S. B. Patil College of Engineering 

Department of Electrical Engineering  

103004 : Basic Electrical Engineering

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2SBPCOE Dept. of Electrical Engineering 2013-14

Electrostatics The branch of engineering which deals with

charges at rest is called Electrostatics.

The branch of engineering which deals with

the flow of electrons is called Current

Electricity.

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Coulomb’s Laws for Electrostatics

1) First Law:- “Like charges repel each other and

unlike charges attract each other.”

2) Second Law:- “The Force between two charges

Q1 & Q2 separated by a distance ‘d’ is

proportional to product of Magnitudes of

charges and inversely proportional to square

of the distance between them.

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Expression for Force between Charges

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Properties of Electric Lines of Force

Originates on the positive charge & Terminates on

negative charge.

Never touch each other

Enter or leave a conducting surface at right angle.

Lines opposite direction ---attract each other

Lines in same direction --- repel each other.

Do not form a closed loop.

Pass through a dielectric medium, but do not pass

through a charged body.

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Electric Field

“The electric or electrostatic field is defined as

the region around a charged body where

another charged body experiences a mechanical

force.”

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Electric Flux (ψ)

“The electric flux (ψ) is defined as the number of

lines of force in any particular electric field.”

It is measured in coulomb (C).

“One coulomb of electric flux is defined as the

flux that originates from a positive charge of

1C.”

Electric Flux Density (D/δ)

“The electric flux density (D) is defined as the flux

per unit area measured at right angles to the

direction of flux.”

It is measured in coulomb/sq. meter (C/m2).

D= ψ/A

The Electric Flux Density is also called as

“Displacement Density.”

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Electric Field Strength or Field Intensity (E)

The Electric Field strength or field intensity is

defined as the mechanical force experienced by a

unit positive charge when it is placed at any

point in the electric field.”

E=F/Q

F=E Q

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The direction of electric field strength at any

point is same as the direction of force on a

positive charge situated at that point.

In other words the direction of electric field

strength at any point is the direction of the

electric lines of force passing through that point.

It is a vector Quantity.

Electric Field Strength or Field Intensity (E)

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Expression for Electric Field Intensity (E)

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Relation between D & E

• The flux density at any point is proportional to the intensity

of electric field at that point.

– Therefore, D α E

D= K E

Where K is constant of proportionality

And K= ε

D= ε E and ε = ε0 εr

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Permittivity

“Permittivity is the property of medium which

allows the electric flux to be established in it.”

Followings are the Permittivity

Absolute Permittivity

Permittivity of Free space or Vacuum

Relative Permittivity

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Absolute Permittivity

It is define as the ratio of electric flux density in

particular medium to the electric field strength

producing that flux density.

E

D

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Permittivity of Free space or Vacuum

It is define as the ratio of electric flux density in

air or vacuum to the electric field strength

producing that electric flux density.

o

ooE

D

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Relative Permittivity

It is define as the ratio of electric flux density in

a dielectric medium to the electric flux density in

air or vacuum, provided the electric field

strength is same in both the cases.

orD

D

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Electric Potential

• It is the quantity which decides the direction of flow of

electric charges.

infinite distance

“The electric potential at point p is defined as the work done

in bringing the unit positive charge from infinity to point P

against the electric field.”

P

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Capacitor or Condenser

• Any two conducting surfaces separated by an

insulating material is called a CAPACITOR or

CONDENSER.Dielectric Material

Conducting Plates

Conducting Plates

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Capacitor or Condenser

• Such an arrangement is used to store large quantity of

charge at low potential. That is, it has the property to

store electrical energy in the form of electrostatics.

• All capacitors consist of the two parallel conducting

plates separated by an insulating material called

Dielectric.

• “The ability of a capacitor to store a charge is

called capacitance.”

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Principle of Capacitor

SW G

C

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Principle of Capacitor

SW G

C

Direction of conventional Current

A

B

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Principle of Capacitor

SW G

C

Direction of conventional Current

An Electric Field exist between two charged plates

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Principle of Capacitor

SW G

C

No current flows when capacitor fully charged.

V

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Principle of Capacitor

SW G

C

A Capacitor can retain charge for a definite time

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Principle of Capacitor

SW G

C

A Charged Capacitor

Direction of conventional Current

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Capacitance

• “The ability of a capacitor to store a charge is called

capacitance.”

• And charge stored in capacitor is directly proportional

to the potential difference across it.

Or

C is constant called

Capacitance

V Q

VCQ *

CV

Q

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Capacitance of Parallel Plate Capacitor with uniform medium

rE

V

d

+Q -Q

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Capacitance of Parallel Plate Capacitor with uniform medium

rE

V

d

+Q -Q

drAo

C Farads

rC

AC

dC

1

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Capacitance of Parallel Plate Capacitor with composite medium

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1V 2V 3V

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1V 2V 3V

r

d

oAC

Capacitance of Parallel Plate Capacitor with composite medium

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Energy Stored in a Capacitor

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Energy Stored in a Capacitor

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Capacitors in Series

V1 V2 V3

C1 C2 C3Q Q Q

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Capacitors in Parallel

V

C1

C2

C3

Q1

Q2

Q3

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Capacitor Charging & Discharging