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Magnetism
Magnetism – is the ability to attract iron,
nickel and cobalt.
Magnetism is perhaps more difficult to understand than
other characteristic properties of matter, such as mass,
energy, and electric charge, because magnetism is
difficult to detect and measure. We can feel mass,
visualize energy, and be shocked by electricity, but we
cannot sense magnetism.
Classification of magnets:
• Natural magnets
• Artificial permanenet
• Electromagnets
Natural Magnet
Natural magnets-lodestone
Lodestones became magnetized by lying in
earth for thousands of years
Artificial permanent magnets
• Hard steel
• Magnetic compass
• Alnico
Hard steel
Magnetic compass
The magnetic compass
was brought to Europe
in the Middle Ages from
the Chinese who had
been using the compass
for over 1500 years
Alnico is an acronym referring to iron alloys which in addition to iron are
composed primarily of aluminium (Al), nickel (Ni) and cobalt (Co). Alnico alloys
are ferromagnetic, with a high resistance to loss of magnetism and are used to
make permanent magnets. Before the development of rare earth magnets in the
1970s, they were the strongest type of magnet.
Aluminium cannot be magnetized
Electromagnets – magnets produced by
means of electric.
The magnetic field of a charged particle such as an electron in motion is
perpendicular to the motion of that particle. The intensity of the magnetic field
is represented by imaginary lines
If the electron's motion is a closed loop, as with an electron circling a nucleus,
magnetic field lines will be perpendicular to the plane of motion.
Electrons behave as if they rotate on an axis clockwise or counterclockwise. This
rotation creates a property called electron spin. The electron spin creates a
magnetic field, which is neutralized in electron pairs. Therefore, atoms that have
an odd number of electrons in any shell exhibit a very small magnetic field.
The lines of a magnetic field do not start or end as the lines of an electric field
do. Such a field is called bipolar or dipolar; it always has a north and a south
pole. The small magnet created by the electron orbit is called a magnetic
dipole
Spinning electric charges also induce a magnetic field. The proton in a hydrogen
nucleus spins on its axis and creates a nuclear magnetic dipole called a
magnetic moment. This forms the basis of MRI.
MRI electromagnet
Laws of magnetism:
• Every magnet has two poles
• Like magnetic poles repel unlike attract
• The force of attraction/repulsion follows inverse square
law (The magnetic force is proportional to the product of
the magnetic pole strengths divided by the square of the
distance between them.)
Every magnet has two poles
The lines of a magnetic field are always closed loops.
Like magnetic poles repel
Unlike poles attract
Breaking a magnet will result in each magnet
becoming a whole
Magnetic domain –accumulation of many
dipoles.
The magnetic dipole in bar magnet generate
imaginary lines of magnetic field.
Characteristics of the lines of force:
•
•
•
•
From N to F outside of the magnet
From S to N inside of the magnet
Lines in the same direction will repel each other
Magnetic field is distorted by magnetic material
but not affected by non-magnetic materials
Lines of force
LINES OF FORCE
Magnetic field is distorted by magnetic material but
not affected by non-magnetic materials.
This principle is employed with many MRI systems that use an iron magnetic
shield to reduce the level of the fringe magnetic field. Ferromagnetic material acts
as a magnetic sink by drawing the lines of the magnetic field into it.
A magnetic element can exist in magnetized
or non-magnetized form.
Magnetization:
• Induction
• Touch
MAGNETIC INDUCTION
(MAGNETIZATION)
Magnetization and demagnetization
Magnetic permeability
Ease with which the material can be
magnetized. Soft iron
Magnetic retentivity
Material resistance to magnetization.
Hard steel
Four Magnetic States of Matter
State
Material
Characteristics
Nonmagnetic
Wood, glass, copper
Unaffected by a
magnetic field
Water, plastic
Weakly repelled from
both poles of a
magnetic field
Paramagnetic
Gadolinium
Weakly attracted to
both poles of a
magnetic field
Ferromagnetic
Iron, nickel, cobalt
Can be strongly
magnetized
Diamagnetic
The SI unit of magnet field strength is the tesla. An older
unit is the gauss. One tesla (T) = 10,000 gauss (G).
The use of a compass might suggest that the Earth has a strong magnetic
field, but it does not. The Earth's magnetic field is approximately 50 μT at the
equator and 100 μT at the poles. This is far less than the magnet on a
cabinet door latch, which is approximately 100 mT.