Two large circular discs separated by a distance of 0 . 01m are...

Two large circular discs separated by a distance of $0 . 01m$ are connected to a battery via a switch as shown in the figure. Charged oil drops of density $900kg{m}^{- 3}$ are released through a tiny hole at the centre of the top disc. Once some oil drops achieve terminal velocity, the switch is closed to apply a voltage of $200V$ across the discs. As a result, an oil drop of radius $8 \times {10}^{- 7}m$ stops moving vertically and floats between the discs. The number of electrons present in this oil drop is __________. (neglect the buoyancy force, take acceleration due to gravity $= 10m{s}^{- 2}$ and charge on an electron (e) $= 1 . 6 \times {10}^{- 19}C$ )

Two large circular discs separated by a distance of 0 . 01m are connected to a battery via a switch as shown in the

Correct answer

Step-by-step solution

$qE = mg$

$q = \dfrac{mg}{E} = \dfrac{900 \times \dfrac{4}{3}\pi{\left(8 \times {10}^{- 7}\right)}^{3} \times 10}{\left(\dfrac{200}{0 . 01}\right)}$

$= \dfrac{900 \times \dfrac{4}{3}\pi \times 512 \times {10}^{- 21} \times 10}{2 \times {10}^{4}}$

$N = \dfrac{900 \times \dfrac{4}{3}\pi \times 512 \times {10}^{- 21} \times 10}{2 \times {10}^{4} \times 1 . 6 \times {10}^{- 19}}$

$N \approx 6$

Practice Electrostatics on Quantrex Academy →

More from Electrostatics

Two charges Q_1 = q and Q_2 = mq are placed at the points P_1(a, b) and P_2(ma, mb)… JEE Advanced 2026 Consider an electric dipole comprising two charges +q and -q each with mass m , separated… JEE Advanced 2026 Two point charges q_1=3\,μ C and q_2=-4\,μ C are placed at points (2 i +3 j +3 k ) and (… JEE Main 2026 The electric potential as a function of x, y is given by V = 5(x^2 - y^2) V. The electric… JEE Main 2026 A thin half ring of radius 35 cm is uniformly charged with a total charge of Q coulomb.… JEE Main 2026 A three coulomb charge moves from the point (0, -2, -5) to the point (5, 1, 2) in an… JEE Main 2026 A particle of charge q and mass m is projected from origin with an initial velocity v = (… JEE Main 2026 A rigid dipole undergoes a simple harmonic motion about its centre in the presence of an… JEE Main 2026 Full Electrostatics list All JEE Advanced PYQs