1. ELECTROSTATICSELECTROSTATICS
THE BRANCH OF PHYSICS DEALING WITHTHE BRANCH OF PHYSICS DEALING WITH
CHARGES AT REST AND THEIR PROPERTIESCHARGES AT REST AND THEIR PROPERTIES
STATIC ELECTRICITY WAS FIRST OBSERVEDSTATIC ELECTRICITY WAS FIRST OBSERVED
BY THALES OF MILETUS IN 600 BC WHEN HEBY THALES OF MILETUS IN 600 BC WHEN HE
FOUND THAT AMBER WHEN RUBBED WITHFOUND THAT AMBER WHEN RUBBED WITH
FUR ACQUIRED THE PROPERTY OFFUR ACQUIRED THE PROPERTY OF
ATTRCACTING TINY PIECES OF SAW DUSTATTRCACTING TINY PIECES OF SAW DUST
ETC.ETC.
ELECTRICITY PRODUCED BY RUBBING ISELECTRICITY PRODUCED BY RUBBING IS
CALLED FRICTIONAL ELECTRICITYCALLED FRICTIONAL ELECTRICITY
SINCE THE CHARGES SO PRODUCED ARE ATSINCE THE CHARGES SO PRODUCED ARE AT
REST IT IS ALSO CALLED STATICREST IT IS ALSO CALLED STATIC
ELECTRICITYELECTRICITY
CHARGES ARE PRODUCED BY TRANSFER OFCHARGES ARE PRODUCED BY TRANSFER OF
ELECTRONSELECTRONS
2. IN 1600 AD, DR. WILLIAM GILBERT, COURT
PHYSICIAN TO QUEEN ELIZABETH I OF ENGLAND,
PUBLISHED THE BOOK (DE MAGNETO) IN WHICH
HE MADE AN ACCOUNT OF ALL THE
EXPERIMENTS AND OBSERVATIONS MADE SO
FAR IN THE FIELD OF ELECTROSTATICS.
GILBERT FOUND THAT THERE ARE TWO KINDS OF
CHARGES AND THAT LIKE CHARGES REPEL AND
UNLIKE CHARGES ATTRACT.
HE NAMED THE TWO KINDS OF CHARGES AS
RESINOUS AND VITREOUS.
THE CHARGE ACQUIRED BY AMBER OR EBONITE
(WHEN RUBBED WITH WOOL OR FUR) WAS
CALLED RESINOUS AND THE OTHER KIND OF
CHARGE WAS CALLED VITREOUS.
4. CONSERVATION OFCONSERVATION OF
CHARGESCHARGES
THE TOTAL CHARGE IN ANY SYSTEM ISTHE TOTAL CHARGE IN ANY SYSTEM IS
ALWAYS CONSERVEDALWAYS CONSERVED
NET CHARGE CAN NEITHER BE CREATEDNET CHARGE CAN NEITHER BE CREATED
NOR BE DESTROYED IN ISOLATIONNOR BE DESTROYED IN ISOLATION
CHARGES CAN ONLY BE PRODUCED ORCHARGES CAN ONLY BE PRODUCED OR
DESTROYED IN EQUAL AND OPPOSITEDESTROYED IN EQUAL AND OPPOSITE
PAIRSPAIRS
THE TOTAL CHARGE BEFORE AND AFTERTHE TOTAL CHARGE BEFORE AND AFTER
ANY REACTION REMAINS THE SAME.ANY REACTION REMAINS THE SAME.
5. QUANTIZATION OF CHARGEQUANTIZATION OF CHARGE
THE CHARGE PRESENT IN ANY BODYTHE CHARGE PRESENT IN ANY BODY
IS ALWAYS THE INTEGRAL MULTIPLEIS ALWAYS THE INTEGRAL MULTIPLE
OF FUNDAMENTAL CHARGEOF FUNDAMENTAL CHARGE THETHE
CHARGE OF AN ELECTRON (CHARGE OF AN ELECTRON (1.6 X 101.6 X 10-19-19
CC ))
NO BODY CAN POSSESS FRACTIONALNO BODY CAN POSSESS FRACTIONAL
ELECTRONIC CHARGE (IN THEELECTRONIC CHARGE (IN THE
MACROSCOPIC WORLD)MACROSCOPIC WORLD)
6. QUARKSQUARKS
ARE PARTICLES CONSIDERED TOARE PARTICLES CONSIDERED TO
POSSESS FRACTIONAL ELECTRONICPOSSESS FRACTIONAL ELECTRONIC
CHARGES --CHARGES -- ± 1/3 e, ± 2/3 e …..± 1/3 e, ± 2/3 e …..
