An oil drop of radius 2 mm with a density 3g cm ⁻³ is held stationary… | JEE Main 2021 (18 Mar, Shift 1)

Q27JEE Main 2021 · 18 MAR · SHIFT 1Medium

Motion of Charges in an Electric Field

An oil drop of radius 2 mm with a density 3g cm ${}^{-3}$ is held stationary under a constant electric field 3.55 $\times$ 105 V m ${}^{-1}$ in the Millikan's oil drop experiment. What is the number of excess electrons that the oil drop will possess? (consider g = 9.81 m/s2)

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Answer: (b)

Mass: $m = \rho\frac{4}{3}\pi r^3 = 3000 \times \frac{4}{3}\pi(2 \times 10^{-3})^3 \approx 1.005 \times 10^{-4}\text{ kg}$
Balance: $neE = mg \Rightarrow n = \frac{mg}{eE}$
$n = \frac{1.005 \times 10^{-4} \times 9.81}{1.6 \times 10^{-19} \times 3.55 \times 10^{5}} \approx 1.73 \times 10^{10}$
So the answer is (b) $1.73 \times 10^{10}$ .

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Motion of Charges in an Electric Field question from JEE Main 2021 (18 Mar, Shift 1)

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