CAIE A-Level Physics 18.3.2 Coulomb's Law

Practise stating and applying Coulomb’s law to calculate the force between point charges in free space, including direction and sign.

Exam points

calculate electric force between point charges using Coulomb’s law
rearrange the law to determine charge, separation or permittivity and use consistent units
interpret the direction, sign and inverse-square dependence of the force between charges

[Maximum number: 6]

(a)

State Coulomb's law.

[ 2 ]

(b)

Positronium is a system in which an electron and a positron orbit, with the same period, around their common centre of mass, as shown in Fig. 2.1.

Fig. 2.1 (not to scale)

The radius r of the orbit of both particles is $1.59 \times 10^{-10} \mathrm{~m}$ .

[ 4 ]

(i)

Explain how the electric force between the electron and the positron causes the path of the moving particles to be circular.

[ 2 ]

(ii)

Show that the magnitude of the electric force between the electron and the positron is $2.28 \times 10^{-9} \mathrm{~N}$ .

[ 2 ]

$F=e^{2} / 4 \pi \varepsilon_{0} X^{2}$

C1

$\begin{aligned} =\left(1.60 \times 10^{-19}\right)^{2} /\left[4 \pi \times 8.85 \times 10^{-12} \times\left(2 \times 1.59 \times 10^{-10}\right)^{2}\right] =2.28 \times 10^{-9} \mathrm{~N} \end{aligned}$

A1

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18.3.2—Coulomb’s law F = Q1Q2 / (4πε0 r 2) for the force between two point charges question 1

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Question (b)

Question (i)

Question (ii)