Edexcel A-Level Physics Coulomb’s Law

Practise Edexcel A-Level Physics Coulomb’s law by calculating force, charge or separation for point charges, nuclei and ions.

Exam points

calculate electrostatic force between point charges using Coulomb’s inverse-square law
rearrange Coulomb’s law to find unknown charge from force and separation data
compare force changes when charges or separation are scaled in multiple-choice contexts

[Maximum number: 2]

The surface of a balloon can become charged by rubbing it against clothing. The charge on the surface of the balloon can be assumed to act as if it was concentrated at the balloon's centre.

Two balloons are suspended by cotton threads so that they just touch each other as shown in diagram 1. The two balloons are then given an equal negative charge and repel as shown in diagram 2. The dots represent the centre of each balloon.

Diagram 1

Diagram 2

Show that the charge on each balloon is about 2 × 10^-7 C.

Use Coulomb's law, $F=\dfrac{1}{4\pi\varepsilon_0}\dfrac{Q^2}{r^2}=k\dfrac{Q^2}{r^2}$ . Thus $1.16\times10^{-3}=(8.99\times10^9)Q^2/(0.47)^2$ , giving $Q=1.69\times10^{-7}\,\mathrm{C}$ , about $2\times10^{-7}\,\mathrm{C}$ .

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4.4.94 - Coulomb’s law question 1