Consider that 1010 electrons are transferred to an oil drop. Which one of the following statements is CORRECT?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Both mass and magnitude of the charge on the oil drop will increase.
Charge is carried by particles, so transferring charge means transferring matter. Each electron has charge \( e = 1.6\times10^{-19} \) C and mass \( m_e = 9.1\times10^{-31} \) kg.
\( \displaystyle q = ne = 10^{10} \times 1.6\times10^{-19}\,\mathrm{C} = 1.6\times10^{-9}\,\mathrm{C} \)
\( \displaystyle \Delta m = n\,m_e = 10^{10} \times 9.1\times10^{-31}\,\mathrm{kg} = 9.1\times10^{-21}\,\mathrm{kg} \)
The mass increase is far too small to measure, but it is not zero, so both the mass and the magnitude of the charge increase.
Why the other options are incorrect
Option (1) Only the magnitude of the charge on the oil drop will be increased, while the mass of the drop will remain unchanged. — Incorrect. The added electrons do have mass, however small.
Option (2) Only the mass of the drop will increase while the charge will remain unaffected. — Incorrect. Electrons carry charge, so the charge must change.
Option (4) No effect on the mass and magnitude of the charge of the oil drop. — Incorrect. Both the charge and the mass change.