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Magnetism

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History
Natural and artificial magnets
Poles of a magnet
Magnetic field
Magnetic field created by a bar magnet and
by a U-shaped magnet
Resultant magnetic field
Magnetization
Earth magnetic field
Magnetic field created by an electric current
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In ancient China, the earliest literary
reference to magnetism lies in a 4th century
BC book called Book of the Devil Valley
Master (鬼谷子): "The lodestone makes iron
come or it attracts it”
Alexander Neckham was the first in Europe to
describe the compass and its use for
navigation in 1269.
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Magnets are found in nature.
Magnetite is a ferromagnetic mineral with
chemical formula Fe3O4.
Magnetite is the most magnetic of all the
naturally occurring minerals on Earth.
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Artificial magnets are more powerful than
natural magnets.
There are many shapes of artificial magnets:
1- Bar magnet:
2- U-shaped magnet:
3- Needle magnet:
4- Other:
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A magnet has two poles: North and South.
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Unlike poles attract and like poles repel.
1. What is the name of the substance in a natural
magnet? And what is its formula?
Magnetite, Fe3O4
2. What are the types of magnets?
U-shaped, bar, needle and other
3. Identify the pole X of the first magnet:
X is South
Magnetic fields are produced by electric
currents, which can be macroscopic currents in
wires, or microscopic currents associated with
electrons in atomic orbits.
 The symbol of the magnetic field is B, and its
(SI) unit is Tesla (T).
 A magnetic field is detected by a magnetic
needle.
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Nikola Tesla (10 July 1856 – 7 January 1943)
was a Serbian-American inventor, physicist,
mechanical engineer, electrical engineer, and
futurist.
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If at any point on a field line we place an ideal
compass needle, free to turn in any direction then the
needle will always point along (tangent to) the field
line.
Field lines converge where the magnetic field is
strong, and spread out where it is weak.
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The magnetic field inside a U-shaped magnet is
uniform. (it has the same direction and value).
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In a point of the space many magnetic fields
may superpose.
The resultant magnetic field is the vector sum
of all the magnetic fields. B = B1 +B2
1.
2.
3.
4.
The unit “Tesla” refers to the Sweden physician Nicolas
Tesla.
Field lines converge where the magnetic field is strong.
The magnetic field between the poles of a bar magnet is
uniform.
The resultant of magnetic fields B1 and B2 is B in the
figure below where B1 = 3mT, B2 = 4mT and B = 7mT.
Temporary:
Permanent
Earth's magnetic field is the magnetic field
that extends from the Earth's inner core to
where it meets the solar wind (a stream of
energetic particles emanating from the Sun). It
is approximately the field of a magnetic dipole
tilted at an angle of 11 degrees with respect to
the rotational axis—as if there were a bar
magnet placed at that angle at the center of the
Earth. However, unlike the field of a bar
magnet, Earth's field changes over time
because it is generated by the motion of
molten iron alloys in the Earth's outer core.
The solar wind is a stream of ionized gases
that blows outward from the Sun at about 400
km/second and that varies in intensity with
the amount of surface activity on the Sun.
The Earth's magnetic field shields it from
much of the solar wind. When the solar wind
encounters Earth's magnetic field it is
deflected like water around the bow of a ship.
The Earth's magnetic field is key for stopping dangerous
things called cosmic rays. It deflects most of them and
keeps them from reaching the earth. Cosmic rays are mostly
protons, with a few electrons and a bit of gamma radiation
thrown in. And the first two are charged particles, which
will be deflected when moving through a magnetic field.
The can (and does) actually funnel the charged particles
toward the magnetic poles, and these charges can "slip
down" along the earth's magnetic field lines and interact
with the upper atmosphere. This is the mechanism behind
the aurora.
Another effect of the loss of the field would be that a
magnetic compass would no longer help us navigate. (the
iron core of the earth is responsible for the existence of he
magnetic fields)
http://wiki.answers.com/
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If the magnetic needle is free to rotate in
any direction it will take the direction of the
terrestrial magnetic field which is not
horizontal.
BT = Bh + Bv
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If the needle is able to rotate in the
horizontal plane only so it will take the
direction of Bh.
The magnetic meridian in the vertical plane
containing the terrestrial magnetic field.
History
 Magnetic field created by an electric current in
a long rectilinear wire
 Magnetic field created by an electric current in
a loop
 Magnetic field created by an electric current in
a flat coil
 Magnetic field created by an electric current in
a solenoid
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Hans Christian Oersted was a professor of science at
Copenhagen University. In 1820 he arranged in his
home a science demonstration to friends and students.
He planned to demonstrate the heating of a wire by an
electric current, and also to carry out demonstrations of
magnetism, for which he provided a compass needle
mounted on a wooden stand. While performing his
electric demonstration, Hans Christian Oersted noted to
his surprise that every time the electric current was
switched on, the compass needle moved. He kept quiet
and finished the demonstrations, but in the months that
followed worked hard trying to make sense out of the
new phenomenon.
However, Hans Christian Oersted could not explain
why. The needle was neither attracted to the wire nor
repelled from it. Instead, it tended to stand at right
angles. In the end he published his findings without any
explanation.
Characteristics of B:
- Point of application: A
- Line of action: perpendicular to the plane
(wire, point A)
- Direction: given by the right hand rule
- Magnitude:
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Where I is the intensity of the current in (A) and
d is the distance between the point and the
wire.
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If a vector is directed inwards:
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If a vector is directed outwards:
I
A
A
I
A
I
Characteristics of B:
- Point of application: center of the coil
- Line of action: perpendicular to the
plane of the coil
- Direction: given by the right hand rule
- Magnitude:
Where I is the intensity of the current in (A) and R is the
radius of the coil in (m) and N is the number of loops.
Characteristics of B:
- Point of application: Any point of the
axis of the solenoid.
- Line of action: Parallel to the axis of the
solenoid.
- Direction: given by the right hand rule
- Magnitude:
Where I is the intensity of the current in (A) and L is the
length of the solenoid in (m) and N is the number of
loops
Remarks:
1is a physical constant noted
and called vacuum permeability or magnetic
constant.
2- Sometimes it is given the number of loops
per meter n, so the formula becomes:
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Now open your books to the page …
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Electromagnetic interaction is one of the four
fundamental interactions of nature. The other
three are the strong interaction, the weak
interaction and gravitation. Electromagnetism
is the force that causes the interaction
between electrically charged particles; the
areas in which this happens are called
electromagnetic fields.
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Electromagnetism is responsible for practically all
the phenomena encountered in daily life, with the
exception of gravity. Ordinary matter takes its
form as a result of intermolecular forces between
individual molecules in matter. Electromagnetism
is also the force which holds electrons and
protons together inside atoms, which are the
building blocks of molecules. This governs the
processes involved in chemistry, which arise from
interactions between the electrons orbiting
atoms.
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Electromagnetism manifests as both electric
fields and magnetic fields. Both fields are
simply different aspects of electromagnetism,
and hence are intrinsically related. Thus, a
changing electric field generates a magnetic
field; conversely a changing magnetic field
generates an electric field. This effect is
called electromagnetic induction, and is the
basis of operation for electrical generators,
induction motors, and transformers.
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Center of gravity of the part of the rod found
in the field.
Perpendicular to the plane formed by B and L
Given by the three fingers of the right hand.
F = I.B.L.sinα, with α = (B,L)
In Faraday's apparatus, a copper wire hangs
with one end near a magnet in a dish of
mercury. When charged with electricity, the
wire revolves around the magnet. The simple
experiment unites electricity, magnetism, and
motion
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