Physics (AS & A Level) · May / June 2020 · Paper 13 · Question 32

A negatively charged oil drop of mass m is between two horizontal parallel metal plates a distance d apart. [diagram: horizontal metal plates with + and - terminals, oil drop of mass m between plates separated by distance d] When the potential difference (p.d.) between the plates is V1 the oil drop rises at a constant speed. When the p.d. is decreased to a value V2 the oil drop falls at the same constant speed. Air resistance acts on the drop when it is moving. The upthrust on the drop is negligible. The acceleration of free fall is g. What is the charge on the oil drop?

1. A\frac{mdg}{V_1 - V_2}
2. B\frac{mdg}{V_1 + V_2}
3. C\frac{2mdg}{V_1 - V_2}
4. D\frac{2mdg}{V_1 + V_2}

As printed in the paper