The force between two charges placed in a medium of relative permittivity 4, compared with the force in a vacuum, is
- A. four times greater
- B. one quarter
- C. unchanged
- D. twice as great
Explanation
The medium appears in the denominator of Coulomb's law, so a relative permittivity of 4 reduces the force to a quarter of its vacuum value. Water has a relative permittivity of about 80, which is why ionic compounds dissociate so readily in it: the attraction between the ions is weakened eightyfold. Air is close enough to 1 that the vacuum formula is normally used for it.
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