CAIE A-Level Physics 18.5.4 Electric Potential Energy

Practise calculating electric potential energy of two point charges, interpreting its sign and relating changes to work and energy transfer.

Exam points

explain how electric potential leads to potential energy for two point charges
calculate electric potential energy using EP = Qq/(4πε₀r) and interpret its sign
relate potential-energy changes to work and energy transfer in a point-charge field

[Maximum number: 4]

In an $\alpha$ -particle scattering experiment, $\alpha$ -particles are directed towards a thin film of gold, as illustrated in Fig. 9.1.

Fig. 9.1

gold film

The apparatus is in a vacuum.
The gold-197 ( ${ }_{79}^{197} \mathrm{Au}$ ) nuclei in the film may be considered to be fixed point charges.
The $\alpha$ -particles emitted from the source each have an energy of 4.8 MeV .
Calculate:

the distance d of closest approach of an $\alpha$ -particle to a gold nucleus.

\begin{aligned} & d= \\ & \text { m [4] } \end{aligned}

$E_{\mathrm{P}}=\mathrm{Qq} / 4 \pi \varepsilon_{0} d$

C1

Q=79 e and q=2 e

C1

$7.68 \times 10^{-13}=\left(79 \times 2 \times\left\{1.60 \times 10^{-19}\right\}^{2} /\left(4 \pi \times 8.85 \times 10^{-12} \times d\right)\right.$

C1

$d=4.7 \times 10^{-14} \mathrm{~m}$

A1

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18.5.4—The concept of electric potential leads to the electric potential energy of question 1