Download magnetic field

MAGNETISM
What is the cause of magnetism?
• The motion of charges cause magnetism.
Example: Currents, spinning electrons.
An electromagnetic phenomenon where moving charges produce a magnetic field.
Magnetic Field: A region where a moving charge can experience a magnetic force.
The terms magnetism and magnetic field are synonymous with each other.
Types of Magnetism
• The magnetism exhibited by materials can
be classified as diamagnetic, paramagnetic,
or ferromagnetic.
Diamagnetic Materials
• All electrons are paired so there is no net
magnetic field.
• In the presence of an extremely strong
external magnetic field, the electron paths
are slightly altered, causing a small
repulsion of the material.
• Magnetic properties are lost after an
external magnetic field is removed.
Paramagnetic Materials
• A few electrons are unpaired so there is a
very weak magnetic field.
• In the presence of an extremely strong
external magnetic field, the electron paths
are slightly altered, causing a small
attraction of the material.
• Magnetic properties are lost after an
external magnetic field is removed.
Ferromagnetic Materials
• Many electrons are unpaired so there is a
moderate magnetic field in various regions of the
material called domains.
• A domain is a region with approximately 1020
electrons, the size of approximately 1 mm2.
• In the presence of an strong external magnetic
field, the electron domains align resulting in a
strong magnetic field within the material.
• Magnetic properties are retained long after the
external magnetic field is removed.
• Iron, cobalt, and nickel are ferromagnetic.
Why aren’t all materials ferromagnetic?
An electron pair.
In most elements the electron pairs spin in
opposite directions, cancelling the magnetic
field produced by the other electron.
Electron Spin
In ferromagnetic materials:
There are many more unpaired electrons thus
resulting, in a strengthened magnetic field.
Ex Iron
Which elements are ferromagnetic?
Iron, Cobalt, and Nickel
Domain: A region with approximately 1020 electrons (approximately 1 mm2).
In magnetic materials, the
magnetic fields in the domains
are arbitrarily orientated causing
the element to have a weak
magnetic field.
An external
magnetic field.
A magnet is created by subjecting a magnetic material to a strong external magnetic
field causing the domains to become aligned.
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Heat and/or sharp blows can cause the domains to become unaligned.
An alloy ALNICO (Aluminum, Nickel, and Cobalt) will make a magnet permanent
once the domains are aligned.
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Magnetic field lines are imaginary
lines to show the magnitude and
direction of magnetic force.
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The direction of a magnetic field
is the direction that a compass will
point if placed in the magnetic field.
Magnetic field lines are always
directed away from the north
towards the south.
The magnetic field lines always
form closed loops (continuous).
The magnetic field is said to
be north-seeking.
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North-seeking refers to the
geographic north of the Earth.
There are always two poles to
magnetic fields, north and south.
Earth’s Magnetic Field
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Earth’s geographic north is near the magnetic south.
Earth’s geographic south is near the magnetic north.
Various Magnetic Field Line Orientations
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The magnetic field between equal strength unlike magnetic poles is uniform.
Magnetic Field Lines between two North Poles.
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Magnetic Pole Force Law
• Like poles repel.
• Unlike poles attract
The Magnetic Field
•The magnetic field is denoted by the letter B.
•The SI unit for the magnetic field is measured in the unit called a
Tesla (T).
•The Tesla is part of the MKS system of units
•The older unit used for the magnetic field was a Weber/m2
(Wb/m2). 1 T = 1 Wb/m2
•A Tesla made up of the fundamental units of kg/As2 which come
from a (kg m/s2)/[(As)m/s]
•The magnetic field is a vector quantity.
•1 T=104 Gauss
A Gauss is the cgs measurement of a magnetic field.
To put magnetic strengths in perspective, here is a table of magnetic field strengths
The Earths magnetic field, which
deflects compass needles
(measured at the N magnetic pole)
0.6
Gaus
s
A common iron magnet
like those used to stick papers on a
refrigerator
100
Gaus
s
The strongest fields achieved so
far in the laboratory
Sustained (steady)
4X
105
The strongest fields achieved so
far in the laboratory
Ephemeral (made using explosives; lasts
only milliseconds)
107
The maximum field observed on
ordinary stars
-
106
Typical magnetic field of radio
pulsars
(the ordinary, familiar kind of neutron star;
(hundreds are known to astronomers)
1012
Magnetars
Gauss
(soft gamma repeaters)
Gauss
Gauss
Gauss
1014 1015
Gauss