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MDCAT Physics QUESTION #4393
Question 1
If a plastic sheet of relative permittivity $\varepsilon_r = 2.5$ is inserted between two point charges placed in vacuum, then the electrostatic force between them:
  • Increases by a factor of 2.5
  • Decreases by a factor of 2.5✔️
  • Increases by a factor of 5
  • Decreases by a factor of 5
Correct Answer Explanation
Coulomb's law in a medium: $F = \dfrac{kq_1q_2}{\varepsilon_r r^2}$. Inserting a dielectric with $\varepsilon_r = 2.5$ reduces the force by the factor $\varepsilon_r$. So the force decreases by a factor of $2.5$.