Download - Physics

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 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
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
uses

ex. 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