JEE ADVANCED PHYSICS QUESTION SEPTEMBER 2020
Answers
Two large circular discs separated by a distance of 0.01 m are connected to a battery via a switch as shown in the figure. Charged oil drops of density 900 kg/m³ are released through a tiny hole at the center of the top disc. Once some oil drops achieve terminal velocity, the switch is closed to apply a voltage of 200 V across the discs. As a result, an oil drop of radius 8 × 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 =10 ms−2
and charge on an electron is 1.6 × 10^-19 C.
solution : potential difference = electric field between plates × seperation
⇒200 Volts = electric field × 0.01
⇒electric field = 2 × 10⁴ N/C
now when electron achieves terminal velocity,
electrostatic force = weight of electron
⇒qE = mg = Vρg
⇒n × 1.6 × 10^-19 × 2 × 10⁴ = 4/3 π(8 × 10^-7)³ × 900 kg/m³ × g
where n is number of electrons.
after solving we get, n ≈ 6
Therefore there are six electrons present in this oil drop .