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An oil drop of radius 2 mm with a density 3 g cm⁻³ is held stationary under a constant electric field 3.55 × 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]

1. 48.8 × 10¹¹
2. 1.73 × 10¹⁰
3. 17.3 × 10¹⁰
4. 1.73 × 10¹²

Correct answer: 1.73 × 10¹⁰

Solution

m = (4/3)pi(2e-3)^3(3000) ~ 1.005e-4 kg; q = mg/E = (1.005e-4 x 9.81)/(3.55e5) ~ 2.78e-9 C; n = q/e ~ 2.78e-9/1.6e-19 ~ 1.73x10^10 electrons.

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