(a)[1]
Define what electric field strength is.
(b)[2]
A sphere S has radius $1.2 \times 10^{-6}\,\text{m}$ and density $930\,\text{kg m}^{-3}$. Show that the weight of S comes to $6.6 \times 10^{-14}\,\text{N}$.
(c(i))[2]
Two horizontal metal plates are separated by $14\,\text{mm}$ in a vacuum. A potential difference (p.d.) of $1.9\,\text{kV}$ is applied across the plates, as illustrated in Fig. 3.1. A uniform electric field exists between the plates. In (b), sphere S is charged and is kept still between the plates by the electric field. Calculate the electric field strength between the plates.
(c(ii))[2]
Calculate the size of the charge on S.
(c(iii))[2]
The magnitude of the p.d. applied across the plates is increased. Explain why S accelerates towards the top plate.