8/16/2019 Electrostatics Notes 1 Final by Vinay Sharma

1/4

Electrostatics

1.Electrostatics or Static Electricity or

Frictional Electricity

 The branch of physics, which deals withstudy of charges at rest (static charge),the force between the static charges

elds and potentials due to thesecharges is called as Electrostatics orStatic Electricity.

Frictional Electricity  is the electricityproduced by rubbing two suitable bodiesand transfer of electrons from one bodyto other.

Historically, this phenonmenon wasdiscovered by a greek Thales of !iletusaround "##$%. The name electricity wastaken from the &reek word Elecktron.

2.What is Electric Charges

Charge is the fundamental property of material.

Charge exist only in material

Charge is something possessed bymaterial objects that make itpossible for them to exert electricalforce and to respond to electricalforce.

 There e'ists only two types of charges,namely positive and negative. ike charges repel and unlike chargesattract each other. %harge is a scalar uantity.

*hen a body gets the static charge byany means then body is said to beelectried or charged. *hen it has nocharge it is said to be neutral.

+undamental Electronic Charge (e =.! x "#$ C%. - unit of charge is

ampere ' sec that is called Coulomb ( C )and is generally denoted by q. 

&ote' ecently, the e'istence of uarksof charge / e and 0 e hasbeen postulated. -f the uarks aredetected in any e'periment withconcrete practical evidence, then the

minimum value of 1uantum of chargewill be either / e or 0 e. However, thelaw of uantiation will hold good

3.Two Kins o! Charges

-t was observed that if two glass rodsrubbed with wool or silk cloth are broughtclose to each other, they repel eachother Fig. a. The two strands of wool or

two pieces of silk cloth, with which therods were rubbed, also repel each otherHowever, the glass rod and wooattracted each other.

imilarly, two plastic rods rubbed withcat2s fur repelled each other Fig. b  butattracted the fur.3n the other hand, the plastic rodsattract the glass rod Fig. c and repel thesilk or wool with which the glass rod isrubbed. The glass rod repels the fur.

-f a plastic rod rubbed with fur is made totouch two small pith balls suspended bysilk or nylon thread, then the balls repeeach other Fig. d and are also repelledby the rod.

4 similar e5ect is found if the pith ballsare touched with a glass rod rubbed withsilk Fig. e. 4 dramatic observation is that a pith baltouched with glass rod attracts anotherpith ball touched with plastic rod Fig. f .

8/16/2019 Electrostatics Notes 1 Final by Vinay Sharma

2/4

-t was concluded, that there are only twokinds of an entity which is called theelectric charge. The e'periments onpith balls suggested that there are twokinds of electrication and we nd that

like charges repel and

6nlike charges attract each other.

y con)ention* the charge on glassrod or cat+s fur is called positi)e andthat on plastic rod or silk is termednegati)e.

 The property which di5erentiates the twokinds of charges is called the polarity of charge.

&ote' 3nly rubbed area of nonconducting body gets charged, and thischarge does not move to other parts ofthe body. The charge is static on rubbedportion only.

 _____________________________________________________________________________ 

Electron theory o! Electri!ication

• Nucleus of atom is positively charged.

• The electron revolving around it is negatively

charged.

• They are equal in numbers, hence atom iselectrically neutral.

• With friction there is transfer of electrons, hence

net charge is developed in the particles.

• It also explains that the charges are compulsorily

developed in pairs equally . ve in one body and

  !ve in second.

• It establish conservation of charges in the

universe.

• The loss of electrons develops ve charge.

While excess of electrons develop !ve charge

• " proton is #$%& times heavier than electron

hence it cannot be transferred. Transferring

lighter electron is easier.

• Therefore for electrification of matter, only

electrons are active and responsible.

