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Chapter Thirty Six Notes:
Magnetism
July 1820: Oersted and
electromagnetism
Hans Christian Oersted
By the end of the 18th century, scientists had noticed many electrical phenomena and
many magnetic phenomena, but most believed that these were distinct forces. Then
in July 1820,
Danish natural philosopher Hans Christian Oersted published a
pamphlet that showed clearly that they were in fact closely related.
During a lecture demonstration, on April 21, 1820, while setting up his
apparatus, Oersted noticed that when he turned on an electric current by connecting
the wire to both ends of the battery, a compass needle held nearby deflected away
from magnetic north, where it normally pointed. The compass needle moved only
slightly, so slightly that the audience didn’t even notice. But it was clear to Oersted
that something significant was happening.
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Even in this day and age, most of the public regards magnetism
as a mystery. That has led to magnetic bracelets and similar "health
products," to magnets taped to fuel lines for better gas mileage, and
to widespread worries about possible reversal of the Earth's field,
encouraged by Hollywood movies.
In the minds of most Americans magnetism is forever associated
with specially treated iron, with patterns of iron filings and with the
way the compass needle lines up with the north-south direction. Few
schools teach much more, because, (1) physics is an elective, and (2)
magnetism is covered near the end of the textbook, the school year
is short, and teachers are happy if they just make it to Ohm's law.
Some people may also know that a current-carrying wire coil
wrapped around an iron bar turns it into a magnet, and about use of
electromagnets in electric machinery. But it's always with iron, or
with some magnetic substance. Why sunspots would be magnetic
remains completely unclear.
In ancient times, both Greeks and Chinese knew about natural
magnets, rare chunks of iron-rich mineral known as lodestones. The
Chinese also knew that if you rubbed a steel needle against a
lodestone, in a fixed direction, it also became a magnet. Around the
year 1000, they furthermore found that if a magnet or lodestone
was placed on a little "boat" floating in a bowl of water, it always
pointed in a fixed direction--and for a magnetized iron bar, that
direction was always north-south. You could rotate the bowl, but the
magnet would keep pointing in the same
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The reason, we now know, is that the Earth, too, is magnetic.
From that came the magnetic compass, quickly copied by Arab
navigators and then by Europeans. We may wonder today--if
lodestones did not exist, the compass might have stayed
undiscovered for a long time, and would Columbus have ventured so
far from land without it?
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Every magnet has two poles. This is where most of its magnetic
strength is most powerful. These poles are called north and south or
north-seeking and south seeking poles. The poles are called this as
when a magnet is hung or suspended the magnet lines up in a north
- south direction. When the north pole of one magnet is placed near
the north pole of another magnet, the poles are repelled. When the
south poles of two magnets are placed near one another, they also
are repelled from one another. When the north and south poles of
two magnets are placed near one another, they are attracted to one
another.
The attraction and repelling of two magnets towards one another
depends on how close they are to each other and how strong the
magnetic force is within the magnet. The further apart of the
magnets are the less they are attracted or repelled to one another.
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The magnetic and electric fields are both similar and different. They
are also inter-related.
Similar: Just as the positive (+) and negative (−) electrical charges
attract each other, the N and S poles of a magnet attract each other.
In electricity like charges repel, and in magnetism like poles repel.
Different : The magnetic field is a dipole field. That means that every
magnet must have two poles.
On the other hand, a positive (+) or negative (−) electrical charge
can stand alone. Electrical charges are called monopoles, since they
can exist without the opposite charge.
◦ • Monopole – a single magnetic pole or electric charge
◦ • Dipole – a pair of opposite poles
◦ • The so-called magnetic moment is the measure of the strength of the dipole.
The magnetic moments are expressed as multiples of Bohr Magnetons. A Bohr
magneton has a value of 9.27 x 10-24 joules/tesla.
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When a magnet is broken into little pieces, a north pole will appear
at one of the broken faces and a south pole. Each piece, regardless
of how big or small, has its own north and south poles.
