Diapositiva 1

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Electromagnetism
Plasma lamp The central sphere is an electrode
and the glass sphere is filled with gases
and driven by an alternating current.
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Contents

• Electricity
• By Sala Luca
• Magnets and the magnetic force
• By Meroni Davide
• Electromagnetism
• By Casati Denis

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Electricity

• Electricity is the science associated with the
  presence and flow of electric charges.
• Electricity is due to several types of
  physics
• Electric Charge
• Electric Current
• Electric Field
• Electric Potential

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Electric Charge

• Electricity is the flow of electric charges. The
  basic units of charge are the proton and
  electron the proton charge is positive while the
  electron charge is negative. Two particles which
  have the same charges, positive or negative,
  repel each other, while two particles which have
  different charges attract each other according to
  Coulombs law the charges on electrons and
  protons, which are equal and opposite, are
  defined as
• e1,610-19 Coulombs

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Charles-Augustin de Coulomb

• The presence of charges gives rise to the
  electromagnetic force. These phenomena were
  investigated by Charles-Augustin de Coulomb, who
  deduced that a charge manifests itself in two
  opposing forms
• like-charged objects repel and opposite-charged
  objects attract.

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• The magnitude of the electromagnetic force,
  whether attractive or repulsive, is given by
  Coulomb's law The scalar form of Coulomb's law
  is an expression for the magnitude and sign of
  the electrostatic force between two idealized
  point charges, small in size compared to their
  separation. This force (F) acting simultaneously
  on point charges (q1) and (q2), is given by

where r is the separation distance and ke is a
proportionality constant.
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• A positive force implies it is repulsive,
  while a negative force implies it is attractive.
• The coulomb (symbol C) is the SI derived
  unit of electric charge.
•  

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Electric Current

• The movement of electric charge is known as
  an electric current, the intensity of which is
  usually measured in amperes. Current can consist
  of any moving charged particles most commonly
  these are electrons, but any charge in motion
  constitutes a current.

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Hans Christian Ørsted

• One of the most important discoveries related
  to current was made accidentally by Hans
  Christian Ørsted in 1820. During a lecture,
  Ørsted noticed a compass needle deflected from
  magnetic north when an electric current from a
  battery was switched on and off, confirming a
  direct relationship between electricity and
  magnetism. He said that an electric current
  produces a circular magnetic field as it flows
  through a wire

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André-Marie Ampère

• André Marie Amperé in France discovered that
  the fundamental nature of magnetism was not
  associated with magnetic poles or iron magnets,
  but with electric currents.

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Electric field

• The concept of the electric field was
  introduced by Michael Faraday. An electric field
  is created by a charged body in the space that
  surrounds it, and results in a force exerted on
  any other charges placed within the field. The
  field may be visualised by a set of imaginary
  lines of force whose direction at any point is
  the same as that of the field. The field lines
  are the paths that a point positive charge would
  seek to make as it was forced to move within the
  field they are however an imaginary concept with
  no physical existence, and the field permeates
  all the intervening space between the lines.
• Field lines emanating from stationary
  charges have several key properties firstly they
  originate at positive charges and terminate at
  negative charges secondly they must enter any
  good conductor at right angles and finally they
  may never cross nor close in on themselves.

Michael Faraday
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Electric Potential

• The concept of electric potential is closely
  linked to that of the electric field. A small
  charge placed within an electric field
  experiences a force, and to have brought that
  charge to that point against the force requires
  work. It is usually measured in volts, and one
  volt is the potential for which one joule of work
  must be expended to bring a charge of one coulomb
  from infinity.

An electric field has the special property that
it is conservative, which means that the path
taken by the test charge is irrelevant all paths
between two specified points expend the same
energy, and thus a unique value for potential
difference may be stated.
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Magnetism

• - History of magnetism
• Compass
• Magnets
• Magnetic force
• Magnetic field lines
• Earths magnetic field is fading
• Magnetic shield
• Magnetic fields on the Sun
• - Right-hand rule

