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# A paisa coin is made up of Al-my alloy and weights $0.75\;g$ 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 only $Al$, find the magnitude of equal number of positive and negative charges. What conclusion do you draw from this magnitude ?

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A)
Let us consider that the coin is made of Aluminium
Given , weight of the diagonal $=17 \;mm$
Atomic mass of Al = 27 g
Atomic number of Al =13
One molar mass of substance has $6.023 \times 10^{23}$ number of atoms
$27\;g$ Al contains $6.023 \times 10^{23}$ atom
1g of Al contains $=\large\frac{6.023 \times 10^{23}}{27}$ atom
$0.75g$ of Al contains $\large\frac{6.023 \times 10^{23}}{27}$$\times 0.75$
$\qquad= 1.67 \times 10^{23}$
One atom of Al contains 13 electrons 13 protons
$1.67 \times 10^{22}$ atom of Al contains
$13 \times 1.67 \times 10^{22}= 21.71 \times 10^{22}$ electron
Hence number of proton $=21.71 \times 10^{22}$
The charge on one electron $= 1.6 \times 10^{-19}$
Charge on $21.71 \times 10^{22}$ electrons.
$\qquad= 1.6 \times 10^{-19} \times 21.17 \times 10^{22}$
$\qquad= 34.736 \times 10^3 \;C$
Thus the mount of positive and negative charge on the coin $=34.736 \times 10^{3}\;C$
This is a very large amount of charge .
This concludes that all the atoms contain a very large amount of positive and negative charges.