An oil drop is spherical, with radius $1.2 \times 10^{-6}\ \text{m}$, and its density is $940\ \text{kg m}^{-3}$. Show that the mass of the oil drop is $6.8 \times 10^{-15}\ \text{kg}$.
The oil drop is charged. Explain why a charge magnitude of $8.0 \times 10^{-20}\ \text{C}$ is impossible.
A charged oil drop moves in a vacuum between two horizontal metal plates that are $8.0\ \text{mm}$ apart. The electric field between the plates is uniform and has a field strength of $2.1 \times 10^{5}\ \text{V m}^{-1}$. The oil drop travels vertically downwards at constant speed. Calculate the potential difference $V$ between the plates.
Explain how the motion of the oil drop indicates that it is in equilibrium.
Determine the charge on the oil drop. State both the magnitude of the charge and its sign.
The magnitude of the potential difference between the plates in part (b) is reduced. Explain why the oil drop accelerates downwards.
Describe how the pattern of the field lines (lines of force) that represent the uniform electric field changes as the potential difference decreases.
Two forces, X and Y, may act on an oil drop in air, but they do not act on an oil drop in a vacuum. Force X can act whether the oil drop is stationary or moving. State the name of force X.
Two forces, X and Y, may act on an oil drop in air, but they do not act on an oil drop in a vacuum. Force Y can act only when the oil drop is moving. State the name of force Y.