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Magnetism
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History of Magnetism
The ancient Greeks > city of Magnesia > magnetit
The early Chinese > the power to attract iron
Around 1000 the Chinese found that such a needle,
when freely suspended, pointed north-south.
The magnetic compass soon spread to Europe.
Columbus used it when he crossed the Atlantic ocean
Around 1600 William Gilbert, physician to Queen
Elizabeth I of England, proposed an explanation:
the Earth itself was a giant magnet
The Magnetosphere
On Earth one needs a sensitive needle to detect magnetic
forces, and out in space they are usually much, much weaker.
But beyond the dense atmosphere, such forces have a much
bigger role, and a region exists around the Earth where they
dominate the environment, a region known as the Earth's
magnetosphere. That region contains a mix of electrically
charged particles, and electric and magnetic phenomena rather
than gravity determine its structure. We call it the Earth's
magnetosphere
Only a few of the phenomena observed on the ground come
from the magnetosphere: fluctuations of the magnetic field
known as magnetic storms and substorms, and the polar aurora
or "northern lights," appearing in the night skies of places like
Alaska and Norway. Satellites in space, however, sense much
more: radiation belts, magnetic structures, fast streaming
particles and processes which energize them. All these are
described in the sections that follow.
Types of magnets
 There are three main types of magnets:



Permanent magnets
Temporary magnets
Electromagnets
 Permanent Magnets

Permanent magnets are those we are most familiar with, such as the magnets hanging onto our
refrigerator doors. They are permanent in the sense that once they are magnetized, they retain a level
of magnetism. As we will see, different types of permanent magnets have different characteristics or
properties concerning how easily they can be demagnetized, how strong they can be, how their
strength varies with temperature, and so on.
 Temporary Magnets

Temporary magnets are those which act like a permanent magnet when they are within a strong
magnetic field, but lose their magnetism when the magnetic field disappears. Examples would be
paperclips and nails and other soft iron items.
 Electromagnets

