Unit 11

Magnetism

Magnets

magnetism – force of attraction or repulsion

not all objects are affected by the force of magnetism

ex. wood, glass, paper, plastic

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

Magnets

poles – two ends of a magnet

every magnet has two polesnorth (N) polesouth (S) pole

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

Magnets

the north pole of a magnet points North

it is often called “North-seeking”

Properties of Magnets

if a north pole and a south pole are brought together, they will attract each other

opposites attract

Properties of Magnets

if the north pole of one magnet is brought near the north pole of another magnet, they will repel each other

if two south poles are brought together, they will repel each other

Properties of Magnets

like magnetic poles repel each other

unlike magnetic poles attract each other

Magnetic Fields

magnetic field – area around a magnet where magnetic forces can act

a magnetic field is made up of magnetic lines of force

Magnetic Fields

magnetic lines of force – lines that show the shape of a magnetic field

the magnetic lines of force are closest together at the poles of the magnet

this is where the magnet is strongest

Making a Magnet

some magnets occur in nature

these magnets are called natural magnets

ex. magnetite (also called lodestone)

Making a Magnet

materials that are not natural magnets can be magnetized

Making a Magnet

magnetic induction – process by which a material can be made into a magnet

Making a Magnet

some materials are easy to magnetize

ex. iron

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 magnet

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 magnet

The Earth as a Magnet

William Gilbert

a British scientist

the Earth has north and south poles like a bar magnet

The Earth as a Magnet

the Earth has a North Magnetic Pole and a South Magnetic Pole

the North Magnetic Pole is located near the geographic North Pole

the South Magnetic Pole is located near the geographic South Pole

The Earth as a Magnet

the North Magnetic Pole is like the south pole of a bar magnet

the South Magnetic Pole is like the north pole of a bar magnet

The Earth as a Magnet

the Earth is surrounded by a magnetic field which extends far into space

magnetosphere – region of the Earth’s magnetic field

The Earth as a Magnet

the magnetosphere traps charged particles from the sun

when these particles enter the atmosphere, an aurora is formed

auroras are also called the northern and southern lights

Electricity and Magnetism

Hans Oersted

about 200 years ago

an electric current flowing in a wire causes a compass needle to move

Electricity and Magnetism

an electric current passing through a wire causes a magnetic field

Electricity and Magnetism

electromagnetism – relationship between electricity and magnetism

Electricity and Magnetism

Michael Faraday (British scientist)Joseph Henry (American scientist)

when a wire is moved across a magnetic field, an electric current is induced in the wire

Electricity and Magnetism

electromagnetic induction – process by which an electric current is produced by moving a wire in a magnetic field

Electricity and Magnetism

a wire carrying an electric current always has a magnetic field around it the magnetic field in a straight wire is not

very strong if the wire is wound into a coil, the magnetic

field becomes much stronger as the individual magnetic fields overlap

the greater the number of coils, the stronger the magnetic field

Electromagnets

electromagnet – temporary magnet made by wrapping a current-carrying wire around an iron core

the center of an electromagnet is called the core

it is often made of iron

Electromagnets

as long as current is flowing, an electromagnet has a magnetic field

when current is turned off, there is no longer a magnetic field

Electromagnets

there are two ways to make an electromagnet stronger

increasing the number of coils

increasing the amount of current

Electromagnets

electromagnets are useful because they can be turned on and off

electromagnets have many important usesex. radios, telephones, computers

Electromagnets

changing the direction of an electric current causes the poles of an electromagnet to reverse

this feature is important in the production of electric motors

Transformers

transformer – device in which alternating current in one coil of wire induces a current in a second coil

Transformers

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

a transformer only works with alternating current

Transformers

a step-up transformer increases voltage

it consists of more coils of wire in the secondary coil than the primary coil

Transformers

a step-down transformer decreases voltage

it consists of more coils of wire in the primary coil than the secondary coil

Electric Motors

electric motor – device that changes electrical energy into mechanical energy

Electric Motors

an electric motor is made up of an electromagnet and a permanent magnet

Electric Motors

the electromagnet is free to turn and is hooked up to a source of alternating current

changing the direction of an electric current causes the poles of an electromagnet to reverse

Electric Motors

attraction and repulsion between the electromagnet and the permanent magnet cause the electromagnet to spin

Electric Motors

an electric motor can use direct current if a special switch is used

Generators

generator – device that changes mechanical energy into electrical energy

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

Generators

an electric generator is made up of an insulated loop of wire and a U-shaped magnet

spinning the loop of wire in the magnetic field of the magnet produces an electric current

Generators

most of the energy we use every day comes from generators

Generators

mechanical energy for many generators is supplied by turbines

a turbine is a large wheel that is turned by moving steam or water