ELECTROSTATIC FORCE. ELECTROSTATICS - I Physics Without Fear.

Transcription

1 ELECTROSTATIC FORCE ELECTROSTATICS - I Physics Without Fear

2 CONTENTS Frictional Electricity Properties of Electric Charges Coulomb s Law Coulomb s Law in Vector Form Units of Charge Relative Permittivity or Dielectric Constant

3 FRICTIONAL ELECTRICITY The electric Charge produced by rubbing two suitable dissimilar bodies is called Frictional electricity. Glass Flannel Silk Ebonite Cause: The bodies acquire charge due to transfer of electrons from one body to other. Electrons in glass are loosely bound than the electrons in silk. So, when glass and silk are rubbed together, some of the loosely bound electrons get transferred from glass to silk. As a result, glass acquires positive charge and silk gains negative charge.

4 FRICTIONAL ELECTRICITY Glass Silk Flannel Ebonite 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.

5 FRICTIONAL ELECTRICITY Remember: The electrification of the bodies (Positive or Negative) is due to transfer of electrons from one object to another. The deficiency the electrons due to transfer of the electrons from a body makes the body positive. The gain of electrons by a body causing an excess of the electrons makes a body negative. An object may acquire positive or negative charge depending on its relative property of attracting electron w.r.t. the body with which it is being rubbed. Fur, Glass, Silk, Cotton, Wood, Sealing wax, Amber, Rubber, Ebonite. If the two bodies from the above list are rubbed, then the body appearing earlier in the list gains positive charge whereas the later becomes negatively charged.

6 FRICTIONAL ELECTRICITY Positive Charge Negative Charge Glass Wool, Flannel Ebonite Dry hair Silk Amber, Ebonite, Rubber, Plastic Polythene Comb PROPERTIES OF ELECTRIC CHARGE: There are only two types of electric charge, the positive and the negative charge. Like charges repel and unlike charges attract each other. Charge is a scalar quantity. Electric charge does not change with velocity.

7 Charge is additive in nature i.e. +C + 2 C 4 C = - C. Charge is conserved* i.e. Electric charge can neither be created nor destroyed. It can only be transferred fro one object to another. Hence the algebraic sum of positive and negative charges in an isolated system remains constant. Charge is quantized i.e. Charge can exist only in discrete packets. It can t have arbitrary continuous values. It can exist only as integral multiples of the fundamental electronic charge (e =.6 x 0-9 C ## ) OR q = ± ne PROPERTIES OF ELECTRIC CHARGE where n =, 2, 3, *When a glass rod is rubbed with silk, the negative charge appearing on silk is equal to the amount of positive charge developed on the glass rod. The net charge on a system before and after electrification remains zero. ##Researchers have postulated the existence of particles called quarks with charge ⅓ e & ⅔ e. In case of a concrete practical evidence of the quarks, the minimum value of a quantum of charge will be reduced to ⅓ e or ⅔ e. However, the law of quantization will still hold good.

8 COULOMB S LAW Force between two point electric charges: The electrostatic force of attraction/ repulsion between two point charges is directly proportional to the product of the charges & inversely proportional to the square of the distance between them. It is a central force i. e. it acts along the line joining the two charges. or For point charges q & q 2 held a distance r apart; we have: F α F α / r 2 F α r 2 or F = k where k is a positive constant of proportionality called electrostatic force constant or Coulomb constant. r 2 q r q 2 In vacuum, k = q F = q 2 where ε 4π 0 r 2 0 is the permittivity of free space. = N m 2 C -2 ε 0 = x 0-2 C 2 N - m -2.

9 COULOMB S LAW In medium, k = 4πε Positive force signifies REPULSION & negative signifies ATTRACTION. where ε is the absolute electric permittivity of the dielectric medium. F = k = r 2 4π r 2 where ε is the permittivity of the medium. = 0 r = Kε 0 is the permittivity of the medium. K= / 0 = r=fva c /Fme d is the dielectric constant of the medium. A graph between (F & q ); (F & q 2 ); (F & /r 2 ) is a st. line. Dielectric constant or Relative permittivity of a medium is defined as the ratio of the force between two charges placed a certain distance apart in vacuum to the force between the two charges placed an equal distance apart in the medium. Dielectric constant has no unit.

10 COULOMB S LAW IN VECTOR FORM r F 2 q r 2 q F 2 2 For similar charges in vacuum, i. e. for > 0, r F 2 -q r 2 -q F 2 2 F 2 = r 2 r 2 and F 2 = r 2 r 2 REPULSIVE FORCE F 2 = -F 2 () Hence both for attractive as well as repulsive force; F 2 & F 2 are equal & opposite. Coulomb s law is compatible with Newton s III law.

11 COULOMB S LAW IN VECTOR FORM F 2 r F 2 F 2 = For dissimilar charges in vacuum, i. e. for > 0, r 2 r 2 q -q 2 r 2 and F 2 = r 2 FOR ATTRACTIVE FORCE; F 2 = -F 2 (2) Hence both for attractive as well as repulsive force; F 2 & F 2 are equal & opposite. Coulomb s law is compatible with Newton s III law. The expression for force may also be written as: F = r 2 r r = r 3 r..{multiply & divide RHS by r}. r 2

12 S I Unit of Electric Charge The expression for force below is used to define SI unit of charge. F = r 2 r r If q =q 2 = coulomb; r=m with the charges in vacuum; then by above expression F= =9x0 4πε 9 N. 0 Hence one C is the charge which when placed at rest in vacuum at a distance of one metre from an equal and similar stationary charge repels it with a force of 9 x 0 9 newton. The symbol used for coulomb is C. One coulomb is a very large unit of charge. The smaller unit used are micro C = μc = 0-6 C. nano C = nc = 0-9 C.

13 VISIT: for more topics