# NCERT Physics Class 12 Exemplar Chapter 1 Electric Charges and Fields Part 8 (For CBSE, ICSE, IAS, NET, NRA 2023)

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Q. 20 A paisa coin is made up of alloy and weighs . It has asquare shape and its diagonal measures . It is electrically neutral and contains equal amounts of positive and negative charges.

Treating the paisa coins made up of only , find the magnitude ofequal number of positive and negative charges. What conclusion do you draw from this magnitude?

Solution:

Molar mass of has atoms.

mass of paisa coin has atoms

Now,

Hence

of charge.

This is an enormous amount of charge. Thus we see that ordinary neutral matter contains enormous amount of charges.

Q. 21 Consider a coin of Example 1.20. It is electrically neutral and contains equal amounts of positive and negative charge of magnitude . Suppose that these equal charges were concentrated in two point charges separated by (i) , (ii) length of a long building , and (iii) (radius of the earth) . Find the force on each such point charge in each of the three cases. What do you conclude from these results?

Solution: we know force between two point separated at a distance .

Here,

(i)

(ii)

(ii)

Conclusion: When separated as point charges these charges exert an enormous force. It is not easy to disturb electrical neutrality.

Q. 22 Fig. represents a crystal unit of cesium chloride, . The cesium atoms, represented by open circles are situated at the corners of a cube of side , whereas a atom is situated at the centre of the cube. The atoms are deficient in one electron while the atom carries an excess electron.

(i) What is the net electric field on the atom due to eight atoms?

(ii) Suppose that the Cs atom at the corner A is missing. What is the net force now on the atom due to seven remaining atoms?

Solution:

(i) Zero, from symmetry.

(ii) Removing a ion is equivalent to adding singly charged ion at that location.

Net force then is

Where

Hence,

Ans , directed from to