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Physical Science

Chapter 18 Magnetism

Bill Nye-Magnetism Video

• What happens when the poles of two

magnets are thought close together?

• What causes a magnet to attract or repel

another magnet?

• How is Earth’s magnetic field oriented?

18.1 Magnets and

Magnetic Fields

What is an example of how you use magnets?

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Magnets around the House:Headphones

Computer speakers

Telephone receiversPhone ringers

Microwave tubesSeal around refrigerator door

Credit card magnetic strip

TV deflection coilComputer monitor

Computer hard drive

Shower curtain weights / attach to tubPower supply transformers

Magnets inside of motors:

CD and DVD spinner and head positioner

Audio and VHS tape transport

VHS tape loader Microwave stirring fans

Kitchen exhaust fans Garbage disposal motor

Sump pump Furnace blower and exhaust

Garage door opener Bathroom exhaust fan

Electric toothbrush Ceiling fan

Pager or cell phone vibrator

Clocks (not the wind-up type or LCD type)

Computers

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Magnets in your Car:Starter motor

A/C clutch

Interior fan motor

Electric door locks

Windshield wiper motor

Electric window motor

Side-view mirror adjuster motor

CD/tape player motor and playback

Engine speed sensors

Alternator

Starter relay

Windshield washer pump motor

More than 3,000 years ago Greeks discovered deposits of a mineral that was a natural magnet.

The mineral is now called magnetite.

MagnetsMagnets

� These magnets were used by the ancient peoples as compasses to guide sailing vessels.

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• Today, the word magnetism refers to the properties and interactions of magnets.

MagnetsMagnets

� magnet – any material that

attracts iron and materials

that contain iron

� magnetism – the

attraction or repulsion of

magnetic materials

� Magnets attract or repel

other magnets.

Common metals affected by magnetism are iron, nickel, and cobalt

IRON NICKEL

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Magnets

• Nonmagnetic materials, such as plastic, glass

and wood, have electrons that flow in

different directions.

• Magnetic poles are where the magnetic force exerted by the magnet is strongest.

• the two ends on any magnet, no matter what its shape

• All magnets have a north pole and a south pole.

Magnetic Poles

• magnetic force – the attraction or repulsion between magnetic poles

Magnets

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� Magnetic poles

that are alike

repel each other.

� Magnetic poles that

are unlike attract

each other.

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� every magnet has two poles

� north (N) pole

� south (S) pole

� even if you break a magnet in half, each half will have a north pole and a south pole

Magnetic Poles

Poles- two ends of a magnet

No Monopoles Allowed

It is not possible to end up with a single North pole or a single South

pole, which is a monopole ("mono" means one or single, thus one

pole).

S N

• Recall that even individual atoms of magnetic materials act as tiny magnets.

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Making a Magnet

� materials that are hard to magnetize

will also stay magnetized for a long

time

� a magnet that is hard to magnetize

but tends to keep its magnetism is

called a permanent magnetEx: Cobalt, Nickel, Aluminum

Making a Magnet

� a material that is easily magnetized

tends to lose its magnetism quickly

� a magnet made of this kind of material

is called a temporary magnetEx: Iron nails

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• The two ends of a horseshoe-shaped magnet are the north and south poles.

• A magnet shaped like a disk has opposite poles on the top and bottom of the disk.

Where are the Poles on different shaped magnets?

• Magnetic field lines always connect the north pole and the south pole of a magnet.

Magnetic Fields

A magnet has a

‘magnetic field’

distributed throughout

the surrounding space

Michael Faraday realized that ...

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• A magnet is surrounded by a magnetic field.

• A magnetic field exerts a force on other magnets and objects made of magnetic materials.

• The magnetic field is strongest close to the magnet and weaker far away.

A magnetic field also has a

direction & illustrated by

arrows.

� magnetic field lines – the lines that map

out the magnetic field around a magnet

Magnetic field lines spread out from one pole, curve

around the magnet, and return to the other pole

forming a closed loop.

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Strong

Field

Weak

Field

The density of field

lines indicates the

strength of the field

Magnetic Field

Video

Field Lines Around Magnets

Bar Sphere

Repelling Attracting

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� magnetic domain – a

grouping of billions of atoms that have their

magnetic fields lined up in the same

direction

� Atom’s electrons

spin around when

placed near a

magnetic field.

An unmagnetized substance looks

like this…

While a magnetized substance looks

like this…

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Earth as a Magnet• Earth has a magnetic field with two main poles.

• This magnetic field enables us to use compasses

• Our planet's rotation

causes molten iron-

nickel in its outer core

to circulate

• creating electrical

currents and a

magnetic field.

Earth as a Magnet

� William Gilbert

� a British scientist

� Showed that the Earth behaves as a magnet

� Earth has north and south poles like a bar magnet

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The Earth’s Magnetic Field

The Earth is surrounded by a magnetic field which extends far into

space. Called the magnetosphere

�Magnetic poles of the Earth are

different from the geographic poles of the Earth

Earth as a Magnet

� the North Magnetic Pole is located near the geographic South Pole

� the South Magnetic Pole is located near the geographic North Pole

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• Earth’s magnetic poles move slowly with time.

Earth’s Magnetic Poles

• Sometimes Earth’s magnetic poles switch places so that Earth’s south magnetic pole is the southern hemisphere near the geographic south pole.

Video

Compasses�Compasses are used to determine direction

�Compass needles are magnetized and respond to the magnetic field of the Earth

�We use the Earth’s magnetic field to find direction.

� Migration of birds

The needle of a compass always

points toward the magnetic

south pole.

