(a) Quantization of charge
(i) Electric charge can exist only as an integral multiple of charge on an electron (-e) i.e.
q = ± ne, where n is an integer.
(ii) The Possible values of electric charge are q = ± 1e; ± 2e; ± 3e...
(iii) Charge less than the charge on an electron (i.e. e = 1.6 * 10-19 C) is not possible.
(b) Conservation of charge
(i) On an isolated system, total electric charge always remains constant.
(ii) Total charge on a body is equal to the algebraic sum of all the charges present on it. Every atom is electrically neutral as it contains as many electrons as the number of protons in it.
(iii) When we rub a glass rod with a piece of silk, the positive charge acquired by the glass rod is equal to negative charge acquired by the silk piece. Thus charges are produced in equal and unlike pairs.
Example (1): What is the possible value of electric charge?
(a) 1 X 1.6 X 10-19 C
(b) 2.4 X 1.6 X 10-19 C
(c) -8 X 1.6 X 10-19 C
(d) 1 X 1.8 X 10-19 C
As we know that, electric charge can exist only as an integral multiple of charge on an electron (-e) i.e.
q = ±ne, where n is an integer. So q = ±1 X 1.6 X 10-19 C
Example (2): If n=2, what will be the value of electric charge? (Given e =1.6 X 10-19 C)
(a) ±0.8 X 10-19 C
(b) ±3.2 X 10-19 C
(c) ±4.3 X 10-19 C
(d) ±6.3 X 10-19 C
We know that
q = ±ne
= 2 X 1.6 X 10-19 C
= ±3.2 X 10-19 C
Hence option (b) is correct.
Example (3): Is charge less than the charge (i.e. e = 1.6 X 10-19 C) on an electron possible?
(a) Yes (b) No
Solution :( b) As we know
q = ±ne where n is an integer i.e. n= 1, 2, 3,...
Example 4): What is the total charge of all of the protons in 1.00 kg of carbon?
(a) 4.82 X 107 C
(b) 3.96 X 107 C
(c) 4.82 X 109 C
(d) 3.96 X 1012 C
Solution: (a) We can find the number of coulombs of positive charge there are in 1.00 kg of carbon from Q=6nce, where nc is the number of atoms in 1.00 kg of carbon and the factor of 6 is present to account for the presence of 6 protons in each atom. We can find the number of atoms in 1.00 kg of carbon by setting up a proportion relating Avogadro’s number NA , the mass of carbon, and the molecular mass of carbon to nc.
Example 5): Determine the electric current in an electric circuit where the total electric charge is 6 C over 5 seconds.
Isolating I from Q = I ∙ t
I = Q/t = 6 / 5 = 1.2 A