An electromagnet is a tightly wound helical coil of wire, usually with an iron core, which acts like a
permanent magnet when current is flowing in the wire. The strength and polarity of the magnetic
field created by the electromagnet are adjustable by changing the magnitude of the current flowing
through the wire and by changing the direction of the current flow.
Properties of a magnet
For each north pole , there is a south pole as well!
Monopoles do not exist!
The opposite poles are
attracted to each other,
N
S
N
S
S
N
the same poles repel each
other.
N
S
Magnetic Field
Michael Faraday realized that a
magnet has a magnetic field
distributed
throughout
the
surrounding space.
- if Earth itself is considered as a
magnet, the south pole of that
magnet would be the one nearer
the north magnetic pole, and
vice-versa.
- the north magnetic pole is so
named not because of the
polarity of the field there but
because of its geographical
location
Magnetic Field
-Michael Faraday proposed a widely used method for visualizing magnetic
fields.
-Imagine a compass needle freely suspended in three dimensions, near a
magnet or an electrical current.
-We can trace in space (in our imagination, at least!) the lines one obtains
when one "follows the direction of the compass needle." Faraday called them
lines of force, but the term field lines is now in common use.
Compass needles outlining field lines!
-Field lines of a bar magnet are commonly illustrated by iron filings sprinkled on a
sheet of paper held over a magnet.
-Similarly, field lines of the Earth start near the south pole of the Earth, curve
around in space and converge again near the north pole.
-However, in the Earth's magnetosphere, currents also flow through space and
modify this pattern: on the side facing the Sun, field lines are compressed
earthward, while on the night side they are pulled out into a very long "tail," like
that of a comet.
-Near Earth, however, the lines remain very close to the "dipole pattern" of a bar
magnet, so named because of its two poles.
Magnetic field lines from an idealized model.
Magnetic field lines.
Magnetic field can be shown with the help of magnetic field lines.
If we put a compass/needle in a magnetic field, than the needle
will point along the field line.
Field lines converge
where the magnetic
field is strong and
spread out where it
is weak.
For example bar
magnet the lines
spread out from the
north pole and
converge
and
closes in the south
pole.
The magnetic field
is the strongest
where the lines are
closer together.
Magnetic field can be
characterized with two physical
quantities:
Magnetic field strength vector
H
and magnetic induction
Both are vector quantities which
means they have direction and
magnitude.
B
Diverse materials
Ferromagnetic materials are the ones normally thought of as 'magnetic';
they are attracted to a magnet strongly enough that the attraction can be
felt. Expl: refrigerator magnet.
Ferrimagnetic materials, which include ferrites and the oldest magnetic
materials magnetite and lodestone, are similar to but weaker than
ferromagnetics.
Paramagnetic substances such as platinum, aluminum, and oxygen are
weakly attracted to a magnet. This effect is hundreds of thousands of times
weaker than ferromagnetic materials attraction, so it can only be detected
by using sensitive instruments, or using extremely strong magnets..
Diamagnetic means repelled by both poles. Compared to paramagnetic
and ferromagnetic substances, diamagnetic substances such as carbon,
copper, water, and plastic are even more weakly repelled by a magnet. The
permeability of diamagnetic materials is less than the permeability of a
vacuum.
Untill now we talked about natural magnets and
their magnetic fields.
But not only magnets can have magnetic fields but
moving electric charges also create magnetic field.
Moving charges can be found in wires under
electric current .
Oersted’s experiment
Until 1821, only one kind of magnetism was known, the one produced by
iron magnets. Then a Danish scientist, Hans Christian Oersted, while
demonstrating to friends the flow of an electric current in a wire, noticed that
the current caused a nearby compass needle to move. The new
phenomenon was studied in France by Andre-Marie Ampere, who concluded
that the nature of magnetism was quite different from what everyone had
believed. It was basically a force between electric currents: two parallel
currents in the same direction attract, in opposite directions repel. Iron
magnets are a very special case, which Ampere was also able to explain.
In nature, magnetic fields are produced in the rarefied gas of space, in the
glowing heat of sunspots and in the molten core of the Earth. Such
magnetism must be produced by electric currents, but finding how those
currents are produced remains a major challenge.
In 1820 Oersted made an experiment:
Without current: needle/compass showed to the
Earth’s magnetic poles.
With current: Needle/compass showed in the current
generated magnetic field.
An electric current produced the deflection of a compass needle.
Another magnetic field than the Earth’s is affecting the
needle
Electric current produces magnetic field!!!!!!!
Magnetic field of a current-carrying wire
Straight wire: right-hand rule
0 I
B
2R
I
R
B
Loop:
I
B  0
2R
Solenoid:
A coiled wire
= lots of loops
I N
B  0
l
In a magnetic field:
Place a linear wire in the magnetic field of a horseshoe magnet.
Switch current on the wire. The wire will move perpendicularly to the
induction lines of the magnetic field.
If we change the direction of the current than the movement’s
direction will change also in the opposite direction.
B
S
Direction of the movement
F
l
B
I
I
N
-
Lorentz force
+
F
Force on wire under
current
 

F  I l  B
Force on a moving charge

 
F  qv  B
In general (in case of a solenoid frame) we can determine
the moment of rotation:
The force acting on one
side of the frame is:
Torque:

The angle between the
solenoid frame and the
magnetic field direction
of the shoemagnet.
F  I l  B
M  F  d  sin  A  l  d
M  B  I  l  d  sin   B  I  A  sin 
If we have more turns in our solenoid, the torque will change like this:
M  N  B  I  A  sin 

B
F  I l  B
F
B
I l
Magnetic induction
Unit:
Vs
B  2  T
m
tesla
  B A
Magnetic flux
In homogenous magnetic field the
magnetic flux is the number of the
induction lines passing through the
surface A perpendicular to the induction
lines.
  Vs  Tm
2
W
weber
Electromagnetic induction
If we move a wire in a magnetic field and it crosses induction
lines than electric potential can be measured at the ends of
the wire.
S
Direction of the movement
l
B
I
N
-
+
Electric potential
This phenomenon is called
electromagnetic
induction
and the measured potential is
called induced potencial.
Induced voltage
 Vi=Blv



v-velocity of the movement (m/s)
L-length
of the conductor (m)
In case of an
solenoid which has N coils:
Vi=BlvN
Lenz’s law
The direction of the potential enducing movement is opposite to the
direction of the movement enduced by the changing current.
The direction of the induced current is such that the magnetic
influence of it inhibits the inducing movement or changes.
S
S
l
B
l
B
I
I
N
N
moving
-
+
currentflow
-
+
An induced current is always in such a direction as to oppose the
motion or change causing it. (according to energy conservation law)
Magnitude of the induced potential
U  B l v