The compass needle points

toward the geographic “North”

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Magnetic Effects

The most visible effect of the earths magnetic field is a colorful light display, called an aurora

Collisions between the charged particles and other particles in the upper atmosphere create glowing lights

Video

18.2 Magnetism from Electric

Currents

• What happens to a compass near

a wire that is carrying a current?

• Why are electric motors useful?

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Electromagnetism

• In 1820, Han Christian Oersted, a Danish physics teacher, found that electricity and magnetism are related.

• Oersted hypothesized:

• Electric current must produce a magnetic field around the wire, and the direction of the field changes with the direction of the current.

• When a compass was brought

near electric current, the

compass needle no longer

pointed north

• When an electric current passes through a wire a

magnetic field is formed.

• The magnetic field depends on the direction of

the current in the wire.

Electromagnetism

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• The direction of the magnetic field around the wire reverses when the direction of the current in the wire reverses.

• As the current in the wire increases the strength of the magnetic field increases.

Electromagnetism

• Electromagnetism – relationship between electricity

and magnetism

• Oersted’s discovery is responsible for the invention of

new tools based on the principles of electromagnetism.

Electromagnetism

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When an electric current is passed through a coil of wire wrapped

around a metal core, a very strong magnetic field is produced. This is

called an electromagnet.

The strength depends on the number of turns in the coil, the

amount of current, and the size of the iron core.

Electromagnetism

Electromagnetism

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YOU CAN MAKE AN ELECTROMAGNET

Electromagnets can be turned on and off

You can make an electromagnet by wrapping a wirearound a piece of ironand sending a currentthrough the wire with a battery.

Classroom electromagnet

vs. commercial electromagnet

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• A single wire wrapped into a cylindrical wire coil is called a solenoid.

• A type of electromagnet

• The solenoid’s magnetic field magnetizes the iron core.

Electromagnetism

• The magnetic field inside a solenoid is stronger than the field in a single loop.

• The field inside the solenoid with the iron core can be more than 1,000 times greater than the field inside the solenoid without the iron core.

Electromagnetism

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Properties of Electromagnets

• Electromagnets are temporary magnets because the magnetic field is present only when current is flowing in the solenoid.

• The strength of the magnetic field increases:

• by adding more turns of wire to the solenoid

• by increasing the current passing through the wire.

Electromagnetism

• Electromagnet has a north pole and a south pole.

• If placed in a magnetic field, an electromagnet will align itself along the magnetic field lines, just as a compass needle will.

• An electromagnet also will attract magnetic materials and be attracted or repelled by other magnets.

Properties of Electromagnets

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A galvanometer gives a measurement of

the current.

an electromagnet that interacts with a

permanent magnet.

The greater the current

passing through the wires,

the stronger the

galvanometer interacts

with the permanent

magnet.

Electromagnetic Devices

Galvanometer

Use of a GalvanometerGalvanometers are used as gauges in cars and many other applications.

• Functions as an ammeter to measure current, when other circuit elements are used.

• Functions as a voltmeter to measure voltage.

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Electric Motors

• Electric Motor – device that changes electrical

energy into mechanical energy

• An electric motor is made up of an

electromagnet and a permanent magnet

Electromagnetic Devices

• A fan uses an electric motor

• The motor in a fan turns the fan blades, moving air past your skin to make you feel cooler.

• Almost every appliance in which something moves contains an electric motor.

Electric Motors

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Electric Motor

• A simple electric motor also includes components called brushes and a commutator.

• The brushes carry current to coil using the commutator.

• The commutator is used to change the currents direction.

• Form a closed electric circuit between the battery and the coil.

Video

18.3 Electric Currents from

Magnetism

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?

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The process of inducing a current by moving a magnetic field through a wire coil without touching it.

Electromagnetic Induction

Causes charges to move within the wire.

• Working independently in 1831, Michael Faraday in Britain and Joseph Henry in the United States both found that:

• Moving a loop of wire through a magnetic field caused an electric current to flow in the wire.

• They also found that moving a magnet through a loop of wire produces a current.

Electromagnetic Induction

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Generators

• electric generator – a

device that converts

mechanical energy into

electrical energy

• A generator uses motion in

a magnetic field to

produce an electric

current

Electromagnetic Induction

How a Generator Works

– a current can be

induced in a loop of

wire by spinning the

loop inside a

magnetic field

– due to the changing

direction of the wire,

an alternating

current is produced

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• The type of generator in a car, is called an alternator.

• The alternator provides electrical energy to operate lights and other accessories.

Using Electric Generators

Electromagnetic Induction

Induced current in a Generator

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• Electrical energy comes from a power plant with huge generators.

• Produces the electrical energy that you use in your home.

• The coils in these generators have many coils of wire wrapped around huge iron cores.

Electromagnetic Induction

• The rotating magnets are connected to a turbine(TUR bine)a large wheel that rotates when pushed by water, wind, or steam.

Electromagnetic Induction

• Thermal energy is then converted to mechanical energy as the steam pushes the turbine blades.

Video

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Electromagnetic Force

• Electromagnetic force: Electricity and

magnetism are two aspects of a single force.

• Electromagnetic energy results

– Light is an example of electromagnetic energy.

EM Waves are made of

both electric and

magnetic fields.

Electric field

Magnetic field

Transformers

• A transformer is a device that increases or

decreases voltage alternating current.

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• a transformer consists of two coils of wire around a magnet

– the first coil, called the primary coil, is connected to the power source

– the second coil, called the secondary coil, is connected to the load

• a load is something that uses electricity, such as a light bulb or a motor

Transformers

Step-down transformer

a transformer that

decreases voltage

Transformers

Two types of transformers:

Step-up transformer

a transformer that

increases voltage

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Transmitting Alternating Current

• This figure shows how step-up and step-down transformers are used in transmitting electrical energy from power plants to your home.

Transformers