THERE ARE SIX TYPES OF QUARKSTHERE ARE SIX TYPES OF QUARKS UP,UP,
DOWN, TOP, BOTTOM, CHARM ANDDOWN, TOP, BOTTOM, CHARM AND
STRANGESTRANGE
BUT THE EXISTENCE OF QUARKS DONOTBUT THE EXISTENCE OF QUARKS DONOT
VIOLATE THE LAW OF CONSERVATION OFVIOLATE THE LAW OF CONSERVATION OF
CHARGE. IT ONLY CHANGES THECHARGE. IT ONLY CHANGES THE
MAGNITUDE OF FUNDAMENTAL CHARGEMAGNITUDE OF FUNDAMENTAL CHARGE
TO THAT OF THE LOWEST POSSIBLETO THAT OF THE LOWEST POSSIBLE
CHARGE ON QUARKS.CHARGE ON QUARKS.
ALSO, QUARKS CANNOT EXIST FREELY.ALSO, QUARKS CANNOT EXIST FREELY.
THEY ARE ALWAYS FOUND COMBINED TOTHEY ARE ALWAYS FOUND COMBINED TO
FORM INTEGRAL MULTIPLES OFFORM INTEGRAL MULTIPLES OF
8. RELATIVE PERMITIVITYRELATIVE PERMITIVITY
Is defined as the ratio of the force betweenIs defined as the ratio of the force between
two point charges separated in vacuum totwo point charges separated in vacuum to
the force between the same two chargesthe force between the same two charges
separated by the same distance whileseparated by the same distance while
kept in the medium.kept in the medium.
i.e.i.e. εεrr= F= F00 /F/Fmm
9. PRINCIPLE OF SUPER POSITIONPRINCIPLE OF SUPER POSITION
States that when there are a number ofStates that when there are a number of
point charges, the net force on any one ofpoint charges, the net force on any one of
the charges is equal to the vector sum ofthe charges is equal to the vector sum of
the forces due to the individual charges.the forces due to the individual charges.
i.e.i.e.
FF11 = F= F1212 + F+ F1313 + F+ F1414 + ……+ ……
10. DEFINE 1 COULOMBDEFINE 1 COULOMB
One coulomb is defined as that chargeOne coulomb is defined as that charge
which when kept one metre apart from anwhich when kept one metre apart from an
equal and similar charge in vacuum, repelsequal and similar charge in vacuum, repels
it with a force of 9 x 10it with a force of 9 x 1099
N.N.
11. ELECTRIC FIELDELECTRIC FIELD QualitativelyQualitatively
The region of space around a charge where itThe region of space around a charge where it
can exert a force of electrical origin on anothercan exert a force of electrical origin on another
charge.charge.
QuantitativelyQuantitatively
The intensity of ELECTRIC FIELD at any pointThe intensity of ELECTRIC FIELD at any point
is defined as the force exerted per unit chargeis defined as the force exerted per unit charge
by a positive test charge kept at that point.by a positive test charge kept at that point.
=
→
0
0
lim
q
F
E
oq
12. ELECTRIC LINES OF FORCEELECTRIC LINES OF FORCE
Are imaginary lines of force such that theAre imaginary lines of force such that the
tangent to it at any point gives thetangent to it at any point gives the
direction of electric field at that point.direction of electric field at that point.
A positive point charge free to move willA positive point charge free to move will
move in the direction of electric field and amove in the direction of electric field and a
negative point charge will move in anegative point charge will move in a
direction opposite to the direction ofdirection opposite to the direction of
electric field along an electric line of force.electric field along an electric line of force.
13. The lines of force to represent uniform electric field
are as shown below
The electric lines of
force due to point
charge q < 0 are as
shown below
The electric lines of
force due to point
charge q > 0 are as
shown below
14. PROPERTIES OF ELECTRIC LINES OF FORCEPROPERTIES OF ELECTRIC LINES OF FORCE
Start from a positive charge and end in a negativeStart from a positive charge and end in a negative
charge.charge.