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The area around a magnet can also behave like a magnet. This is
called a magnetic field. The larger the magnet and the closer the
object to the magnet, the greater the force of the magnetic field.
Magnetic Materials
The term magnetism is derived from Magnesia, the name of a region
in Asia Minor where lodestone, a naturally magnetic iron ore, was
found in ancient times. Iron is not the only material that is easily
magnetized when placed in a magnetic field; others include nickel
and cobalt.
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The magnetic field is the central concept used in describing
magnetic phenomena.
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• A region or a space surrounding a magnetized body or currentcarrying circuit in which resulting magnetic force can be detected.
• A magnetic field consists of imaginary lines of flux coming from
moving or spinning electrically charged particles. Examples include
the spin of a proton and the motion of electrons through a wire in
an electric circuit.
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Magnetic field or lines of flux of a moving charged particle
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A magnetized bar has its power concentrated at two ends, its poles;
they are known as its north (N) and south (S) poles, because if the
bar is hung by its middle from a string, its N end tends to point
northwards and its S end southwards. The N end will repel the N end
of another magnet, S will repel S, but N and S attract each other. The
region where this is observed is loosely called a magnetic field.
Either pole can also attract iron objects
such as pins and paper clips. That is
because under the influence of a nearby
magnet, each pin or paper clip becomes
itself a temporary magnet, with its poles
arranged in a way appropriate to
magnetic attraction.
But this property of iron is a very special type of magnetism, almost an
accident of nature!
Out in space there is no magnetic iron, yet magnetism is widespread. For
instance, sunspots consist of glowing hot gas, yet they are all intensely
magnetic. The Earth's own magnetic powers arise deep in its interior, and
temperatures there are too high for iron magnets, which lose all their power
when heated to a red glow. What goes on in those magnetized regions?
It is all related to electricity.
MAGNETIC FORCE
The magnetic field of an object can create a magnetic force on other
objects with magnetic fields. That force is what we call magnetism.
When a magnetic field is applied to a moving electric charge, such as
a moving proton or the electrical current in a wire, the force on the
charge is called a Lorentz force.
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Attraction
When two magnets or magnetic objects are close to each other,
there is a force that attracts the poles together.
Force attracts N to S
Magnets also strongly attract ferromagnetic materials such as iron,
nickel and cobalt.
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Repulsion
When two magnetic objects have like poles facing each other, the
magnetic force pushes them apart.
Force pushes magnetic objects apart
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Magnetic and electric fields
The magnetic and electric fields are both similar and different. They
are also inter-related.
Each atom that makes up a substance is a time magnet. When atoms
are arranged not in random directions but all in the same direction,
the substance is a permanent magnet.
Magnetism and electricity are very closely related, so that the flow of
electricity through a conductive wire generates a magnetic field, and
conversely a change in a magnetic field produces a flow of current in
a conductor.
How is Magnetism Produced?
The electrons in an atom spin as they rotate about the nucleus. This
spinning motion creates a magnetic effect in each electron, which
together forms a magnetic field around the atom.
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Normally, the atoms in any
substance are oriented in
random fashion throughout
the substance. This means
that
their
individual
magnetic fields cancel each
other, and the substance as
a whole does not appear
magnetically charged.
In a permanent magnet, all
of the electrons are oriented
in the same direction. This
means that the substance as
a whole acts as a magnet.
The lines that show the
direction of the magnetic
field are called magnetic
lines.
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Even when the atoms are oriented randomly, exposure to a nearby
magnet may cause them to line up with the magnetic field.
Substances that do this easily, such as iron or nickel, can be
'magnetized.‘
Other substances, in which the atoms remain randomly oriented
even when exposed to a magnet, such as copper, wood or plastic,
cannot be magnetized.
When a coil is wrapped around an iron bar and electric current is
passed through the coil, the iron bar picks up a magnetic field, and
becomes an 'electromagnet.' The strength of the magnetic field is
proportional to the size of the current flow. The relation is similar
to the way wind passing through a windmill (current flow) flows at
right angles to the plane of rotation of the windmill blades
(creation of the magnetic field).
