Ncert Exercises & Solutions

Consider a coin in Question 20. It is electrically neutral and contains equal amounts of positive and negative charge of magnitude 34.8 kC. Suppose that these equal charges were concentrated in two-point charges separated by:

(i) 1 cm ( $\approx \frac{1}{2}$ x diagonal of the one paisa coin)

(ii) 100 m ( $\approx$ length of a long building)

(iii) 10⁶ m (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?

Hint: Use the coulomb's law.

Step 1: Find the force between the charges in each case.

Here,

q = \pm 34.8 k C = \pm 3.48 \times 10^{4} C

r_{1} = 1 cm = 10^{- 2} m, r_{2} = 100 m, r_{3} = 10^{6} m and \frac{1}{4 π ε_{0}} = 9 \times 10^{9}

F_{1} = \frac{| q |^{2}}{4 π ε_{0} r_{1}^{2}} = \frac{9 \times 10^{9} \times {(3.48 \times 10^{4})}^{2}}{{(10^{- 2})}^{2}} = 1.09 \times 10^{23} N

F_{2} = \frac{| q |^{2}}{4 π ε_{0} r_{2}^{2}} = \frac{9 \times 10^{9} \times {(3.48 \times 10^{4})}^{2}}{(100)^{2}} = 1.09 \times 10^{15} N

F_{3} = \frac{| q |^{2}}{4 π ε_{0} r_{3}^{2}} = \frac{9 \times 10^{9} \times {(348 \times 10^{4})}^{2}}{{(10^{6})}^{2}} = 1.09 \times 10^{7} N

Step 2: Find the conclusion by observing the force between the charges.

When charges in an electrically neutral matter are separated by a distance, they exert a large force on each other and hence, it is no possible to separate the charges in an electrically neutral matter.

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