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A dielectric is the insulator between a capacitor's plates. Its main purpose is to raise capacitance: the field polarizes it, which weakens the field between the plates and lets them hold more charge at the same voltage. Capacitance grows by a factor κ (the dielectric constant), so C = κε0A/d. It also boosts the breakdown voltage and keeps the plates apart.
Dielectrics are basically insulators, materials that are poor conductors of electric current. Unlike the free electrons in a conductor, its electrons are tethered to its atoms. Consequently, no current can flow through it.
Such a material has no place in conductive devices, unless it is used to insulate itself, of course. However, if you think that dielectrics are despised by engineers, you are terribly mistaken. In fact, dielectrics are as ubiquitous as transistors. Between every capacitor is sandwiched a dielectric, the same capacitors without which your touchscreen would merely be a sheet of glass. But how does an insulator enhance the efficacy of a capacitor?
First we need to understand how a capacitor works.
The Capacitor
A capacitor is a device that consists of two parallel metallic plates placed extremely close to one another. The primary objective of a capacitor is to store charge. The charge can later be released to drive other circuits. This property renders it very useful in devices such as inverters. However, before releasing charge, it must first acquire it.
A capacitor is fed charge by connecting its plates to the terminals of a battery. Now, because metals are a sea of free electrons, when the electrons emanating from the negative terminal reach the metal, they violently repel the electrons on its surface. The repulsive force neutralizes the force exerted on the electrons by the battery, deterring them from accumulating on the plate.
The extra electrons could be accommodated if they were somehow attracted by a force greater than the force of repulsion. This is achieved by placing another metal plate parallel to it. The parallel plate is connected to the positive end of the battery. At that point, the positive terminal attracts the electrons from the plate to which it is connected, rendering it positively charged.
This positively charged plate will now provide the force of attraction we desired, meaning that it will attract the extra electrons transmitted by the negative terminal. In this way, the capacitor will store charge. This charge on the plate can be used to drive another circuit in the absence of a battery by simply connecting the wires to the negative and positive plates as they typically are to the two terminals of a battery.
How much charge a capacitor can hold for a given push from the battery is its capacitance, written as C = Q/V, where Q is the charge stored and V is the voltage across the plates. The higher the capacitance, the more charge the device banks at the same voltage. So anything that lets the plates pile up more electrons without raising the voltage is a win, and that is exactly where the dielectric comes in.
Why Dielectrics Enhance Capacitance
Even though atoms in a dielectric cannot be ionized to generate a current, they can certainly be polarized. Slip the dielectric between the charged plates and the field reaching across the gap tugs on every atom in it. The electrons are nudged toward the positive plate while the nuclei lean toward the negative one, so each atom stretches into a tiny dipole. This piles up a net negative charge on the face of the dielectric next to the positive plate, and a net positive charge on the face next to the negative plate.
Those induced charges set up their own electric field, one that points against the field of the plates. The two partly cancel, so the net field between the plates (and with it the voltage) drops. Now, because an army of positive charges faces the negative plate, the electrons on that plate are bound even more tightly, and the battery can push still more of them on before the voltage climbs back up. Since capacitance is C = Q/V, squeezing more charge Q in while holding V down means the overall capacitance shoots up!
Capacitance is given by the ratio of the plates’ cross-sectional area and the distance between them. Capacitance will increase if we increase the cross-section of the plate for the obvious reason that a larger plate can accommodate more charges. Capacitance decreases with an increase in the distance between the plates for the simple reason that an increased distance weakens the attractive forces that lure and bind the electrons to the second plate.
However, we have just found that capacitance also depends on the medium between the plates, specifically on how strongly that medium polarizes. That property is captured by the permittivity ε of the material. For an empty (vacuum) gap the permittivity is ε0 = 8.85 × 10−12 F/m, and the capacitance is:
C0 = ε0A/d
Fill that gap with a dielectric and the capacitance grows by a clean factor called the dielectric constant (or relative permittivity), written κ:
C = κε0A/d = κC0
A vacuum has κ = 1 by definition and air is barely different at κ ≈ 1.0006, which is why the two behave almost identically. Solid insulators do far better: paper sits around κ ≈ 3.5, while water is a striking κ ≈ 80. So a parallel-plate capacitor that stored, say, 10 pF with an air gap would jump to about 35 pF (10 pF × 3.5) if you slid a sheet of paper of the same thickness between the plates, without changing their size or spacing at all.
So why bother with a dielectric at all? It earns its keep in three ways. First, as we have seen, it multiplies the capacitance by κ, packing more charge into the same volume. Second, a good dielectric is not merely an insulator but a material that refuses to ionize at any cost, so it withstands a far stronger field than air before it breaks down and arcs over. That higher breakdown voltage lets engineers push the plates closer together (a smaller d means yet more capacitance) and run the capacitor at a higher working voltage. Third, the dielectric physically keeps the plates apart, averting the short that would ruin the device. An even better dielectric is also robust and capable of working at higher temperatures.
