Question

Identical point charges of 9.3 nC are 0.47 mm apart. Calculate the force between the two charges. 

Answer 1

Explanations & Calculations

• You need to use Coulomb's law to calculate the force on each other due to their charges.
• You also need to convert the given charges into Coulombs and millimeters into meters.

$\qquad\qquad \begin{aligned} \small 1\,nC&=\small 1\times10^{-9}\,C\\ \therefore\,\,\,\small 9.3\,nC&=\small 9.3\times10^{-9}\,C\\\\ \small 1mm&=\small 10^{-3}\,m\\ \therefore \,\,\,\small 0.47\,mm&=\small 0.47\times10^{-3}m \end{aligned}$

• Prediction — the medium the charges are in is a vacuum in which vacuum permittivity $\small \epsilon_0=8.854\times10^{-12}C^2/Nm^2\implies\frac{1}{4\pi\epsilon_0}\approx9.0\times10^9Nm^2C^{-2}$ .
• By using the law, you can get the answer.

$\qquad\qquad \begin{aligned} \small F&=\small \frac{1}{4\pi\epsilon_0}.\frac{Q_1Q_1}{r^2}\to\frac{1}{4\pi\epsilon_0}.\frac{Q_1^2}{r^2} \end{aligned}$

• You can now give it a try.