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History Of Magnetism

• In ancient China, the earliest literary reference
  to magnetism lies in a 4th century BC book called
  Book of the Devil Valley Master. The ancient
  Chinese scientist Shen Kuo (10311095) was the
  first person to write of the magnetic needle
  compass.
• Alexander Neckham, by 1187, was the first in
  Europe to describe the compass and its use for
  navigation. In 1269, Peter Peregrinus de
  Maricourt wrote the Epistola de magnete, the
  first extant treatise describing the properties
  of magnets.
• In 1282, the properties of magnets and the dry
  compass were discussed by Al-Ashraf, a physicist,
  astronomer, and geographer. In 1600, William
  Gilbert published his De Magnete, Magneticisque
  Corporibus, et de Magno Magnete Tellure (On the
  Magnet and Magnetic Bodies, and on the Great
  Magnet the Earth).
• In this work he describes many of his experiments
  and he concluded that the Earth was itself
  magnetic and that this was the reason compasses
  pointed north. An understanding of the
  relationship between electricity and magnetism
  began in 1819 with work by Hans Christian Oersted.

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Compass

• A compass is a navigational instrument that
  measures directions in a frame of reference that
  is stationary relative to the surface of the
  Earth.
• The frame of reference defines the four cardinal
  directions (or points) north, south, east, and
  west. Usually, a diagram called a compass rose,
  which shows the directions (with their names
  usually abbreviated to initials), is marked on
  the compass.
• There are different types of compass
• the magnetic compass contains a magnet that
  interacts with the earth's magnetic field and
  aligns itself to point to the magnetic poles
• the gyro compass (sometimes spelled with a
  hyphen, or as one word) contains a rapidly
  spinning wheel whose rotation interacts
  dynamically with the rotation of the earth.

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Magnets

• A magnetized bar has its power concentrated at
  two ends, its poles known as north(N) and
  south(S).
• 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 colled
  magnetic field. Either pole can also attract iron
  objects such as pins and paper clips.
• That is because under the influence of a magnet,
  each pin or paper clip becomes itself a temporary
  magnet.

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Magnetic Force

• In 1821 Hans Christian Oersted (1777-1851) in
  Denmark found that an electric current produced a
  magnetic force. Andrè-Marie Ampère (1775-1836) in
  France discovered that the fundamental nature of
  magnetism was associated with electric currents.
  The magnetic force was a force between electric
  currents.

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Magnetic Field Lines

• Michael Faraday proposed a method for visualizing
  magnetic fields.
• Field lines of a bar magnet are commonly
  illustrated by iron filings sprinkled on a sheet
  of paper held over a magnet.
• The filings line up in the space of the field.

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Earths Magnetic Field Is Fading

• Earths magnetic field is fading.
• Today is about 10 percent weaker than it was
  when German mathematician Gauss started keeping
  tabs on it in 1845, scientists say.
• If the trend continues, the field may collapse
  altogether and then reverse. Compasses would
  point south instead of north.

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Magnetic Shield

• The geo-dynamo is the mechanism that creates our
  planets magnetic field, maintains it, and causes
  it to reverse.
• Earths geo-dynamo creates a magnetic field that
  shields most of the habited parts of our planet
  from charged particles that come mostly from the
  sun.
• The field deflects the speeding particles toward
  Earths Poles.
• Without our planets magnetic field, Earth would
  be subjected to more cosmic radiation.
• The increase could knock out power grids,
  scramble the communications systems on spacecraft.

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Magnetic Fields On The Sun

• Many of the interesting features observed on the
  Sun by Yohkoh are magnetic. Indeed, much of the
  structure of the Sun's corona is shaped by the
  magnetic field, just like the pattern of the iron
  filings.
• Although it varies over time and from place to
  place on the Sun, the Sun's magnetic field can be
  very strong.
• Inside sunspots, the magnetic field can be
  several thousand times the strength of the
  Earth's magnetic field.