The tangent to it at any point gives the direction ofThe tangent to it at any point gives the direction of
electric field at that point.electric field at that point.
They never intersect each otherThey never intersect each other
They tend to contract longitudinally and expandThey tend to contract longitudinally and expand
laterally.laterally.
They always enter or emerge normal to the surface ofThey always enter or emerge normal to the surface of
a charged conductor.a charged conductor.
They are close together in regions of strong electricThey are close together in regions of strong electric
field and far apart in regions of weak electric field.field and far apart in regions of weak electric field.
15. ELECTRIC DIPOLEELECTRIC DIPOLE
Two equal and opposite point chargesTwo equal and opposite point charges
separated by a very small distanceseparated by a very small distance
constitute an electric dipole.constitute an electric dipole.
Electric dipole moment of a dipole isElectric dipole moment of a dipole is
defined as the product of the magnitude ofdefined as the product of the magnitude of
either of the charges and the distanceeither of the charges and the distance
between the charges.between the charges.
Dipole moment,Dipole moment,
qlp ×= 2
16. TORQUE ON A DIPOLETORQUE ON A DIPOLE
ττ = pE sin= pE sinθθ
OrOr
ττ = p X E= p X E
wherewhere pp is the electric dipole momentis the electric dipole moment
andand EE is the intensity of electric field.is the intensity of electric field.
17. DERIVATION (DERIVATION (ττ = PE sin= PE sinθθ))Force on charge +q atForce on charge +q at
A .A .
force on charge - q at Bforce on charge - q at B
Forces F A and FBForces F A and FB
equal and oppositeequal and opposite
form a couple whichform a couple which
tends to rotate thetends to rotate the
dipoledipole
torque acting on dipoletorque acting on dipole
AF q E=
BF q E= −
τ = ×force arm of couple
18. so from -------- ( 1 )so from -------- ( 1 )
No torque acts when dipole moment alignsNo torque acts when dipole moment aligns
parallel to electric field ( i.eparallel to electric field ( i.e θθ = 0 )= 0 )
from ( 2 )from ( 2 )
τ = × − − − − −qE AC ( )1
In ABC∆
AC
AB
=sinθ AC AB= sinθ AC l= 2 sinθ
τ θ= ×qE l2 sin
= ×( ) sinq l E2 θ
τ θ= × − − − −pE sin ( )2 p q l dipole moment= × 2
τ = pE sin0 = ×pE 0
19. ELECTRIC FLUXELECTRIC FLUXIs the total lines of force passingIs the total lines of force passing
normal to a given surfacenormal to a given surface
φφEE = E A= E A for uniform electric fieldfor uniform electric field
Electric flux is a scalar quantityElectric flux is a scalar quantity
∫=
s
E SdE
.φ
20. GAUSS’ THEOREMGAUSS’ THEOREM
States the total electric flux through aStates the total electric flux through a
closed surface (surface integral ofclosed surface (surface integral of
electric field over a closed surface)electric field over a closed surface)
is equal to 1/is equal to 1/εεoo times the totaltimes the total
charge enclosed by the surface.charge enclosed by the surface.
MathematicallyMathematically
( )enclosed
s
qSdE∫ =
0
1
.
ε
21. ACTION OF POINTSACTION OF POINTS
The surface charge density is not uniformThe surface charge density is not uniform
in the case of uneven metal surfaces. It isin the case of uneven metal surfaces. It is
maximum at sharp points and hence themaximum at sharp points and hence the
intensity of electric field will also beintensity of electric field will also be
maximum at these points. This is knownmaximum at these points. This is known
as action of points.as action of points.
22. CORONA DISCHARGECORONA DISCHARGE
When a metal with sharp points isWhen a metal with sharp points is
charged, the sharp points acquire a highcharged, the sharp points acquire a high
electric field and ionizes the air moleculeselectric field and ionizes the air molecules
nearby and then repels them away. Thenearby and then repels them away. The
charged air molecules moving away fromcharged air molecules moving away from
the sharp points constitute an electric windthe sharp points constitute an electric wind
and the discharge of electricity from sharpand the discharge of electricity from sharp
points like this is known aspoints like this is known as coronacorona
discharge.discharge.