".#asic $ro%erties o! Charge

-f the si7es of charged bodies are verysmall as compared to the distancesbetween them, we treat them as point

charges. 4ll the charge content of thebody is assumed to be concentrated atone point in space. ome otherproperties of the electric charge are

#. ,dditi)e of charges '

Charge is the Scalar -uantity

8 %harges can be added by simple rules

of algebra. 4ddition of eual positive andnegative charge makes 9ero charge

-f a system contains n charges q:, q;, q<…, qn, then the total charge of thesystem is q: + q; + q< + … + qn .

%harge has magnitude but no direction=roper signs have to be used whileadding the charges in a system.

8/16/2019 Electrostatics Notes 1 Final by Vinay Sharma

3/4

'. Charge is conser)ed -t is notpossible to create or destroy netcharge carried by any isolatedsystem.

-solated system (-ndividual) Electriccharge can neither be created nordestroyed, it can only be transferred.

%harges in pair can be created ordestroyed.

 

%. uanti/ation of charge '

%harge on an electron (>e) and chargeon a proton (?e) are eual and

opposite, and are the minimum chargethat is called basic unit of charge .

%harges are integral multiples of abasic unit of charge denoted by e.

 Thus charge on a body is alwaysgiven by  - = ne. where n is anyinteger, positive or negative.

 $y convention, the charge on anelectron is taken to be negative@therefore charge on an electron is writtenas –e and that on a proton as +e.

 The fact that electric charge is always anintegral multiple of e is  termed as-uanti/ation of charge.

-n the -nternational ystem (-) of 6nits,a unit of charge is called a coulomb andis denoted by the symbol %.-n thissystem, the value of the basic unit ofcharge is e = :."#;:A; B :#C:A %. ______________________________________________________________________________________ 

". Charge &s Trans!erable

'. Charge &s lways ssociate With ass

*.#y Charging ass &s Change

+.Charge &s &n,ariant -

charge does not depend on frame of reference

(charge does not depends on speed )

com%arison o! Charge n ass

C0,12EElectric charge on the body may bepositive or negative.

!as

%harge carried by the body doesnot depend upon the velocity.

%haupo

%harge is uanti7ed !asElectric charge is always conserved !as

is n

+orce may be attractive orrepulsive, according to the charge

&ramas

 __________________________________________________________________________________________

Electron theory o! Electri!ication

• Nucleus of atom is positively charged.

• The electron revolving around it is negatively

charged.

8/16/2019 Electrostatics Notes 1 Final by Vinay Sharma

4/4

• They are equal in numbers, hence atom is

electrically neutral.

• With friction there is transfer of electrons, hence

net charge is developed in the particles.

• It also explains that the charges are compulsorily

developed in pairs equally . ve in one body and

  !ve in second.

• It establish conservation of charges in the

universe.

• The loss of electrons develops ve charge.

While excess of electrons develop !ve charge

• " proton is #$%& times heavier than electron

hence it cannot be transferred. Transferring

lighter electron is easier.

• Therefore for electrification of matter, only

electrons are active and responsible.

Charge and 3ass relation

 8 %harge cannot e'ist without matter.

8 3ne carrier of charge is electron which hasmass as well.

8 Hence if there is charge transfer, mass isalso transferred.

8 ogically, negatively charged body isheavier then positively charged body.

Conuctors &nsulators an /ielectrics

 Those substances which allowelectricity to pass through them

easily are called conductors. Theyhave electric charges (electrons) that arecomparatively free to move inside thematerial.

!etals, human and animal bodies andearth are conductors.

 Those substances which do  not allo4electricity to pass through themeasily are called insulators.

!ost of the non>metals like glassporcelain, plastic, nylon, wood o5er highresistance to the passage of electricitythrough them.

5nsulators are also called 6ielectric3bviously, dielectric cannot conduct

electricity. However, when a electriceld is applied* induced chargesappear on the surface of thedielectric. Hence we may denedielectric as the insulating material whichtransmits electric e5ects withoutconducting.

*hen we bring a charged body in contactwith the earth, all the e'cess charge onthe body disappears by causing a

momentary current to pass to the groundthrough the connecting conductor (suchas our body). This process of sharing thecharges with the earth is calledgrounding or earthing.