Transcript What is magnetism?

Chapter 2
Magnetism and
Electromagnetism
By : Dayang khadijah
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THE MYSTERY OF
MAGNETISM
•Each time you turn on a light, listen to your stereo, fly in an airplane, or watch
TV, you are depending on the principles of magnetism to work for you.
•Take a look at the pictures below. All of the items in these pictures have
something to do with magnetism.
Hydroelectric Dam
Video Cassette Tape
Magnetic Particle Inspection Unit
Fan
Airplane Navigational Panel
What is magnetism?

Just like when the Greeks of the old times discovered
the first naturally occurring magnetic stones, or natural
magnets, you have been observing a property of matter
called magnetism.

Magnetism is the force of attraction or repulsion in and
around a material. Magnetism is present is all materials
but at such low levels that it is not easily
detected. Certain materials such as magnetite, iron,
steel, nickel, cobalt and alloys of rare earth elements,
exhibit magnetism at levels that are easily detectable.
• Magnetism is a basic force of attraction and
repulsion in nature that is created, by moving
charges.
• A magnet is an object, which has a magnetic field
that causes a push or pulling action.
• Similar to electric charges, unlike poles attract,
while like poles repel
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• Magnetism is a basic force of attraction and
repulsion in nature that is created, by moving
charges.
• A magnet is an object, which has a magnetic field
that causes a push or pulling action.
• Similar to electric charges, unlike poles attract,
while like poles repel
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What is a magnet?

A magnet is any piece of material that has the property
of attracting iron (or steel). Magnetite, also known as
lodestone, is a naturally occurring rock that is a magnet.

This natural magnet was first discovered in a region
known as magnesia and was named after the area in
which it was discovered.

Magnetism may be naturally present in a material or the
material may be artificially magnetized by various
methods. Magnets may be permanent or temporary.

After being magnetized, a permanent magnet will retain the
properties of magnetism indefinitely.

A temporary magnet is a magnet made of soft iron, that is
usually easy to magnetize; however, temporary magnets lose
most of their magnetic properties when the magnetizing cause
is discontinued.

Permanent magnets are usually more difficult to magnetize,
but they remain magnetized. Materials which can be
magnetized are calledferromagnetic materials
What is happening when you cut a magnet?


A magnet can be cut into smaller and smaller pieces
indefinitely, and each piece will still act as a small
magnet.
Thus, the cause of magnetism must be from a
property of the smallest particles of the material, the
atoms.
• magnets do not come in separate charges
• Any magnetic/magnetized object has a North and South
pole
• If you break a magnet in half, each piece will have a
North and a South end
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•What is a magnetic field and how is it created?

A magnetic field describes a volume of space where there is a
change in energy. As Ampere suggested, a magnetic field is
produced whenever an electrical charge is in motion.

The spinning and orbiting of the nucleus of an atom produces
a magnetic field as does electrical current flowing through a
wire.

The direction of the spin and orbit determine the direction of
the magnetic field. The strength of this field is called
the magnetic moment.

The motion of an electric charge producing a
magnetic
field
is
an
essential
concept
in
understanding magnetism.

The magnetic moment of an atom can be the result of
the electron's spin, which is the electron orbital
motion and a change in the orbital motion of the
electrons caused by an applied magnetic field.
• Magnetic field lines – 3D lines which tiny bar magnets
lie along. Magnetic field lines run from N to S.
• A compass can be used to map out the magnetic field.
• Field forms closed “flux lines” around the magnet
(lines of magnetic flux never intersect)
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MAGNETIC LINES OF FORCE
What do the lines around the bar magnet indicate?

The lines that we have mapped out around the
magnet, called the magnetic lines of force, indicate
the region in which the force of the magnet can be
detected.

This region is called the magnetic field. If an iron
object is near a magnet, but is not within the magnetic
field, the object will not be attracted to the magnet.



When the object enters the magnetic field, the force
of the magnet acts, and the object is attracted.
The pattern of these lines of force tells us something
about the characteristics of the forces caused by the
magnet.
The magnetic lines of force, or flux, leave the north
pole and enter the south pole.
Magnetic field
• The strength of the magnetic field is greater where
the lines are closer together and weaker where they
are farther apart.
• Field is strongest in regions of dense field lines.
• Field is weakest in regions of sparse field lines.
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The density
of field lines
indicates the
strength of
the field
Weak
Field
Strong
Field
Magnetic field
Magnetic flux
• Magnetic flux is measurement of the quantity of
magnetism, the description of how certain materials
relate to magnetic fields.
• Specifically, it describes the strength and extent of
the object's interaction with the field.
• Magnetic lines of force (flux) are assumed to be
continuous loops.
• Magnetic flux measured in Webers (Wb)
• Symbol

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How is the earth like a magnet?

Since the earth is a huge magnet with a magnetic
north and south pole, the lines of magnetic force
around the earth look like there is a huge vertical bar
magnet running through the center of the earth.
The Earth is a Magnet
• A magnetic compass aligns itself along the magnetic
field lines (produced by the Earth in the absence of a
stronger field)
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• The North pole of the compass points to the Earth
magnetic South pole (generally toward geographic
north) and vice‐versa
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FERROMAGNETIC MATERIALS

Notice that even though the paper clip alone are not
magnetic, when they are attached to a magnet, they
become a little magnets themselves.
How does an object become a magnet?

We have determined in previous discussions that
magnets can be permanent or temporary. A permanent
magnet is more difficult to magnetize but will retain
the properties of magnetism indefinitely.

A temporary magnet is generally made of soft iron
and will remain magnetized only as long as the
magnetizing cause is present.

Bringing a ferromagnetic object, like a nail, into the
magnetic field of a strong magnet can cause the
molecules of the iron material to line up and the nail
to become a temporary magnet.

As long as it is in the magnetic field of the bar
magnet, the nail acts like a magnet and picks up other
ferromagnetic materials.

In this case it is the paper clip. Then, the paper clip
becomes a magnet and can pick up another paper clip,
and so forth.
• Magnetism can be transferred or induced into
other materials, this is known as Magnetic
Induction
• The induction of magnetism into a material
can be permanent or temporary
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• Magnetic materials (ferromagmagnetic): iron, steel,
cobalt, nickel and some of their alloys.
• Non magnetic materials: water, wood, air, quartz.
• In an un-magnetised state, the molecular magnets lie in
random manner, hence there is no resultant external
magnetism exhibited by the iron bar.
Iron bar
Magnetic
molecules
wood
Non-magnetic
molecules
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• When the iron bar is placed in a magnetic field or
under the influence of a magnetising force, then these
molecular magnets start turning their axes and
orientate themselves more or less along a straight
lines.
N
S
N
S
Iron bar
Magnetic molecules
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• When the iron bar is placed in a very strong magnetic
field, all these molecular magnets orientate
themselves along a straight lines (saturated).
N
S
N
Iron bar
S
Magnetic molecules
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