Two large circular discs separated by a distance of 0.01 m are connected to a battery via a switch a shown in the figure. Charged oil drops of density `900 kg m^(-3)` are releasd through a tinly hole at the centre of the top disc . Once some oil drops achieve terminal velocity , the swith is closed to apply a voltage of 200 V across the disc. As a result an oil drop of radius `8 xx 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 (e) ` = 1.6 xx 10^(-9) (C)`

Answer

6

Explanation

`qE = mg `
` q = (mg)/(E) = 900 xx (4)/(3) pi (8 xx 10^(-7))^(3) xx 10`
` = 900xx (4)/(3) pi xx 512 xx 10^(-21) xx 10`
` 2 xx 10^(4)`
` N = 900 xx (4)/(3) pi xx 512 xx 10^(-21) xx 10`
` 2 xx 10^(4 xx 1.6 xx 10^(-19)`
N = 6