Two identical point charges kept apart experience an electrostatic force of in vacuum…

PHYSICS · Multi-part · A Level · Physics · Fields and their consequences · Fields and their consequences

Problem

Two identical point charges kept

0.30\,\text{m}

apart experience an electrostatic force of

1.0\,\text{N}

in vacuum and

0.25\,\text{N}

when the region between them is completely filled with an insulating oil. (i) Derive an expression that relates the relative permittivity

\varepsilon_r

of the oil to the magnitudes of the forces measured in vacuum and in the oil. (ii) Using the given data, calculate the numerical value of

\varepsilon_r

for the oil. (iii) Keeping the charges the same, the separation in oil is now increased to

0.60\,\text{m}

. Predict the new magnitude of the force between the charges.