Question

Absolute mass of an electron is equal to ________.
(A) $9.109\times {{10}^{-31}}kg$
(B) $-1.602\times {{10}^{-31}}kg$
(C) $1.672\times {{10}^{-27}}kg$
(D) $1.647\times {{10}^{-27}}kg$

Answer 1

Hint: Recollect the theory of discovery of electrons. Millikan had established a method known as oil drop experiment to determine the charge on the electron. Thomson had determined the charge to mass ratio of electrons. Find the relation between both the parameters and calculate the mass of the electron.

Complete answer:
- Absolute mass is the term used to describe mass of subatomic particles like protons, neutrons, electrons, etc.
- Sir J. J. Thomson conducted an experiment to measure the ratio of electrical charge (e) to the mass (m) of cathode ray particles (electron) by using a specially designed cathode ray tube.
- He determined the charge to mass ratio of electrons as,
\[\dfrac{e}{m}=1.75882\times {{10}^{11}}C/kg\]
where m is the mass of the electron in kg and e is the charge in coulomb (C).
- He also concluded that cathode ray particles are negatively charged and therefore, the charge on the electron is represented as -e.
- Later, scientist Millikan conducted the oil drop experiment to determine the charge on electrons. He found that the charge on electrons is equal to $-1.6022\times {{10}^{-19}}C$.
- By knowing the charge to mass ratio value, he determined the mass of electron as,
\[{{m}_{e}}=\dfrac{e}{{}^{e}/{}_{m}}=\dfrac{1.6022\times {{10}^{-19}}C}{1.75882\times {{10}^{11}}C/kg}\]
where e is the electronic charge and m its mass.
\[{{m}_{e}}=9.1094\times {{10}^{-31}}kg\]
- Therefore, the absolute mass of an electron is $9.109\times {{10}^{-31}}kg$.

Therefore, the answer is option (A).

Note:
Remember absolute mass of electron is equal to absolute mass of proton. Protons and electrons differ only with respect to charge from each other. Protons are positively charged and electrons are negatively charged.