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Charged drop held still by a vertical electric field: charge, number of electrons or plate potentials

This type of question, asked 3 times: 2021 Paper 9, 14 Jun 2022, 2 Apr 2026 S2. All 3 questions below, oldest first.

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  1. Question 1

    2021 Paper 9

    1
    An oil drop of radius 2 mm2\text{ }mm with a density 3 g3\text{ }g cm−3cm^{- 3} is held stationary under a constant electric field 3.55×105 V m−13.55 \times10^{5}\text{ }V{\text{ }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/s2g = 9.81\text{ }m/s^{2} )

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  2. Question 2

    14 Jun 2022

    8
    A vertical electric field of magnitude 4.9×105 N/C4.9 \times10^{5}\text{ }N/C just prevents a water droplet of a mass 0.1 g0.1\text{ }g from falling. The value of charge on the droplet will be: (Given g=9.8 m/s2g = 9.8\text{ }m/s^{2} )

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  3. Question 3

    2 Apr 2026 S2

    14
    Two metal plates ( A,BA,B ) are kept horizontally with separation of (12π)cm\left( \frac{12}{\pi} \right)cm, with plate A on the top. An atomizer jet sprays oil (density 1.5 g/cm31.5\text{ }g/cm^{3} ) droplets of radius 1 mm horizontally. All oil droplets carry a charge 5 nC . The potentials VAV_{A} and VBV_{B} are required on plates A and B respectively in order to ensure the droplets do not descend. The values of VAV_{A} and VBV_{B} are ____\_\_\_\_ . (Neglect the air resistance to the droplets and take g=10 m/s2g = 10\text{ }m/s^{2} )

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