Physics 9702 · AS & A Level · Electric force between point charges
Electric force between point charges — practice question
An $\alpha$-particle is moving through a vacuum towards the middle of a gold nucleus, as shown in Fig. 5.1. The gold nucleus carries charge $79e$. Both the gold nucleus and the $\alpha$-particle may be treated as point charges. When the $\alpha$-particle is a large distance from the gold nucleus, its energy is $7.7 \times 10^{-13}\,\text{J}$.
(a)[3]
The $\alpha$-particle does not strike the gold nucleus. Show that the radius of the gold nucleus must be less than $4.7 \times 10^{-14}\,\text{m}$.
(b)[3]
Determine the acceleration of the $\alpha$-particle when the centre-to-centre separation between the gold nucleus and the $\alpha$-particle is $4.7 \times 10^{-14}\,\text{m}$.
(c)[1]
In an $\alpha$-particle scattering experiment, a beam of $\alpha$-particles is directed at a very thin gold foil. Suggest why the gold foil must be very thin.
Worked solution & mark scheme
This 7-mark question has a full step-by-step worked solution and mark scheme. One marking point: “reduction in KE equals electrical potential energy $E = \frac{Qq}{4\pi\varepsilon_0 r}$” …