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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
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