Electrostatics is the study of electric charges at rest and so it is also called static electricity.
Electric Charge
Electric charge, like mass, is one of the fundamental attributes of the particle of which the matter is made. Charge is the physical property of certain fundamental particles (like electron, proton) by virtue of which they interact with the other similar fundamental particles.
To distinguish the nature of interaction charges are divided into two parts :
(i) positive
(ii) negative
SI unit of charge is coulomb and CGS unit is esu.
1 C = 3 × 109 esu.
Magnitude of the smallest known charge is e = 1.6 × 10-19C (charge of one electron or proton).
Note : True test of electrification is repulsion and not attraction as attraction may also take place between a charged and an uncharged body and also between two oppositely charged bodies.
Charging of the body
Basically charging can be done by two methods; by
(i) Conduction
(ii) Induction
Ordinarily, matter contains equal number of protons and electrons. A body can be charged by the transfer of electrons or redistribution of electrons.
The process of charging from an already charged body can happen either by conduction or induction.
Conduction : Conduction from a charged body, involves transfer of like charges. A positively charged body can create more bodies, which are positively charged, but the sum of the total charge on all positively charged bodies will be the same as the earlier sum.
Induction : Induction is a process by which a charged body accomplishes the creation of other charged bodies, without touching them or losing its own charge. An uncharged conductor gets charged by induction due to redistribution of its own charge.
Induced Charge , $\large q’ = -q(1-\frac{1}{k})$
Where k = dielectric constant of material of uncharged body .
For metals , k = ∞ ⇒ q’ = – q
Properties of Charge :
(i) Like charges repel and unlike charges attract each other .
(ii) Charge is a scalar quantity .
(iii) Charge is additive in nature . e.g. +2C + 5C-3C = 4C
(iv) Conservation of charge :
Charge is conserved, i.e. total charge on an isolated system is constant. By isolated system, we mean here a system through the boundary of which no charge is allowed to escape or enter. This does not require that the amount of positive and negative charges separately conserved; only their algebraic sum is conserved.
(v) Quantization of charge :
Charge exists in discrete packets rather than in continuous amount.
i.e. charge on any body is the integral multiple of the charge on an electron
Q = ± ne, where n = 0, 1, 2, ……………..
(vi) Charges on a conductor :
Static charges reside on the surface of the conductor.
(vii) Distribution of charges :
The concentration of the charges is maximum on a surface with greater curvature.
Note: If a body possesses n1 protons and n2 electrons, then net charge on it will be (n1 – n2)e , i.e., n1(e) + n2(-e) = (n1 – n2)e
Illustration : Is a charge of 5.8 × 10-18 possible ?
Sol: Applying Quantization of charge ,
Q = n e
$\large n = \frac{Q}{e} = \frac{5.8 \times 10^{-18}}{1.6 \times 10^{-19}}$
n= 36.25 , this is not an integer . Hence this charge is not possible .
Illustration : One million electrons are added to a glass rod. What is the total charge on the rod ?
Sol: here , n = 106
Applying Quantization of charge ,
Q = n e
Q = 106 × (-1.6 × 10-19)
Q = – 1.6 × 10-13 C