Electromagnetism:
basic principles and
applications
Massimiliano Bazzi
LNF – INFN
Stage International Masterclass
2011
What is electromagnetism?
Electromagnetism is the force that causes the interaction
between electrically charged particles.
It is as well one of the four fundamental interactions of
nature. The other three are the
strong interaction
weak interaction
gravitation
Among the four electromagnetism is the most present in
daily life and the most well known.
Electric charge
“Electric charge is a physical property of matter which causes it to
experience a force when near other electrically charged matter.”
Beyond plain definition, electric charge depends on electrons, which are
the fundamental bricks of electromagnetism.
The electron is a subatomic particle carrying a negative electric charge
whose value is −1.602176487×10−19 Coulomb.
Electrons form the outermost shell of the atom.
They balance the positive charge of protons
inside the nucleus, so matter tends to be
electrically neutral.
Some materials tend to acquire or to loose
electrons breaking the electric balance (ie:
rubbing bakelite or glass with wool).
Electric charge
bakelite
glass
bakelite
bakelite
By rubbing bakelite sticks with cloth, you can produce either
attractive or repulsive phenomena
Electric field
The region of space in which an electromagnetic force is present is
called electric field.
It is generally represented by means of force lines.
Electric Field
• Electric field is by definition the force a particular source
may apply on a fictitious charge of 1Coulomb.
• Electric field intensity depends on the distance from the
source and on the dielectric medium. Dimensionally it is
expressed in Volt/meter (V/m).
Electric potential
•
Like the gravitational field, we can introduce the concept
of electric potential defined as the level of potential
energy detected by a unitary charge immersed in an
electric field in a certain position.
•
The electric potential is mathematically defined by the
following
•
Single point potential is meaningless, but if we consider
potential differences it becomes extremely useful.
•
Potential difference is known as voltage.
Voltage
A
B
E
•
Given two arbitrary locations A and B in an electric field,
the voltage represents the work necessary to bring a
unitary charge from A to B.
•
WAB = VB – VA
Voltages and potentials are measured in Volt (V).
Electric current
•
When charges flow from one position to another this
phenomenon is called electric current
•
The current verse is always from positive to negative
(when positive is the higher potential point)
•
Current unit is Ampère (A), and corresponds to a charge
of 1C through the conductor section in 1s.
Magnetic field
•
An electric current produces a magnetic field according
to the right hand rule
•
Magnetic field is a force field whose force lines are
closed
•
Its unit of measurement is ampère/meter (A/m)
Voltage Source
A voltage source is an element that
maintains an electrical potential
difference between its terminals and,
therefore produces an electromotive
force (e.m.f.)
•
The first voltage source ever realized
was the Voltaic Pile in 1800
The voltaic pile transforms chemical
potential in voltage
•
Before that the only way to dispose
of electrical energy was the Leyden
jar
Electrical Resistance
The electrical resistance of an electrical
element measures its opposition to the
passage of an electric current.
•
The resistance of an object can be
defined as the ratio of voltage to current.
This ratio is called Ohm's law.
•
V=R∙I
Electrical Resistance
•
This effect can be explained by the
presence of “obstacles” inside the
conductor.
•
During their passage electrons
collide with conductor molecules
loosing their kinetic energy.
•
This energy loss is measured as a
voltage drop at conductor
extremities.
•
Electric energy loss becomes
thermal energy (Joule's first law)
Capacitance
Capacitance is the ability of a body to
hold an electrical charge.
Capacitance is also a measure of the
amount of electrical energy stored for
a given electric potential.
C=Q/V
A common form of energy storage
device is a parallel-plate capacitor
whose capacitance is directly
proportional to the surface area of the
conductor plates and inversely
proportional to the separation
distance between the plates.
Capacitance
Inductance
Inductance is the property of an electrical circuit causing
voltage to be generated proportional to the rate of change
in current in a circuit.
It is a consequence of magnetic field generated by
current. The inductor is a magnetic energy storage device
The solenoidal shape increases inductance
Inductor
•
The inductor stores magnetic
energy by means of its shape
It is the basic element of
electromagnets, motors and
transformers.
By means of a magnetic core
we can intensify magnetic flux
Capacitance & Inductance
Capacitance and inductance are frequently called reactances,
since they react to an energy variation.
The capacitor reacts to voltage variations (so to electric field).
In static condition it works as an open circuit.
The inductor react to current variations (so to magnetic field).
In static condition it works as short circuit.
The equation describing both devices contains derivatives!
Direct current
• Direct current (DC) is the
unidirectional flow of electric charge
constant in time.
• It is typically generated from electric
sources as batteries, thermocouples,
solar cells.
• Conventionally it flows from positive
pole to negative
• It is used to give energy to small
devices
Alternating current
In alternating current (AC) the movement of electric
charge periodically reverses direction.
The usual waveform of an AC power circuit is a sine wave.
Alternator
•
Alternator produces alternating
current
•
By rotating a conductor winding
embedded in a magnetic field you
produce an e.m.f.
•
This principle is known as
Faraday's law
Alternator
•
magnetic flux is maximum when
the winding plane is perpendicular
to the field and null when parallel
•
By rotating the winding at a
frequency F you produce a
voltage sine wave at the same
frequency
•
This is the most efficient way to
transform mechanical energy into
electric energy
Alternator
Alternators may have more independent windings and
produce more waves
•
Each sine wave is called phase
The angle between windings corresponds to the delay
between phases
AC / DC
AC may be rectified, that is to say converted in DC!!!
Transformer
A transformer is a static device that transfers electrical
energy from one circuit to another through inductively
coupled conductors.
A varying current in the first or primary winding creates a
varying magnetic flux in the transformer's core and thus
a varying magnetic field through the secondary winding.
Ideal Transformer
Real Transformer
The physical limitations of the practical transformer may
be brought together as an equivalent circuit model built
around an ideal lossless transformer
Electric Motor
stator
rotor
We can transform electric
energy back to mechanical energy.
The motor in principle is
similar to an alternator.
Instead of a magnet and a
winding, often we use two
windings, rotor and
stator.
Magnetizing current is the
same for both windings.
Electric Motor
brushes
rotor
stator
switch
Three-phase motor
In the Three-phase motors there are three windings 120°
distant inside the stator. By applying a three-phase
current we obtain a rotating magnetic field.
Maxwell's equations
All electromagnetism is described by Maxwell's equations
!!!!
Electromagnetic wave equation
By combining the previous equations we can obtain a new
way to describe electromagnetism
Through waves!
• C0 is the speed of light!!!
Electromagnetic waves
Electromagnetic waves can be generated by a variety of
methods, such as a discharging spark or by an oscillating
molecular dipole.
Electromagnetic spectrum
Electromagnetism Pioneers
André-Marie
Ampère
1775 – 1836
Alessandro
Volta
1745 – 1827
Electromagnetism Pioneers
Michael
Faraday
1791 – 1867
James Clerk
Maxwell
1831 – 1879