Electromagnetism: basic principles and applications

Electromagnetism:Electromagnetism:basic principles and basic principles and

applicationsapplications

Massimiliano BazziMassimiliano BazziLNF – INFNLNF – INFN

Stage International Masterclass Stage International Masterclass 20112011

What is electromagnetism?What is electromagnetism?Electromagnetism is the force that causes the interaction Electromagnetism is the force that causes the interaction

between electrically charged particles.between electrically charged particles.It is as well one of the four fundamental interactions of It is as well one of the four fundamental interactions of

nature. The other three are the nature. The other three are the

strong interactionstrong interactionweak interactionweak interaction

gravitationgravitation

Among the four electromagnetism is the most present in Among the four electromagnetism is the most present in daily life and the most well known.daily life and the most well known.

Electric chargeElectric charge““Electric charge is a physical property of matter which causes it to Electric charge is a physical property of matter which causes it to

experience a force when near other electrically charged matter.”experience a force when near other electrically charged matter.”Beyond plain definition, electric charge depends on Beyond plain definition, electric charge depends on electronselectrons, which are , which are

the fundamental bricks of electromagnetism.the fundamental bricks of electromagnetism.The electron is a subatomic particle carrying a negative electric charge The electron is a subatomic particle carrying a negative electric charge

whose value is −1.602176487×10−19 Coulomb.whose value is −1.602176487×10−19 Coulomb.

Electrons form the outermost shell of the atom.Electrons form the outermost shell of the atom.They balance the positive charge of protons They balance the positive charge of protons

inside the nucleus, so matter tends to be inside the nucleus, so matter tends to be electrically neutral.electrically neutral.

Some materials tend to acquire or to loose Some materials tend to acquire or to loose electrons breaking the electric balance (ie: electrons breaking the electric balance (ie:

rubbing bakelite or glass with wool).rubbing bakelite or glass with wool).

Electric chargeElectric charge

By rubbing bakelite sticks with cloth, you can produce either By rubbing bakelite sticks with cloth, you can produce either attractive or repulsive phenomenaattractive or repulsive phenomena

bakelite bakelite

bakeliteglass

Electric fieldElectric field

The region of space in which an electromagnetic force is present is The region of space in which an electromagnetic force is present is called called electric fieldelectric field..

It is generally represented by means of It is generally represented by means of force linesforce lines..

Electric FieldElectric Field

• Electric field is by definition the force a particular source Electric field is by definition the force a particular source may apply on a fictitious charge of 1Coulomb.may apply on a fictitious charge of 1Coulomb.

• Electric field intensity depends on the distance from the Electric field intensity depends on the distance from the source and on the dielectric medium. Dimensionally it is source and on the dielectric medium. Dimensionally it is expressed in Volt/meter (V/m).expressed in Volt/meter (V/m).

Electric potentialElectric potential• Like the gravitational field, we can introduce the concept Like the gravitational field, we can introduce the concept

of electric potential defined as the level of potential of electric potential defined as the level of potential energy detected by a unitary charge immersed in an energy detected by a unitary charge immersed in an electric field in a certain position.electric field in a certain position.

• The electric potential is mathematically defined by the The electric potential is mathematically defined by the followingfollowing

• Single point potential is meaningless, but if we consider Single point potential is meaningless, but if we consider potential differencespotential differences it becomes extremely useful. it becomes extremely useful.

• Potential difference is known as Potential difference is known as voltagevoltage..

VoltageVoltage

• Given two arbitrary locations A and B in an electric field, Given two arbitrary locations A and B in an electric field, the voltage represents the work necessary to bring a the voltage represents the work necessary to bring a unitary charge from A to B.unitary charge from A to B.

• WWABAB = V = VBB – V – VAA

Voltages and potentials are measured in Volt (V).Voltages and potentials are measured in Volt (V).

E

A

B

Electric currentElectric current

• When charges flow from one position to another this When charges flow from one position to another this phenomenon is called phenomenon is called eelectric currentlectric current

• The current verse is alwaysThe current verse is always from positive to negative from positive to negative (when positive is the higher potential point)(when positive is the higher potential point)

• Current unit is Ampère (A), and corresponds to a charge Current unit is Ampère (A), and corresponds to a charge of 1C through the conductor section in 1s.of 1C through the conductor section in 1s.

