Question

A paisa coin is made up of Al-Mg alloy and weighs 0.75g. 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 number of positive and negative charges. What conclusion do you draw from this magnitude?

Answer 1

1 Molar mass M of Al has N_A = 6.023 × 10²³ atoms.

∴ m = mass of Al paisa coin has N = N_A m/M atoms

Now, Z_Al = 13, MAl = 26.9815 g

Hence N = 6.02 × 10²³ atoms/mol × 0.75/26.9815g/mol

= 1.6733 × 10²² atoms

∴ q = +ve charge in paisa = N Ze

= (1.67 × 10²²)(13) (1.60 × 10^–19C)

= 3.48 × 10⁴ C

q = 34.8 kC of ±ve charge.

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