ELECTROSTATICS

It is the branch of Physics which deals

with the study of charges at rest and the forces between

static charges.

Charging by induction

In charging by induction, a charged body A imparts some charge to another body B without any actual contact between the two.

Charging by Friction

A Styrofoam plate when rubbed with wool gets charged by friction.

Charging of two bodies by induction when they are in contact with each other.

Deflection in a positively charged electroscope needle when a negatively charged body approaches it.

COLOUMB’S LAW According to this law the force of

interaction between any two charges is directly proportional to the product of the charges and is inversely proportional to the square of the distance between them.

ELECTRIC POTENTIAL 

The work done by an E field as it act on a charge q to move it from point A to point B is defined as Electric Potential Difference between points A and B:

B

A

AB rdEq

W

q

UVV

.

Equipotential surfaces

An equipotential surface is that at every point of which electric potential is the same.

A three dimensional view of equipotential surface.

ELECTRIC DIPOLE

An electric dipole consists of a pair of equal and opposite point charges separated by a small distance.

EXPRESSION FOR ELECRIC DIPOLE MOMENT Dipole Moment is the

measure of the strength of electric dipole.

It is denoted as p p =q(2a) where: ‘q’=magnitude of two

charges. ‘a’= distance between the two charges.

Electric Flux

Electric flux over an area in an electric field represents the total number of field lines crossing this area.

E=electric field

A=surface area

= electric Flux

POINTS TO PONDERPOINTS TO PONDER

•Electric field inside a conductor is zero.

GAUSS THEOREM According to this theorem ,the total electric flux

over the surface S in vacuum is 1/E times the total charge (Q) contained inside S. The area vector of surface S is denoted by A.

IMPORTANT POINTS ON GAUSS LAW The Gauss Law holds good

for any surface regardless of its shape and size.

The surface chosen for application of Gauss Law is known as Gaussian surface.

Total electric flux is zero when no charge is enclosed in surface.

APPLICATIONS OF GAUSS LAW To find the field due to a thin plane sheet of charge.

E =

2 E

Parallel plate capacitorParallel plate capacitor

A capacitor is an A capacitor is an arrangement for arrangement for storing large amounts storing large amounts of electric charge and of electric charge and hence electrical hence electrical energy in a small energy in a small space.space.

It has a symbol:It has a symbol:

CapacitanceCapacitance Capacitance of a capacitor is defined as the Capacitance of a capacitor is defined as the

ratio of charge on the capacitor to the potential ratio of charge on the capacitor to the potential of the capacitor.of the capacitor.

It is depicted as:It is depicted as:

Capacitance is measured in units called farads (F) of which the definition is:

1 Farad is the capacitance of a conductor, which has potential difference of 1 volt when it carries a charge of 1 coulomb.  

Capacitors in parallelCapacitors in parallel The capacitors are said to be in parallel between any two points, if The capacitors are said to be in parallel between any two points, if

we can proceed one point to another through different paths.we can proceed one point to another through different paths.

Expression for Parallel combination:Expression for Parallel combination:CC = = CC1 1 ++ CC22 + C + C33 + ……… C + ……… Cnn

Capacitors in seriesCapacitors in series The capacitors are said to be in series between two points , if we The capacitors are said to be in series between two points , if we

can proceed from one point to another only through one end.can proceed from one point to another only through one end.

Expression for series combination is given as:Expression for series combination is given as:1/C1/C =1/C=1/C1 1 +1/C+1/C2 2 +1/C+1/C3…..3…..+1/C+1/Cnn

Energy and capacitance of a parallel plate capacitor

The expression for capacitance of parallel plate capacitor is given as:

C= E A/d

• The expression for energy stored in a parallel plate capacitor is given as : E = ½QV = ½CV²   

Capacitors of various formsCapacitors of various forms

VAN DE GRAFF GENERATORVAN DE GRAFF GENERATOR It is a device It is a device

used for used for building up high building up high potential potential differences differences which are used which are used to accelerate to accelerate charge particles charge particles like electrons , like electrons , protons etc. protons etc. needed for needed for nuclear physics.nuclear physics.

History : Van de Graff History : Van de Graff generatorgenerator

Robert J. Van de Robert J. Van de Graff invented the Graff invented the Van De Graff Van De Graff Generator.Generator.

THANKS

Efforts by: Ashima Dogra XII – SciFaith Academy2008-09