If the wind reverses direction,
the windmill will also rotate in
the opposite direction.
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If a permanent magnet is inserted and withdrawn through the center
of the coil, it causes an electric current to flow in the coil. The
direction of the current flow is in opposition to the change in the
magnetic field, so that the current is reversed each time the
permanent magnet is inserted or withdrawn. This is the principle of
the electric generator. The relation is similar to the way a windmill
revolves (current flows) in response to the wind passing through it
(movement of the magnet).
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The connection between electric current and
magnetic field was first observed when the
presence of a current in a wire near a
magnetic compass affected the direction of
the compass needle. We now know that
current gives rise to magnetic fields, just as
electric charge gave rise to electric fields.
With positive current, point
your thumb in the
direction of the current
and your fingers wrap
around the wire in the
direction of the B field.
Compass near a current-carrying wire
[B-field = Magnetic field]
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Form a loop with current carrying wire, and the concentration of the
magnetic field within the loop is much stronger. Double the number
of loops, and the magnetic field is twice as strong. The magnetic
field intensity increases with the number of loops. A current
carrying coil of wire with many loops is an electromagnet.
Sometimes a piece of iron is placed inside
the coil of an electromagnet. The magnetic
domains of the iron are induced into
alignment, increasing the magnetic field
intensity. Beyond a certain limit, the
magnetic field in the iron “saturates,” so
iron is not used in the cores of the
strongest electromagnets, which are made
of superconducting material (section 34.4)
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Orbit of charged particle
in a magnetic field
A charged particle moving in a plane
perpendicular to a magnetic field will
move in a circular orbit with the
magnetic force playing the role of
centripetal force.
The direction of the force is given by
the right-hand rule.
Equating the centripetal force with
the magnetic force and solving for R
the radius of the circular path we get
mv2 / R = q v B and
R=mv/qB
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Right Hand Rule:
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Since a charged particle
moving
through
a
magnetic
field
experiences a deflecting
force, a current of
charged
particles
moving
through
a
magnetic
field
also
experiences a deflecting
force. The direction of
that
deflection
is
dependent
upon
the
direction of the current.
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The basic galvanometer,
devised by William Sturgeon in
1825, allows all of the various
combinations of current and
magnetic needle direction to be
tried out. By making suitable
connections to the screw terminals,
current can flow to the right or to
the left, both above and below the
needle. Current can be made to
travel in a loop to double the
effect, and, with the aid of two
identical external galvanic circuits,
the currents in the two wires can
be made parallel and in the same
direction. Note that the wires are
insulated from each other where
they cross.
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Electric Motor
A current-carrying loop in a B field is the basis of an electric motor.
Using the right-hand rule one can see that the forces acting on the
wire will cause the loop to rotate. Changing the current direction at
the right time will cause the loop to continue rotating on the motor
shaft.
DC motor
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In ancient times, both Greeks and Chinese knew about natural
magnets, rare chunks of iron-rich mineral known as lodestones. The
Chinese also knew that if you rubbed a steel needle against a
lodestone, in a fixed direction, it also became a magnet. Around the
year 1000, they furthermore found that if a magnet or lodestone was
placed on a little "boat" floating in a bowl of water, it always pointed
in a fixed direction--and for a magnetized iron bar, that direction
was always north-south. You could rotate the bowl, but the magnet
would keep pointing in the same direction.
The reason, we now know, is that the Earth, too, is magnetic.
From that came the magnetic compass, quickly copied by Arab
navigators and then by Europeans. We may wonder today--if
lodestones did not exist, the compass might have stayed
undiscovered for a long time, and would Columbus have ventured so
far from land without it?
Robert Norman and an early scientific experiment
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Figure 4
This is mainly about explaining a very fundamental concept in
science--the experiment. A scientific experiment is a way of testing
nature, to learn how it behaves.
By 1580, the use and manufacture of compass needles was a well
known art. The maker would take a flat steel needle, find its middle
by balancing it, install a pivot there, and then magnetize it by
stroking it against a magnet or a lodestone. But that was not
enough. The north-pointing end always seemed heavier, and a tip
had to be snipped off, to make the needle balance again.
