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Which of the following values cannot be the dielectric constant of an insulator?

1. 1.5
2. 3
3. 4.5
4. infinity

Correct answer: infinity

Solution

The dielectric constant (relative permittivity) of an insulator is a finite number greater than 1 (vacuum is 1). An infinite dielectric constant corresponds to a conductor (where the field inside vanishes), not an insulator. Hence infinity is impossible for an insulator.

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