# Relative permittivity/Delectric constant

What Is the physical meaning of the dielectric constant ?

(I don't want the mathematical expression)

The dielectric susceptibility (multiplied by $$\epsilon_0$$ to put everything in SI units) is the induced electric dipole moment per unit volume per unit electric field applied.

In general this is a tensor and may have real and complex, frequency-dependent components. But for simple linear, isotropic materials and static fields it is a number.

By convention, the dielectric cconstant is then the susceptibility $$+1$$.

• Then the dielectric constant will never be smaller than 1, except for the vacuum ?? Commented Jun 4, 2020 at 11:58
• @LEARNER, not quite. Depends what you mean by it. In all materials dielectric suscaptibility is dispersive (depends on frequency, and sometimes on wavevector), so we often talk about frequency dependent dielectic contant. Now that can be positive, negative, zero and even complex.
– Cryo
Commented Jun 4, 2020 at 12:04
• @Cryo And why there's this wide range of different relative permittivity's value, I mean why plastic has a relative permittivity equal to 3 while the air has only 1 ? Commented Jun 4, 2020 at 12:14
• @LEARNER Because the dipole moment per unit volume induced by a given electric field is much, much less in air than in plastic. See definition above. Commented Jun 4, 2020 at 12:20
• @RobJeffries I get it now! I have one more question if you may, Is that value (I mean the value of relative permittivity of plastic which is equal to 3) good ? Is it a good value or not ? Commented Jun 4, 2020 at 12:59

In most simple words it is the ratio of permitivity of any medium to permitivity of free space.