If a plastic sheet of relative permittivity 2.5 is inserted between two point charges placed in vacuum, then the electrostatic force between them
Correct answer: B. Decreases by a factor of 2.5
- A. Increases by a factor of 2.5
- B. Decreases by a factor of 2.5
- C. Increases by a factor of 5
- D. Decreases by a factor of 5
Explanation
Relative permittivity appears in the denominator of Coulomb's law, so inserting a medium of relative permittivity 2.5 reduces the force to 1 over 2.5 of its vacuum value. Water, with a relative permittivity of about 80, weakens ionic attraction eightyfold, which is why ionic compounds dissociate so readily in it. Air is close enough to 1 that the vacuum formula is 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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