Transcript magnetic field

Magnets have been known for centuries. The
Chinese and Greeks knew about the “magical”
properties of magnets. The ancient Greeks used
a stone substance called “magnetite.” They
discovered that the stone always pointed in the
same direction. Later, stones of magnetite
called “lodestones” were used in navigation.
What is Magnetism?
Magnetism is the
attraction of a magnet
for another object
Magnets
• A magnet will pull some metals towards itself.
• Metals that contain iron, nickel, and cobalt are
magnetic. In other words, a magnet will pull
them.
• Magnets are made of iron, nickel, or cobalt
The ends of a magnet are where the
magnetic effect is the strongest. These
are called “poles.” Each magnet has
2 poles – 1 north, 1 south.
For Every North, There is a South
Every magnet has at least one north pole and one south
pole. By convention, we say that the magnetic field lines
leave the North end of a magnet and enter the South end of
a magnet.
If you take a bar magnet and break it into two pieces, each
piece will again have a North pole and a South pole. If you
take one of those pieces and break it into two, each of the
smaller pieces will have a North pole and a South pole. No
matter how small the pieces of the magnet become, each
piece will have a North pole and a South pole.
S
N
S
N S
N
Breaking Magnets
How to break a magnet:
Heat it
Drop it
Strike it hard
This causes the
domains to become
random again!
Every magnet has a north pole
and a south pole.
Like repels like
Opposites attract
Magnetic Fields
The region where the magnetic forces
act is called the “magnetic field”
Magnetic Field
• Every magnet has a magnetic field around
it. It can be thought of as a line of force
running from the north end of the magnet to
the south end of a magnet.
• Earth’s magnetic field is what causes the
needle of a compass to point north and
south.
Magnetic Domain
Atoms themselves have magnetic properties due
to the spin of the atom’s electrons.
Groups of atoms join so that their magnetic fields
are all going in the same direction
These areas of atoms are called “domains”
The entire domain acts like a magnet with a north
and a south pole
When an unmagnetized substance is placed in a magnetic
field, the substance can become magnetized.
This happens when the spinning electrons line up in the
same direction.
An unmagnetized substance
looks like this…
While a magnetized substance
Looks like this…
Magnetic Domains
Magnetic force lines.
What do you know of that has
a North and South Pole?
• The Earth is one giant magnet. It has two
magnetic poles and is surrounded by a
magnetic field.
• This magnetic field is what causes the
needle of a compass to point in different
directions and causes the poles of a
magnet to point either north or south.
William Gilbert, an English
physician, first proposed in
1600 that the earth itself is a
magnet, and he predicted that
the Earth would be found to
have magnetic poles.
Like many Planets in our Solar System
the Earth has a Magnetic Field
The Earth acts like a magnet
It exerts magnetic forces
and is surrounded by a
magnetic field that is
strongest near the
North and South
magnetic poles
Magnetic South Pole
Geographic South Pole
Geographic North Pole
Magnetic North Pole
The Earth behaves as if there was a
magnet in its interior.
S
N
The North Pole of the Earth has a
“south” polarity
S
N
As a result a compass will always point
towards the North
S
N
S
N
S
N
A compass is a Magnetized
piece of metal that can spin.
S
A compass is a Magnetized
piece of metal that can spin.
N
North attracts South
S
The north tip of the compass
always points towards the
North Pole of the Earth
N
A Magnetic Field also protects us from
the Solar Wind
The Sun produces a stream of charged
particles that spread throughout the solar
system.
These particles are deflected by the
Earth’s Magnetic Field
The Earth’s magnetic field extends far into
space. It is called the “magnetosphere.”
When the magnetic particles from the sun, called “solar
wind”, strike this magnetosphere, we see a phenomenon
called…
The Aurora Borealis in the Northern Hemisphere
And the Aurora Australis in the Southern Hemisphere
Video Here
Magnetic Declination
Electromagnets
An electromagnet can
be used to move large
quantities of metal.
When the current is on
the magnet will pick up
the metal. When you
want to drop it you turn
off the power and the
electromagnet is
disabled and the metal
drops.
Electromagnet
• A temporary magnet made by passing electric
current through a wire coiled around an iron
bar.
• A junkyard is one place to see an
electromagnet in action. A crane holding a
huge electromagnet can be used to pick up
scrap metal when current flows through it.
When the crane operator wants to drop the
scrap, he or she will simply shut off the current
to the electromagnet.
In an electromagnet the electrons move
around an iron nail
+
-
Magnets and electricity
• Magnets are used to generate, or
produce, electricity.
• Spinning a coil of wire inside a magnetic
field produces an electric force between
the ends of the coil.
• In a similar way, an electric current
produces a magnetic field around it.
Electromagnets
An electromagnet consists of an iron core placed
inside a wire coil. The magnetic field strength of a
wire coil carrying an electric current increases in
direct proportion to the number of turns of the coil.
Electric Motors
An electric motor uses
the sideways push of a
magnetic field to turn a
current-carrying wire
loop. Electric motors use
a commutator to change
the direction of the
current in the loop.
Alternating current
electric motors do not
use commutators.
Oersted’s Experiment
Hans Christian Oersted discovered in 1820 that an electric
current near a compass causes the compass needle to be
deflected. Oersted's experiment showed that every electric
current has a magnetic field surrounding it.
Uses of magnets
• Magnets were first put to use in navigation
because they always point north and south.
• Magnets are used to hold, separate, control,
convey and elevate products and to convert
electrical energy into mechanical energy or
convert mechanical energy into electrical
energy.
Magnet Activity
We use magnets in many ways!
Which of the following items in a home use a
magnet?
Blender
washer
dryer
Radio
can opener
dishwasher
Refrigerator
doorbell
compass
Click here to find out! (select kitchen on the left of the
screen, close window to return to this presentation)
Magnet Activity
Magnets around the House:
Headphones
Refrigerator magnets
Computer speakers
Telephone receivers
Phone ringers
Microwave tubes
Seal around refrigerator door
Plug-in battery eliminators
Floppy disk recording and reading head
Audio tape recording and playback head
Video tape recording and playback head
Credit card magnetic strip
TV deflection coil
Computer monitor
Computer hard drive
Shower curtain weights / attach to tub
Power 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
• http://science.pppst.com/magnets.html