanent magnet pulls on the loop of wireand rotates it

Galvanometers

Galvanometers- uses an electromagnet to measure electric current

Has electromagnet placed between poles of a permanent magnet

Electromagnet also attached to a needle

When current runs through, electromagnet turns on and is pulled by permanent magnet moving needle

Section 2-3 Electricity from Magnetism (Pgs. 54-59)

Electric Current from a Changing Magnetic Field

Oersted discovered that an electric current could produce a magnetic field.

If an electric current can produce a magnetic field, can a magnetic field produce an electric current?

Michael Faraday’s experiments answer this question

Faraday’s Experiment

Wrapped a wire attached to a battery around an iron ring creating an electromagnet

Wrapped a second wire around the other side of the ring and attached it to a Galvanometer.

Galvanometer measured no electric current in the second wire

Would appear magnetism cannot create an electric current, until Faraday discovered something.

Faraday’s Discovery

The instant he connected or disconnected the wire to the battery, the galvanometer pointer moved.

As long as battery was fully connected, no electric current was measured by the

galvanometer.

Electric current is ONLY made when magnetic field is changing.

Electromagnetic Induction- the process by which an electric current is made by

changing a magnetic field.

Inducing Electric Current

Move either the wires or the magnet

Move a magnet through the loops of a wire

Move a wire between the poles of a magnet

Either way, magnetic field is changing and electric current is produced in the wires.

Charges in a wire only move when they pass through a magnetic field.

How to increase the electric current

1. Move magnet through the loops faster

2. Add more loops to the wire

3. Move wire faster between the poles of the magnet

Wednesday 9/23/09

In order for a magnetic field to produce an electric current, what must happen?

Magnetic field must be changing

What can you do to increase the current produced by a magnetic field.

Move the magnet or wires faster

Add more loops of wire

Electric Generators

Electric Generator- uses electromagnetic induction to change mechanical energy into electrical energy

A wire sits between the poles of a permanent magnet.

A crank turns the wire.

Because the wire keeps moving through the magnetic field, a current is produced in the wire.

Generators Produce Alternating current

Every half turn, the wire crosses the magnetic field in a different direction

Current goes opposite direction

As a result, the electric current produced is an alternating current

Alternating current MUST be produced in order for transformers to work.

Power Plant Generators

Use many coils of wire instead of one

Rotate magnets around the wire instead of rotating wire inside of the magnet

Use many sources to turn the turbines that turn the magnets

1. Steam from nuclear reactions

2. Water

3. Wind

Transformers

Transformers- increases or decreases the voltage of

alternating current.

Two separate coils of wire wrapped around an iron ring

Primary (first) coil receives an alternating current from the generator

Current produces a magnetic field in the iron ring

Because current is alternating, magnetic field of iron ring is constantly changing direction.

That changing magnetic field allows an electric current to be produced in the

secondary (second) wire.

If current was not alternating, no current would be produced in second wire.

Thursday 9/24/09

What type of current do generators produce?

Alternating current

What increases and decreases the voltage of alternating current?

Transformers

Step-Up, Step-Down

Number of loops in primary and secondary coils determines whether voltage is

increased or decreased.

Step-up Transformer- increases the voltage and decreases current

More loops in secondary coil causing voltage to increase

Step-Down Transformers – decreases voltage and increases current

Less loops in the secondary coil causes voltage to decrease

Amount of power (W) stays constant

Power = current (I) x voltage (V)

So if voltage goes down, current must go up and visa versa.

Why Increase and Decrease Voltage?

Voltage must be increased after leaving power plant in order to travel long

distances on power lines

Voltage is stepped down before it enters your house to keep appliances from

blowing up