February/March 2026 Paper 42

Define electric field at a point.

An oil droplet has charge of magnitude $Q$. The droplet is held in equilibrium by the electric field between two horizontal parallel plates, as shown in Figure 2.1. The electric field is created by a potential difference (p.d.) $V$ between the plates, with the lower plate at the higher electric potential. The separation of the plates is $x$. On Figure 2.1, draw four field lines to represent the electric field between the plates.

Explain, with reference to forces, whether the sign of $Q$ is positive or negative.

For the oil droplet in 2(b), state an expression, in terms of some or all of $Q$, $V$ and $x$, for the electric field strength $E$.

State an expression, in terms of some or all of $Q$, $V$ and $x$, for the weight $W$ of the droplet.

The plates in 2(b) have a p.d. across them of $4800\text{ V}$ and a separation of $0.024\text{ m}$. The magnitude of the charge on the oil droplet is $3e$, where $e$ is the elementary charge. Calculate the mass of the oil droplet.

The polarity of the p.d. across the plates in 2(b) is reversed. Describe and explain the subsequent motion of the oil droplet.