Physics Class 12 NCERT Solutions: Chapter 1 Electric Charges and Fields Part 1 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Coulomb՚S Law (Electrostatic)

Q: 1. What is the force between two small charged spheres having charges and placed apart in air?


Repulsive force of magnitude

Charge on the first sphere,

Charge on the second sphere,

Distance between the spheres,

Electrostatic force between the spheres is given by the relation,


Hence, force between the two small charged spheres is . The charges are of same nature. Hence, force between them will be repulsive.

Q: 2. The electrostatic force on a small sphere of charge due to another small sphere of charge in air is . (A) What is the distance between the two spheres? (B) What is the force on the second sphere due to the first?


(A) Electrostatic force on the first sphere,

Charge on this sphere,

Charge on the second sphere,

Electrostatic force between the spheres is given by the relation,

Where, Permittivity of free space


The distance between the two spheres is .

(B) Both the spheres attract each other with the same force. Therefore, the force on the second sphere due to the first is .

Q: 3. Check that the ratio is dimensionless. Look up a table of physical constants and determine the value of this ratio. What does the ratio signify?


The given ratio is .


Gravitational constant

Its unit is .

Masses of electron and proton

Their unit is kg.

Its unit is C.

Permittivity of free space

Its unit is

Therefore, unit of the given ratio

Hence, the given ratio is dimensionless.

Hence, the numerical value of the given ratio is

This is the ratio of electric force to the gravitational force between a proton and an electron, keeping distance between them constant.

Q: 4. (A) Explain the meaning of the statement ‘electric charge of a body is quantised’ .

(B) Why can one ignore quantisation of electric charge when dealing with macroscopic i.e.. , large scale charges?


(A) Electric charge of a body is quantized. This means that only integral number of electrons can be transferred from one body to the other. Charges are not transferred in fraction. Hence, a body possesses total charge only in integral multiples of electric charge.

(B) In macroscopic or large scale charges, the charges used are huge as compared to the magnitude of electric charge. Hence, quantization of electric charge is of no use on macroscopic scale. Therefore, it is ignored and it is considered that electric charge is continuous.