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Right-hand rule
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Right-hand rule
In mathematics and physics, the right-hand
rule is a common mnemonic device in order to
understand conventions for vectors in 3
dimensions. It was invented, for use in
electromagnetism, by British physicist John
Ambrose Fleming in the late 19th century.
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A different form of the right-hand rule is used
in situations where a vector must be assigned to
the rotation of a body, a magnetic field or a
fluid. When a rotation is specified by a vector,
and it is necessary to understand the way in
which the rotation occurs, the right-hand grip
rule is applicable. This version of the rule is
used in two applications of Ampère's circuital
law 1. an electric current passes through
a solenoid (a coil forming the shape of a
straight tube, a helix, is called a solenoid),
creating a magnetic field. When you wrap your
right hand around the solenoid with your fingers
in the direction of the  current, your thumb
points in the direction of the magnetic north
pole 2. an electric current passes through a
straight wire. Here, the thumb points in the
direction of the current (from positive to
negative), and the fingers point to the direction
of the magnetic lines of flux.
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Electromagnetism

• Faradays law
• Electromagnet
• Pick up
• AC/DC Generator
• Electric Circuits
• Maxwell

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Electromagnetism

• Electromagnetism is the branch of science concerne
  d with the forces that occur between electrically
  charged particles. In electromagnetic theory
  these forces are explained using electromagnetic
  fields.
• Electric fields are the cause of several common
  phenomena, such as electric potential (such as
  the voltage of a battery) and electric current
  (such as the flow of electricity through a
  flashlight).
•  
• Electromagnetism manifests as both electric
  fields and magnetic fields. Both fields are
  different aspects of electromagnetism. In fact,
  if a change occurs in an electric field, a
  magnetic field is generated, and viceversa if a
  change occurs in an magnetic field, a electric
  field is generated.
• This effect is called electromagnetic induction,
  and is the basis of operation for electrical
  generators, induction motors, and transformers.

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Faraday's Law We suppose that we have
a spire if we take a magnet and we move it in
and out,a current is created in the spire.This
current is called the induced current.This
current is generated even if we keep holding
the magnet and move the spire.If this current in
the coil is generated, we say that there is an
electromotive force.
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Electromagnet An electromagnet is a type
of magnet where the magnetic field is produced by
the flow of electric current. The magnetic field
disappears when the current is turned off.
Electromagnets are widely used as components of
other electrical devices, such as Pick up in an
electric guitar.
The magnetic field of all the turns of wire
passes through the center of the coil, creating a
strong magnetic field there. A coil forming the
shape of a straight tube (a helix) is called
a solenoid. Much stronger magnetic fields can be
produced if a "core" of ferromagnetic material,
such as soft iron, is placed inside the coil. The
ferromagnetic core increases the magnetic field
up to thousands of times the strength of the
field of the coil, due to the high magnetic
permeability (µ0) 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.
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The strenght of the magnetic field around the
coil can be increased by Using a soft iron core
Using more turns of wire on the coil Using a
bigger current If we reverse the direction of the
current, we reverse the magnetic field direction.
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Electromagnets in the Pick-up
The most elementary form of a pick-up is made by
a fixed bar magnet and a very small copper wire
(the size of a hair) that is wrapped around. 
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The wire turns around the bar several times
(thousands), thus creating an electric coil. The
magnet coil generates a magnetic field around
itself. As the pickup is placed just below, the
strings of the guitar interact with it.  When
the string is still, the magnetic field is
inert. As soon as we touch the string, the shape
of the field is altered. The alteration of
the lines of force in the magnetic field causes a
small pulse of electrical energy,
which then reach the amplifier in the form
of alternating current. The movement of
the vibrating string is always different
it depends on the note that is being played.  
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Back in Black
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AC Generator
If we turn a coil in a magnetic field, we
produce motional emfs (electromagnetic forces) in
both sides of the coil. The component of the
velocity, perpendicular to the magnetic field,
changes sinusoidally with the rotation the
voltage, which has been generated, is sinusoidal
or AC. This process can be described in terms
of Faraday's lawwe see that the rotation of the
coil continually changes the magnetic
flux through the coil and therefore it generates
a voltage.
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DC Generator
The essential difference between an AC and a DC
generator is the nature of the connection between
the rotor coils and the external circuit.
In a DC generator, the brushes run on a
split-ring commutator which reverse the
connection between the coil and the external
circuit for every half-turn of the coil.The
voltage in the external circuit fluctuates
between zero and a maximum, while the current
flows in a constant direction.
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Electric circuits

• An electric circuit is an interconnection
  of electric components. The components in an
  electric circuit can take many forms, which can
  include elements such as resistors, capacitors,
  switches, transformers and electronics.
• The resistor is the simplest element of
  passive circuit it resists the current that
  flows through it, dissipating its energy as heat.
  The resistance is a consequence of the motion of
  charge through a conductor.
• Ohm's law is a basic law of circuit theory the
  current through a conductor between two points is
  directly proportional to the potential difference
  across the two points.

