Ncert Exercises & Solutions

A paisa coin is made up of Al-Mg alloy and weighs 0.75 g. It has a square shape and its diagonal measures 17 mm. It is electrically neutral and contains equal amounts of positive and negative charges. Treating the paisa coins made up of only Al, find the magnitude of equal no. of positive and negative charges. What conclusion do you draw from this magnitude?

Hint: Use Q=ne.

Step 1: Find the no. of Al-atoms present in one paisa coin.

Given,

Mass of a paisa coin = 0.75 g

The atomic mass of aluminium =26.9815 g

Avogadro's number = 6.023 x 10²³

$∴$ Number of aluminium atoms in one paisa coin,

$N = \frac{6.023 \times 10^{23}}{26.9815} \times 0.75 = 1.6742 \times 10^{22}$

Step 2: Find the amount of positive or negative charge on one paisa coin.
As the atomic number of Al is 13, each atom of Al contains 13 protons and 13 electrons.

$∴$ The magnitude of positive and negative charges in one paisa coin = Nxzxe

$\begin{array}{l} = 1.6742 \times 10^{22} \times 13 \times 1.60 \times 10^{- 19} C \\ = 3.48 \times 10^{4} C = 34.8 kC \end{array}$

This is a very large amount of charge. Thus, we can conclude that ordinary neutral matter contains an enormous amount of

$\pm$ charges

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