N
t
Neumann’s law: the induced potential is directly
proportional with magnetic induction, with the length of the
wire and the movements speed perpendicular to the
induction lines.
The induced electromotive force or
EMF in any closed circuit is
PROPORTIONAL to the time rate of
change of the magnetic flux through
the circuit.
The electromotive force is directly proportional to the change of
magnetic flux in by wire surrounded area and inverse proportional
with the time needed for the change.
Self-induction
If the current is changing in a conductor, the flux of the
conductor’s magnetic field will change too, which induces
potential in the conductor itself.
IN

A
2

BA

N
A I
I
l
s  N
N
N

 L
t
t
t
l t
t
Vs
L   H
A
H [ Henry] self-induction constant
Electromagnets
In order to concentrate the magnetic field generated by a wire, it is commonly wound
into a coil, where many turns of wire sit side by side.
Much stronger magnetic fields can be produced
if a "core" of ferromagnetic material, such as
soft iron, is placed inside the coil.
The magnetic field of all the turns of wire
passes through the center of the coil. A coil
forming the shape of a straight tube, a
helix (similar to a corkscrew) is called a
solenoid; a solenoid that is bent into a
donut shape so that the ends meet is a
toroid.
The ferromagnetic core magnifies the magnetic
field to thousands of times the strength of the field
of the coil alone, due to the high magnetic
permeability μ of the ferromagnetic material. This is
called
a
ferromagnetic-core
or
iron-core
electromagnet.
The main advantage of an electromagnet over a permanent magnet is that the
magnetic field can be rapidly manipulated over a wide range by controlling the
amount of electric current. However, a continuous supply of electrical energy is
required to maintain the field.
Electromagnet in use
Casette
Hard disks work similar way!
Generator, Electromotor
Electromagnetic waves
Faraday not only viewed the space around a magnet as filled with field
lines, but also developed an intuitive (and perhaps mystical) notion that
such space was itself modified, even if it was a complete vacuum. His
younger contemporary, the great Scottish physicist James Clerk
Maxwell, placed this notion on a firm mathematical footing, including in it
electrical forces as well as magnetic ones. Such a modified space is now
known as an electromagnetic field.
Today electromagnetic fields (and other types of fields as well) are a
cornerstone of physics. Their basic equations, derived by Maxwell,
suggested that they could undergo wave motion, spreading with the
speed of light, and Maxwell correctly guessed that this actually was light
and that light was in fact an electromagnetic wave.
Heinrich Hertz in Germany, soon afterwards, produced such waves by
electrical means, in the first laboratory demonstration of radio waves.
Nowadays a wide variety of such waves is known, from radio (very long
waves, relatively low frequency) to microwaves, infra-red, visible light, ultraviolet, x-rays and gamma rays (very short waves, extremely high frequency).
Radio waves produced in our magnetosphere are often modified by their
environment and tell us about the particles trapped there. Other such waves
have been detected from the magnetospheres of distant planets, the Sun
and the distant universe. X-rays, too, are observed to come from such
sources and are the signatures of high-energy electrons there.
Electromagnetic waves
If we have a capacitor with the capacity of C, and a solenoid with the
inductivity of L connected
the electric energy of the capacitor and the magnetic energy of the solenoid
periodically (quarter period) changes into each other.
If we open the plates of the capacitor to open oscillating circuit the electric
„space” runs free into the environment and a periodocally changing electric
(E) and magnetic (B) „space” appears.
This „space” has energy:
2
1
1B
2
w  0E 
2
2 0
w: energy density (E/V)
These alternating electric (E) and magnetic (B) field converts
from one to the other and so propagates in space.
Thank you for the attention!