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• I is the current through the conductor in units
  of amperes.
• V is the potential difference measured across
  the conductor in units of volts.
• R is the resistance of the conductor in units of
  ohms, symbolised by the Greek letter O.

The capacitor consists of two conducting plates
separated by a thin insulating layer If the
charges on the plates are q and -q, and V gives
the voltage between the plates, then the
capacitance is given by
The unit of capacitance is the farad F
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• The inductor is a conductor, usually a coil
  of wire, that stores energy in a magnetic field
  in response to the current through it. When the
  current changes, the magnetic field does too,
  inducing a voltage between the ends of the
  conductor.
• The relationship between the self
  inductance L of an electrical circuit in henries,
  voltage and current is

where v denotes the voltage in volts and i the
current in amperes. The voltage across an
inductor is equal to the product of its
inductance and the time rate of change of the
current through it.
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Maxwell
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James Clerk Maxwell

• Electromagnetic waves were analysed
  theoretically by James Clerk Maxwell in 1864.
  Maxwell developed a set of equations that could
  unambiguously describe the interrelationship
  between electric field, magnetic field, electric
  charge, and electric current. Maxwell's
  equations describe how electric charges and
  electric currents act as sources for the electric
  and magnetic fields of the four equations, two
  of them, Gauss law and Gauss' law for magnetism,
  describe how the fields are emanated from
  charges, while the Ampère's law (with Maxwell's
  correction) and Faraday's law describe how the
  fields 'circulate' around their respective
  sources.

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Paradox of Ampere theory
On the left CB J0 I
In the centre CB 0
On the rigth CB J0 I
When we use the word centre, we have to consider
that we are inside the space of the capacitor.
How does the current cross the void without a
wire?
Between the two plates, there is only an electric
field.
Maxwell, using the Gauss law, was able
to correct the Amperes equation and combine
every fundamental equations of electricity and
magnetism. Furthermore Maxwell noted also the
trend of CB e CE these move in a sinusoidal
way along planes that are perpendicular to each
other.
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Maxwell equations
Gauss's law
Gauss's law for magnetism
MaxwellFaraday equation(Faraday's law of
induction)
Ampère's circuital law(with Maxwell's
correction)
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Maxwell also discovered the speed of these
waves with the relation
If we are in the void, this speed becomes
C299 792,458 km/s
The speed of light!!!
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If we change the frequency, we have different
types of waves
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SI electromagnetism units SI electromagnetism units SI electromagnetism units SI electromagnetism units SI electromagnetism units
Symbol Name of Quantity Derived Units Unit Base Units
I Electric current ampere A A ( W/V C/s)
Q Electric charge coulomb C As
U, ?V, ?f E Potential difference Electromotive force volt V kgm2s-3A-1 ( J/C)
R Z X Electric resistance Impedance Reactance ohm O kgm2s-3A-2 ( V/A)
? Resistivity Ohm metre Om kgm3s-3A-2
P Electric power Watt W kgm2s-3 ( VA)
C Capacitance Farad F kg-1m-2s4A2 ( C/V)
E Electric field strength volt per metre V/m kgms-3A-1 ( N/C)
D Electric displacement field Coulomb per square metre C/m2 Asm-2
e Permittivity farad per metre F/m kg-1m-3s4A2
?e Electric susceptibility (dimensionless) - -
G Y B Conductance Admittance Susceptance Siemens S kg-1m-2s3A2 ( O-1)
?, ?, s Conductivity siemens per metre S/m kg-1m-3s3A2
B Magnetic flux density, Magnetic induction Tesla T kgs-2A-1 ( Wb/m2  NA-1m-1)
F Magnetic flux Weber Wb kgm2s-2A-1 ( Vs)
H Magnetic field strength ampere per metre A/m Am-1
L, M Inductance Henry H kgm2s-2A-2 ( Wb/A Vs/A)
µ Permeability henry per metre H/m kgms-2A-2
? Magnetic susceptibility (dimensionless) - -
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