Magnetic fieldMagnetic field

• An electric current produces aAn electric current produces a magnetic field magnetic field according according to the right hand ruleto the right hand rule

• Magnetic field is a force field whose force lines are Magnetic field is a force field whose force lines are closed closed

• Its unit of measurement is ampère/meter (A/m)Its unit of measurement is ampère/meter (A/m)

Voltage SourceVoltage SourceA voltage source is an element that A voltage source is an element that maintains an electrical potential maintains an electrical potential difference between its terminals and, difference between its terminals and, therefore produces an electromotive therefore produces an electromotive force (e.m.f.)force (e.m.f.)

• The first voltage source ever realized The first voltage source ever realized was the was the Voltaic PileVoltaic Pile in 1800 in 1800The voltaic pile transforms chemical The voltaic pile transforms chemical potential in voltagepotential in voltage

• Before that the only way to dispose Before that the only way to dispose of electrical energy was the of electrical energy was the Leyden Leyden jarjar

Electrical ResistanceElectrical Resistance

The electrical resistance of an electrical The electrical resistance of an electrical element measures its opposition to the element measures its opposition to the passage of an electric current.passage of an electric current.

• The resistance of an object can be The resistance of an object can be defined as the ratio of voltage to current. defined as the ratio of voltage to current. This ratio is called This ratio is called Ohm's lawOhm's law..

• V = R V = R ∙∙ I I

Electrical ResistanceElectrical Resistance• This effect can be explained by the This effect can be explained by the

presence of “presence of “obstacles”obstacles” inside the inside the conductor.conductor.

• During their passage electrons During their passage electrons collide with conductor molecules collide with conductor molecules loosing their kinetic energy.loosing their kinetic energy.

• This energy loss is measured as a This energy loss is measured as a voltage drop at conductor voltage drop at conductor extremities.extremities.

• Electric energy loss becomes Electric energy loss becomes thermal energy (Joule's first law)thermal energy (Joule's first law)

CapacitanceCapacitanceCapacitance is the ability of a body to Capacitance is the ability of a body to hold an electrical charge. hold an electrical charge. Capacitance is also a measure of the Capacitance is also a measure of the amount of electrical energy stored for amount of electrical energy stored for a given electric potential. a given electric potential.

C = Q / VC = Q / VA common form of energy storage A common form of energy storage device is a parallel-plate capacitor device is a parallel-plate capacitor whose capacitance is directly whose capacitance is directly proportional to the surface area of the proportional to the surface area of the conductor plates and inversely conductor plates and inversely proportional to the separation proportional to the separation distance between the plates.distance between the plates.

CapacitanceCapacitance

InductanceInductance

Inductance is the property of an electrical circuit causing Inductance is the property of an electrical circuit causing voltage to be generated proportional to the rate of change voltage to be generated proportional to the rate of change in current in a circuit.in current in a circuit.It is a consequence of magnetic field generated by It is a consequence of magnetic field generated by current. The inductor is a magnetic energy storage devicecurrent. The inductor is a magnetic energy storage deviceThe solenoidal shape increases inductanceThe solenoidal shape increases inductance

InductorInductorThe inductor stores magnetic The inductor stores magnetic energy by means of its shapeenergy by means of its shapeIt is the basic element of It is the basic element of electromagnets, motors and electromagnets, motors and transformers.transformers.

• By means of a By means of a magnetic core magnetic core we can intensify magnetic fluxwe can intensify magnetic flux

Capacitance & InductanceCapacitance & InductanceCapacitance and inductance are frequently called Capacitance and inductance are frequently called reactancesreactances, ,

since they react to an energy variation.since they react to an energy variation.

The capacitor reacts to voltage variations (so to electric field).The capacitor reacts to voltage variations (so to electric field).In static condition it works as an open circuit.In static condition it works as an open circuit.

The inductor react to current variations (so to magnetic field).The inductor react to current variations (so to magnetic field).In static condition it works as short circuit.In static condition it works as short circuit.

The equation describing both devices contains The equation describing both devices contains derivativesderivatives!!

Direct currentDirect current• Direct current (DC) is the unidirectional Direct current (DC) is the unidirectional

flow of electric charge constant in flow of electric charge constant in time. time.

