First slide

Electric charges and its properties

Question

A copper sphere of mass 2.0 g contains about 2 x 10²² atoms. The charge on the nucleus of each atom is 29 e (e = electronic charge). The mass of an electron is 9.11 x 10^-31kg.How much mass will the sphere lose or gain if it is given a charge of +2 $μ$ C?

Easy

A

Loss a mass of 1.13 x 10^-14 g
B

Gain a mass of 1.13 x 10^-14g
C

Neither gain nor loss any mass
D

Change in mass will depend upon the kinetic energy of the electrons

Solution

To give positive charge to the copper sphere, a certain number of free electrons must be removed.
The total number of electrons is $29 \times (2 \times 10^{22}) = 5.8 \times 10^{23}$
Since a positive charge of 2 $μ$ C is to be given the number of electrons to be removed $= \frac{2 \times 10^{- 6}}{1.6 \times 10^{- 19}} = 1.25 \times 10^{13}$
The mass of the electrons removed
$\begin{array}{l} = 1.25 \times 10^{13} \times 0.91 \times 10^{- 30} kg \\ = 1.13 \times 10^{- 17} kg = 1.13 \times 10^{- 14} g \end{array}$
This mass will be lost by the copper sphere.

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