Question 16.7: What's the dielectric constant κfor a vacuum? How about for ...
What’s the dielectric constant κfor a vacuum? How about for a perfect conductor? Explain, based on the response of each when it’s between the plates of a capacitor. Assume the conductor fills the space between the capacitor plates almost completely, with its faces adjacent to the capacitor plates but not quite touching.
(a) A vacuum is literally nothing (Figure 16.23a), so the electric field is unaffected and κ = 1 , exactly.
(b) Charges in the conductor are free to move, and, as you’ve seen, they move so as to cancel completely any electric field inside the conductor, as shown in Figure 16.23b. So the electric field between the plates becomes essentially zero. In practice there s still a very small region of nonzero field between each plate and the conductor, but that’s negligible if the gap is small. We can therefore conclude that the dielectric constant of a conductor is infinite!
REFLECT The vacuum and the perfect conductor represent the two extremes of dielectric constant.
Normal materials have dielectric constants in the range 1<\kappa<\infty.