• It is typically generated from electric It is typically generated from electric sources as batteries, thermocouples, sources as batteries, thermocouples, solar cells.solar cells.

• Conventionally it flows from positive Conventionally it flows from positive pole to negativepole to negative

• It is used to give energy to small It is used to give energy to small devicesdevices

Alternating currentAlternating current

In alternating current (AC) the movement of electric In alternating current (AC) the movement of electric charge periodically reverses direction. charge periodically reverses direction. The usual waveform of an AC power circuit is a sine wave.The usual waveform of an AC power circuit is a sine wave.

AlternatorAlternator

• AlternatorAlternator produces alternating produces alternating currentcurrent

• By rotating a conductor winding By rotating a conductor winding embedded in a magnetic field you embedded in a magnetic field you produce an e.m.f.produce an e.m.f.

• This principle is known as This principle is known as Faraday's lawFaraday's law

AlternatorAlternator

• magnetic fluxmagnetic flux is maximum when is maximum when the winding plane is perpendicular the winding plane is perpendicular to the field and null when parallel to the field and null when parallel

• By rotating the winding at a By rotating the winding at a frequency F you produce a frequency F you produce a voltage sine wave at the same voltage sine wave at the same frequencyfrequency

• This is the most efficient way to This is the most efficient way to transform mechanical energy into transform mechanical energy into electric energyelectric energy

AlternatorAlternator

Alternators may have more independent windings and Alternators may have more independent windings and produce more waves produce more waves

• Each sine wave is called Each sine wave is called phasephaseThe angle between windings corresponds to the delay The angle between windings corresponds to the delay between phasesbetween phases

AC / DCAC / DC

AC may be rectified, that is to say converted in DC!!!AC may be rectified, that is to say converted in DC!!!

TransformerTransformer

A transformer is a static device that transfers electrical A transformer is a static device that transfers electrical energy from one circuit to another through inductively energy from one circuit to another through inductively coupled conductors. coupled conductors. A varying current in the first or primary winding creates a A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus varying magnetic flux in the transformer's core and thus a varying magnetic field through the secondary winding.a varying magnetic field through the secondary winding.

Ideal TransformerIdeal Transformer

Real TransformerReal Transformer

The physical limitations of the practical transformer may The physical limitations of the practical transformer may be brought together as an equivalent circuit model built be brought together as an equivalent circuit model built around an ideal lossless transformeraround an ideal lossless transformer

Electric MotorElectric Motor

We can transform electricWe can transform electricenergy back to mechani-energy back to mechani-cal energy.cal energy.The motor in principle is The motor in principle is similar to an alternator.similar to an alternator.Instead of a magnet and aInstead of a magnet and awinding, often we use twowinding, often we use twowindings,windings, rotor rotor and andstatorstator..Magnetizing current is theMagnetizing current is thesame for both windings.same for both windings.

rotorrotor

statorstator

Electric MotorElectric Motor

rotorrotorstatorstator

brushesbrushes

switchswitch

Three-phase motorThree-phase motor

In the Three-phase motors there are three windings 120° In the Three-phase motors there are three windings 120° distant inside the stator. By applying a three-phase distant inside the stator. By applying a three-phase

current we obtain a rotating magnetic field.current we obtain a rotating magnetic field.

Maxwell's equationsMaxwell's equationsAll electromagnetism is described by Maxwell's equationsAll electromagnetism is described by Maxwell's equations

!!!!!!!!

Electromagnetic wave equationElectromagnetic wave equationBy combining the previous equations we can obtain a new By combining the previous equations we can obtain a new way to describe electromagnetismway to describe electromagnetism

Through waves!Through waves!• CC00 is the speed of light!!! is the speed of light!!!

Electromagnetic wavesElectromagnetic wavesElectromagnetic waves can be generated by a variety of Electromagnetic waves can be generated by a variety of methods, such as a discharging spark or by an oscillating methods, such as a discharging spark or by an oscillating molecular dipole. molecular dipole.

Electromagnetic spectrumElectromagnetic spectrum

Electromagnetism PioneersElectromagnetism Pioneers

André-Marie Ampère

1775 – 1836

Alessandro Volta

1745 – 1827

Electromagnetism PioneersElectromagnetism Pioneers

Michael Faraday

1791 – 1867

James Clerk Maxwell

1831 – 1879