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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.
Contents
• Electricity
By Sala Luca
• Magnets and the magnetic force
By Meroni Davide
• Electromagnetism
By Casati Denis
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
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 Coulomb’s
law: the charges on electrons and protons, which are equal and opposite, are defined
as:
e=1,6*10^-19 Coulombs
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 oppositecharged objects attract.
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.
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.
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.
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
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.
Electric field
Michael Faraday
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.
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.
Magnetism
- History of magnetism;
- Compass;
- Magnets;
- Magnetic force;
- Magnetic field lines;
- Earth’s magnetic field is fading;
- Magnetic shield;
- Magnetic fields on the Sun;
- Right-hand rule;
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
(1031–1095) 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.
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.
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.
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.
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.
Earth’s Magnetic Field Is Fading
Earth’s 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.
Magnetic Shield
The geo-dynamo is the mechanism that creates our planet’s magnetic field, maintains it, and
causes it to reverse.
Earth’s 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 Earth’s Poles.
Without our planet’s magnetic field, Earth would be subjected to more cosmic radiation.
The increase could knock out power grids, scramble the communications systems on
spacecraft.
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.
Right-hand rule
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.
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.
Electromagnetism
• Faraday’s law
• Electromagnet
• Pick up
• AC/DC Generator
• Electric Circuits
• Maxwell
Electromagnetism
Electromagnetism is the branch of science concerned 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.
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.
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.
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.
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.
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.
Back in Black
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
law:we see that the rotation of the coil continually changes the magnetic flux through the
coil and therefore it generates a voltage.
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.
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.
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 Ω.
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
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.
Maxwell
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.
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 Ampere’s 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.
Maxwell equations
Gauss's law
Gauss's law for magnetism
Maxwell–Faraday equation
(Faraday's law of induction)
Ampère's circuital law
(with Maxwell's correction)
Maxwell also discovered the speed of these waves with the relation:
If we are in the void, this speed becomes:
C=299 792,458 km/s
The speed of light!!!
If we change the frequency, we have different types of waves:
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
A·s
U, ΔV, Δφ; E
Potential difference; Electromotive force
volt
V
kg·m2·s−3·A−1 (= J/C)
R; Z; X
Electric resistance; Impedance; Reactance
ohm
Ω
kg·m2·s−3·A−2 (= V/A)
ρ
Resistivity
Ohm metre
Ω·m
kg·m3·s−3·A−2
P
Electric power
Watt
W
kg·m2·s−3 (= V·A)
C
Capacitance
Farad
F
kg−1·m−2·s4·A2 (= C/V)
E
Electric field strength
volt per metre
V/m
kg·m·s−3·A−1 (= N/C)
D
Electric displacement field
Coulomb per square metre
C/m2
A·s·m−2
ε
Permittivity
farad per metre
F/m
kg−1·m−3·s4·A2
χe
Electric susceptibility
(dimensionless)
-
-
G; Y; B
Conductance; Admittance; Susceptance
Siemens
S
kg−1·m−2·s3·A2 (= Ω−1)
κ, γ, σ
Conductivity
siemens per metre
S/m
kg−1·m−3·s3·A2
B
Magnetic flux density, Magnetic induction
Tesla
T
kg·s−2·A−1 (= Wb/m2 = N·A−1·m−1)
Φ
Magnetic flux
Weber
Wb
kg·m2·s−2·A−1 (= V·s)
H
Magnetic field strength
ampere per metre
A/m
A·m−1
L, M
Inductance
Henry
H
kg·m2·s−2·A−2 (= Wb/A = V·s/A)
μ
Permeability
henry per metre
H/m
kg·m·s−2·A−2
χ
Magnetic susceptibility
(dimensionless)
-
-
The en d