23. LIGHTNING CONDUCTORLIGHTNING CONDUCTOR Is a device made of metal with sharp points fixedIs a device made of metal with sharp points fixed
on the top of huge buildings and earthed by thickon the top of huge buildings and earthed by thick
strips of conductor.strips of conductor.
They protect the building in two ways.They protect the building in two ways.
They avoid the occurrence of lightning by coronaThey avoid the occurrence of lightning by corona
discharge and neutralizing the clouds.discharge and neutralizing the clouds.
Even if lightning strikes, it provides a lowEven if lightning strikes, it provides a low
resistance conducting path for the chargesresistance conducting path for the charges
coming from the clouds and protects the buildingcoming from the clouds and protects the building
from damage.from damage.
24. VAN DE GRAFFVAN DE GRAFF
GENERATORGENERATOR
Is a device used to
produce very high
potential by the action
of points.
It works on the
principle that whenever
a charge is given to a
hollow conductor, the
charge is immediately
transferred to the outer
surface.
26. CAPACITANCECAPACITANCE
The ratio of electric charge toThe ratio of electric charge to
electric potential of a conductor orelectric potential of a conductor or
a device is called capacitancea device is called capacitance
Capacitance C = Q/VCapacitance C = Q/V
Unit is farad (F)Unit is farad (F)
1 farad = 1 coulomb / 1 volt1 farad = 1 coulomb / 1 volt
27. PRINCIPLE OF APRINCIPLE OF A
CAPACITORCAPACITOR
Capacitor is based on the principleCapacitor is based on the principle
that the capacitance of an isolatedthat the capacitance of an isolated
charged conductor increases whencharged conductor increases when
an uncharged earthed conductor isan uncharged earthed conductor is
kept near it and the capacitance iskept near it and the capacitance is
further increased by keeping afurther increased by keeping a
dielectric medium between thedielectric medium between the
conductors.conductors.
28. CAPACITANCE OF A PARALLELCAPACITANCE OF A PARALLEL
PLATE CAPACITORPLATE CAPACITOR
Electric field between the plates,Electric field between the plates,
E =E = σσ//εε00
ButBut σσ=Q/A=Q/A
∴∴E=Q/AE=Q/Aεε00
Potential difference between thePotential difference between the
two plates , V = Ed = Qd/Atwo plates , V = Ed = Qd/A εε00
Capacitance, C = Q/VCapacitance, C = Q/V
C=A ε /d
29. CAPACITANCE OF A PARALLELCAPACITANCE OF A PARALLEL
PLATE CAPACITOR WITH APLATE CAPACITOR WITH A
DIELECTRIC SLABDIELECTRIC SLAB
When a dielectric slab is kept between the platesWhen a dielectric slab is kept between the plates
COMPLETELYCOMPLETELY filling the gapfilling the gap
E’ = EE’ = E00/K where K is the dielectric constant of the/K where K is the dielectric constant of the
medium.medium.
Potential differencePotential difference
V’ = E’d = EV’ = E’d = E00d/K=Qd/Kd/K=Qd/K εε00AA
Capacitance C’ = Q/V’ = KCapacitance C’ = Q/V’ = K εε00A/d = KCA/d = KC
∴∴when a dielectric medium is filled between thewhen a dielectric medium is filled between the
plates of a capacitor, its capacitance isplates of a capacitor, its capacitance is
increased K times.increased K times.
30. DIELECTRIC STRENGTHDIELECTRIC STRENGTH
Dielectric strength of aDielectric strength of a
dielectric is the maximumdielectric is the maximum
electric field that can beelectric field that can be
applied to it beyond which itapplied to it beyond which it
breaks down.breaks down.
31. PRACTICE PROBLEMSPRACTICE PROBLEMS
Calculate the number of electrons inCalculate the number of electrons in
excess in a body with 1 coulomb ofexcess in a body with 1 coulomb of
negative charge.negative charge.
Q = neQ = ne
Q = 1CQ = 1C
e = 1.6 X 10e = 1.6 X 10-19-19
CC
n = Q/e= 1/(1.6 X 10n = Q/e= 1/(1.6 X 10-19-19
C) = 6.25 X 10C) = 6.25 X 101818
