[Physics] The advantages of a dielectric in a capacitor

capacitancedielectricelectricityelectrostatics

In a capacitor a dielectric can be placed in between the two plates. I have trouble understanding the points / advantages of a dielectric from what I have read in a text book. The points written there are:

  1. The mechanical advantage of seperating the plates in practice.
  2. Any insulator will experience dielectric breakdown at some point, so picking a dielectric other than air can make it tolerate higher voltage.
  3. The capacitance is greater with a dielectric than with vacuum in between.

Point 1) is obvious. But I have questions to the next two advantages.

Question to advantage 2)

I understand that an insulator like air will become ionized at some high voltage and become conducting and ruin the capacitor. It therefor makes sense to use some other dielectric than air, which is easily ionized in comparison.

But can vacuum become ionized? Its a weird question, I know, but compared to vacuum, is this point 2) misleading? Isn't it right that a dielectric will not have that advantage over vacuum?

Question to advantage 3)

The capacitance is greater with a dielectric present, since the dielectric (with dielectric constant $K>0$) reduces the electric field (because of polarization) to $E=E_{\text{no dielectric}}/K$, which means a reduced potential difference $V=Ed$ between the plates (where $d$ i seperation). But why is this interesting?

Reducing the voltage does not seem like anything we would prefer. And since it is prefered, why not just reduce the area $A$ of the plates, which will reduce the capacitance by definition: $C=\epsilon A/d$, so the voltage drops accordingly, $C=Q/V$?

Thank you very much for your help with these confusing points.

Best Answer

On point 2: While vacuum itself, being composed of nothing at all, is not expensive, a capacitor structure able to maintain a vacuum when surrounded by air is impractically expensive.

On point 3: Don't think of higher-value capacitors as requiring less voltage. Rather, a higher-value capacitor allows us to "store" more charge at the same voltage. In a power-supply bypass application, for example, that means it's able to supply more transient current to a load without exceeding the allowed voltage ripple.

And, while it's possible to "simply" increase the plate area to increase the capacitance, when you're trying to cram several hundred or thousand capacitors into a miniature device like a cell phone, that strategy doesn't meet your overall needs.

Related Question