A spherical oil drop has a radius of 1.2 × 10⁻⁶ m. The density of the oil is 940 kg m⁻³.

1. (a)(i)Show that the mass of the oil drop is 6.8 × 10⁻¹⁵ kg.[2]
2. (a)(ii)The oil drop is charged. Explain why it is impossible for the magnitude of the charge to be 8.0 × 10⁻²⁰ C.[1]
3. (b)The charged oil drop in (a) is in a vacuum between two horizontal metal plates, as illustrated in Fig. 4.1. [Figure 4.1] The plates are separated by a distance of 8.0 mm. The electric field between the plates is uniform and has a field strength of 2.1 × 10⁵ V m⁻¹. The oil drop moves vertically downwards with a constant speed.
4. (b)(i)Calculate the potential difference V between the plates.[2]
5. (b)(ii)Explain how the motion of the oil drop shows that it is in equilibrium.[1]
6. (b)(iii)Determine the charge on the oil drop. sign of charge[3]
7. (c)The magnitude of the potential difference between the plates in (b) is decreased.
8. (c)(i)Explain why the oil drop accelerates downwards.[2]
9. (c)(ii)Describe the change to the pattern of the field lines (lines of force) representing the uniform electric field as the potential difference decreases.[1]
10. (d)Two types of force, X and Y, can act on an oil drop when it is in air, but cannot act on an oil drop when it is in a vacuum. Force X can act on an oil drop when it is stationary or when it is moving. Force Y can only act on an oil drop when it is moving. State the name of:
11. (d)(i)force X[1]
12. (d)(ii)force Y.[1]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.