Electrostatics of Class 12
Charge is a property of matter that causes it to produce and experience electrical and magnetic effects. The subject of the electrical effect of charges at rest is called electrostatics. When both electrical and magnetic effect are present, the interaction between charges is referred to as electromagnetic.
There exist two types of charges in nature: positive and negative. Like charges repel, and unlike charges attract.
The SI unit of charge is the coulomb (C). It is defined in terms of electric current. One coulomb is the charge transferred in one second across the section of wire carrying a current of one ampere.
q = It
⇒1C = (1A) (1s)
The reason is that one can make precise measurements of the current flowing in wire, whereas the charge on a body tends to leak away.
The coulomb is a very large amount of charge. A typical charge acquired by a rubbed body is 10−8C, whereas a lightning bolt may transfer as much as 20 C between the earth and a cloud.
Quantization of Charge
Electric charges appear only in discrete amounts, it is said to be quantized. The quantum of charge, first directly measured in 1909 by R. A. Millikan, is approximately
e = 1.602 × 10−19 C
A charge q must be an integer multiple of this basic unit. That is q = 0, ±e, ±2e, ±3e etc Although the mass of the proton is about 1800 times greater than that of the electron, their charges have the same magnitude.
qe = − e ; qp = + e
Note that the electron itself is not the charge: Charge is a property, like mass, of elementary particles, such as the electron.
Conservation of Charge
For an isolated system, the total charge remains constant, charge is neither created nor destroyed, it is transferred from one body to the other.
By ‘isolated’ we mean that there are no paths, such as wires or damp air, by which charges can leave or enter the system. To apply the law of conservation of charges, we add up the number of elementary charges before the interaction and then again after it. For example,
In a chemical reaction
Na+ + Cl− → NaCl
(+e) + (−e) = (0)
In a radioactive decay
n → p + e− + ν (antineutrino)
(0) = (+e) + (−e) + (0)
Invariance of Charge
The numerical value of an elementary charge is independent of velocity. It is provided by the fact that an atom is neutral. The difference in masses of an electron and a proton suggests that electrons move much faster in an atom than protons. If the charges were dependent on velocity, the neutrality of atoms would be violated.