The force between two charges placed in a medium of relative permittivity 4, compared with the force in a vacuum, is
Correct answer: B. one quarter
- 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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About Coulomb's Law
Coulomb's law gives the electrostatic force between two point charges in terms of their magnitudes, separation and the medium between them. Questions involve attraction and repulsion, the inverse square relationship, superposition of forces and vector direction, which must not be confused with the electric field produced by a charge.
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