physics notes for class 12|NCERT solution|chapter 1|electric field:

NCERT solution|chapter 1|electric field:

Question 1.1:

What is the force between two small charged spheres having charges of 2 × 10 -7C and

3× 10−7 C placed 30 cm apart in air?

Answer: Electrostatic force between the spheres is given by ,

Where, ∈0 = Permittivity of free space,

 

Hence, force between the two small charged spheres is 6 × 10-3 N

 Hence, force between them will be repulsive because charges are of same nature.

Question 1.2:

The electrostatic force on a small sphere of charge 0.4 μC due to another small sphere of Charge  − 0.8 μC  in air is 0.2 N.

(a). What is the distance between the two sphere?

(b).What is the force on the second sphere due to the first?

Answer: f=0.2N, q1=0.4 μC =.4 x 10-6 C ,q2=− 0.8 μC  =0.8x10-6 C

Electrostatic force between the spheres is

distance between the two spheres is 0.12 m.

Both the spheres attract each other with same force then force for

second sphere due to the first F is 0.2 N(as given)

Question 1.3: Check that the ratio ke2/G me and determine the value of this ratio. What does the ratio signify  ?

Answer:

as given in question,

 

Where,

G = Gravitational constant, Its unit is N m2 kg−2.

,me and mp = Masses of electron and proton. Their unit is kg. e = Electric charge., Its unit is C.

∈0 = Permittivity of free space, Its unit is N m2 C−2

 

Hence it is  dimensionless.

e = 1.6 × 10−19 C

G = 6.67 × 10−11 N m2 kg-2

me= 9.1 × 10−31 kg

mp = 1.66 × 10−27 kg

Hence, the numerical value is

 

Question 1.4:

Explain the meaning of the statement ‘electric charge of a body is quantised’.

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

Answer:

Electric charge of a body is quantized. This means that only integral value (1, 2, ….,) of electrons can be transferred from one body to the other.  In other word Charges are not transferred in fraction . Hence, a body has integral multiple of  total charge.

In macroscopic scale , the charges used are much greater in magnitude than electric charge. Hence, quantization of electric charge is of no use in macroscopic scale. Therefore, it is ignored in macroscopic level.

Question 1.5:

When a glass rod is rubbed with a silk cloth, charges appear on both. A similar phenomenon is observed with many other pairs of bodies. Explain how this observation

consistent with the law of conservation of charge.

Answer: A glass rod is rubbed with a silk cloth, charges appear on both. A similar phenomenon is observed with many other pairs of bodies because, Rubbing produces charges of equal magnitude but opposite in nature,as charges are created in pairs. This phenomenon of charging is known as “charging by Friction”. The net charge on the system of two rubbed bodies is zero. This is because equal

amount of opposite charges cancel each other. When a glass rod is rubbed with a silk

cloth, opposite nature charges appear on both the bodies. This phenomenon is in accordance with the law of conservation of energy. A similar phenomenon is observed with many other pairs of bodies