A spherical oil drop, radius `10^(-4)` cm has on it at a certain a total charge of 40 electrons. Calculate the energy that would be required to place an additional electron on the drop.

Explanation

Intital charge on the oil drop `= 40 xx 1.6 xx 10^(-19)C`
Potential of the oil drop `= ( q)/( 4pi epsilon_(0)r)`
`= 9 xx 64 xx 10^(-4) ` volts
Now energy required = work done in bringing an electron from infinity to a point of potential `9 xx 64 xx 10^(-4)` volts.
As, Potential `= ("Work")/("Charge")`
`:.` Work = Potential ` xx` charge `= 9 xx 64 xx 10^(-4) xx 1.6 xx 10^(-19) = 921.6 xx 10^(-23) ` joules.