Question

In a Millikan’s set-up, a charged oil drop falls under gravity with a certain terminal speed. The drop is held stationary applying suitable electric field and is found to carry 2 excess electrons. Suddenly the drop is found to move upwards with the same terminal velocity. In this observation which of the following possibilities appear to be most fitting ?

• A. The electric field stops acting.
• B. The drop looses the excess electrons.
• C. The drop picks up some additional electrons.
• D. Data insufficient.

Answer 1

Answer: (C)

The drop picks up some additional electrons.

Charged drop with 2 excess electrons stopped falling downwards by applying electric field suggest electrostatic force is upwards.
Now the drop moving upwards means increase in electrostatic force meaning increase in negative charge.
Hence the drop picks up some additional electrons