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The story goes that a compass maker named Robert Norman once
snipped off too much and ruined a needle, so he devised an
experiment, to find what was happening. Before magnetizing the
needle, he balanced it not on a vertical pivot but on a horizontal
one, lined up in the east-west direction. (Figure 4, above). Before the
needle was magnetized, it stayed horizontal. Afterwards, its north
end slanted down. (For some reason, the needle in Figure 4 points
straight down, as it would at the magnetic pole.) Aha! The northpointing magnetic force on the needle was not horizontal, but
pointed into the Earth.
It was a classical scientific experiment, one of
the first, and was published in 1581. Norman's
contemporary was William Gilbert, distinguished
physician and later physician to Queen Elizabeth I.
Gilbert devoted much of his energy and money to
study magnetism, and in 1600 published his
research in a book "De Magnete" (Latin for "On the
Magnet"). The preceding visualization of the
downward "dip angle" of the magnetic force (Slide
5) is from this book.
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Slide 5
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Gilbert devised an experiment which suggested a reason for the
properties of the compass: the Earth itself was a giant magnet.
Using as model for the Earth a lodestone fashioned into a sphere (he
named it "terrella" or "little Earth"), Gilbert reproduced not only the
north pointing properties of the horizontal needle, but also the
downward slanting of the needle which Robert Norman made.
You will find two reviews of Gilbert's
book and a lot more, including most of
what we are telling you here today, in a
web course on Earth magnetism, "The
Great Magnet, the Earth" With home
page by Dr. David P. Stern :
http://www.phy6.org/earthmag/demagint.htm .
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I found the following story, and thought it was extremely interesting!
If you would like to read the entire story go to the following website:
http://science.nasa.gov/headlines/y2003/29dec_magneticfield.htm
December 29, 2003: Every few years, scientist Larry Newitt of the
Geological Survey of Canada goes hunting.
His quarry is Earth's north magnetic pole.
At the moment it's located in northern Canada, about 600 km from
the nearest town: Resolute Bay, population 300, where a popular Tshirt reads "Resolute Bay isn't the end of the world, but you can see
it from here.“
 Scientists have long known that the magnetic pole moves.
Sometimes the field completely flips. The north and the south poles
swap places. Such reversals, recorded in the magnetism of ancient
rocks, are unpredictable.
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At the heart of our planet lies a solid iron ball, about as hot as the
surface of the sun. Researchers call it "the inner core." It's really a
world within a world. The inner core is 70% as wide as the moon. It
spins at its own rate, as much as 0.2° of longitude per year faster
than the Earth above it, and it has its own ocean: a very deep layer
of liquid iron known as "the outer core."
At the heart of our planet lies a solid iron ball, about as hot as the
surface of the sun. Researchers call it "the inner core." It's really a
world within a world. The inner core is 70% as wide as the moon. It
spins at its own rate, as much as 0.2° of longitude per year faster
than the Earth above it, and it has its own ocean: a very deep layer
of liquid iron known as "the outer core."
Earth's magnetic field comes from this ocean of iron, which is an
electrically conducting fluid in constant motion. Sitting atop the hot
inner core, the liquid outer core seethes and roils like water in a pan
on a hot stove. The outer core also has "hurricanes"--whirlpools
powered by the Coriolis forces of Earth's rotation. These complex
motions generate our planet's magnetism through a process called
the dynamo effect.
The next page has some of the figures pertaining to the information
given above. If you would like more, go to the website!
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Right:
a schematic
diagram of Earth's
interior. The outer
core is the source of
the
geomagnetic
field.
The movement of Earth's
north
magnetic
pole
across
the
Canadian
arctic, 1831--2001.
Supercomputer models of Earth's magnetic field.
On the left is a normal dipolar magnetic field,
typical of the long years between polarity reversals.
On the right is the sort of complicated magnetic
field Earth has during the upheaval of a reversal.