Physics 12 Electrostatics Notes

Topics: Electric charge, Electrostatics, Electric field Pages: 27 (5586 words) Published: September 10, 2012
ELECTROSTATICS - I – Electrostatic Force
1. Frictional Electricity 2. Properties of Electric Charges 3. Coulomb’s Law 4. Coulomb’s Law in Vector Form 5. Units of Charge 6. Relative Permittivity or Dielectric Constant 7. Continuous Charge Distribution i) Linear Charge Density ii) Surface Charge Density iii) Volume Charge Density

Frictional Electricity:
Frictional electricity is the electricity produced by rubbing two suitable bodies and transfer of electrons from one body to other. + ++


+ ++


+ ++

Glass Silk
+ ++ + + ++ + ++

- - - -

.- - Ebonite Flannel

Electrons in glass are loosely bound in it than the electrons in silk. So, when glass and silk are rubbed together, the comparatively loosely bound electrons from glass get transferred to silk. As a result, glass becomes positively charged and silk becomes negatively charged. Electrons in fur are loosely bound in it than the electrons in ebonite. So, when ebonite and fur are rubbed together, the comparatively loosely bound electrons from fur get transferred to ebonite. As a result, ebonite becomes negatively charged and fur becomes positively charged.

It is very important to note that the electrification of the body (whether positive or negative) is due to transfer of electrons from one body to another. i.e. If the electrons are transferred from a body, then the deficiency of electrons makes the body positive. If the electrons are gained by a body, then the excess of electrons makes the body negative. If the two bodies from the following list are rubbed, then the body appearing early in the list is positively charges whereas the latter is negatively charged. Fur, Glass, Silk, Human body, Cotton, Wood, Sealing wax, Amber, Resin, Sulphur, Rubber, Ebonite. Column I (+ve Charge) Glass Wool, Flannel Ebonite Dry hair Column II (-ve Charge) Silk Amber, Ebonite, Rubber, Plastic Polythene Comb

Properties of Charges:
1. There exists only two types of charges, namely positive and negative. 2. Like charges repel and unlike charges attract each other. 3. Charge is a scalar quantity. 4. Charge is additive in nature. 5. Charge is quantized. i.e. Electric charge exists in discrete packets rather than in continuous amount. It can be expressed in integral multiples fundamental electronic charge (e = 1.6 x 10-19 C) q = ± ne 6. Charge is conserved. i.e. The algebraic sum of positive and negative charges in an isolated system remains constant. eg. When a glass rod is rubbed with silk, negative charge appears on the silk and an equal amount of positive charge appear on the glass rod. The net charge on the glass-silk system remains zero before and after rubbing. It does not change with velocity also. where n = 1, 2, 3, ………… eg. +2 C + 5 C – 3 C = +4 C

Note: Recently, the existence of quarks of charge ⅓ e and ⅔ e has been postulated. If the quarks are detected in any experiment with concrete practical evidence, then the minimum value of ‘quantum of charge’ will be either ⅓ e or ⅔ e. However, the law of quantization will hold good.

Coulomb’s Law – Force between two point electric charges: The electrostatic force of interaction (attraction or repulsion) between two point electric charges is directly proportional to the product of the charges, inversely proportional to the square of the distance between them and acts along the line joining the two charges. Strictly speaking, Coulomb’s law applies to stationary point charges. F α q1 q2 F α 1 / r2 or Fα q1 q2 r2 or F=k q1 q2 r2 q1 r where k is a positive constant of proportionality called electrostatic force constant or Coulomb constant. q2

In vacuum, k =

1 4πε0

where ε0 is the permittivity of free space

In medium, k =

1 4πε

where ε is the absolute electric permittivity of the dielectric medium

The dielectric constant or relative permittivity or specific inductive capacity or dielectric coefficient is given by ε K = εr = ε0 In vacuum, F = 1 4πε0 1 4πε0εr q1 q2 r2 q1 q2 r2

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