0
$\begingroup$

Okey first of I know dielectric materials used in capacitors to insulate and polarize due to electrical field created by two metal plates and then when you connect this stored energy to a system you will get a high voltage high current for a short period of time with the flow of charged particules in plates but...

Why does this dielectric material is neccessary why dont we just put these metal plates as close as possible without touching each other to reduce to path between them to get the highest storage amount and just left between plates empty doesn't this create an electrical field that we need? after finishing the electrical field between two plates my knowledge from there tells me I dont need a material to create a electrical field so whats the deal with this material? and what would happen if we don't use any material? what would be the difference?

$\endgroup$
  • 2
    $\begingroup$ 'put these metal plates as close as possible without touching' - so, with air (a dielectric) between them? ... By the way, they already exist, they're called 'air variable capacitors'. $\endgroup$ – lemon Feb 28 at 10:14
  • 1
    $\begingroup$ Air is a "material." It has a dielectric constant (admittedly, it's very close to 1, but it's not exactly 1). There is no reason why you can't make a capacitor with no material between the plates - just put the plates in a vacuum chamber. It's not a very practical thing to do, though. $\endgroup$ – alephzero Feb 28 at 10:25
5
$\begingroup$

. . . . why don't we just put these metal plates as close as possible without touching each other . . . .

How is this going to be done?
One of the functions of a solid dielectric is to keep the plates separated.

Air as with other dielectric is an insulator but if the electric field is to large it "breaks down" and becomes a conductor.
So for a given separation of the plates there are dielectrics which have a larger breakdown field strength and so can have, for a given separation of the plates, a larger potential difference across the plates before breaking down than if air were used.

As the aim in the manufacture of a capacitor is to maximise the capacitance in as small a volume as possible and to have a high maximum working voltage as dielectric is often placed between the plates of a capacitor.

Capacitors with air as the dielectric do exist, for example as variable capacitors and as standard capacitors.

$\endgroup$
3
$\begingroup$

A dielectric material is an insulator, which means that it won't conduct a current. It will however, as you say, become polarized and thus create a polarization field. This field will accumulate carriers at the interface between the metal connectors and the dielectrics, which is how the capacitor stores a charge.

Dielectric materials tend to be more insulating than air, and thus by using such a material the plates (in a parallel plate capacitor) can be placed closer together which would yield higher capacitance.

Capacitors with air in between them do exist.

$\endgroup$
2
$\begingroup$

Because of permittivity $\varepsilon$. Capacitance is calculated as:

$$C=\varepsilon_0\varepsilon \frac Ad$$

$\varepsilon_0$ is a natural constant (vacuum permittivity). $A$ is plate area and $d$ is separation.

$A$ and $d$ depend on the conductive materials. But $\varepsilon$ depends on the in-between material - the so-called dielectric.

The dielectric may contain dipoles, asymmetrically charged molecules, that are able to twist and rotate when an electric field is established between the capacitor plates. When all such dipoles are biased and turned the same way, then all their "negative ends" point towards the positive plate, and all their "positive ends" towards the negative plate. Each of them now attracts the charges on the plates. Their combined attraction "strength" may have a significant influence in pulling a few more charges to theplates.

The dipoles relate directly to $\varepsilon$. Water is a good example with a high value around $\varepsilon\approx 80$, because water is an oddly asymmetric molecule.

When a battery or other voltage source is applied to a capacitor, it will push charges to the plates until the accumulated repulsion force of those charges at the plates balances out the battery push. If there is a strong dielectric between the plates, then its dipoles will "help out" the battery by "pulling a few extra charges" to the plate. In this sense, the dielectric material increases the amount of charge that can be stored on those plates - the dielectric increases the capacitance.

Dielectric breakdown

Furthermore, a dielectric helps against so-called dielectric breakdown. Breakdown happens for instance during lightning storms. The voltage (potential difference between cloud and ground) is so large that charges (current) want to move across the separation so badly, that the usually insulating air can't resist them from moving. When charges in this way flow through an insulator, this is dielectric breakdown. The critical limit is called the breakdown voltage.

If you want to store some amount of charge in your capacitor, but your capacitor is not that large (has a low $C$), you will have to apply larger voltage $V$. The relationship is this:

$$Q=CV$$

$Q$ is the amount of charge that you want to store. For a smaller capacitance $C$, you must increase the voltage. Maybe this will make you reach the breakdown voltage, so it is impossible. But with a good dielectric, you can increase $C$ - in other words, you can reach the intended amount of stored charge with a lower voltage.

$\endgroup$
0
$\begingroup$

There are several reasons to use a dielectric material rather than depending on an air gap between capacitor plates:

1) One capacitor plate is positively charged and the other capacitor plate is negatively charged. Unlike charges attract, so a large charge on capacitor plates with a small air gap would tend to close that air gap due to electrostatic attraction. A dielectric material would resist this tendency much more than an air gap.

2) The dielectric material is an insulator, so it's surface molecules become polarized by the electric field between the capacitor plates. This polarization acts to reduce the strength of the electric field between the plates for a given capacitor charge, which allows somewhat more charge to be deposited into the capacitor for a given voltage across the capacitor terminals.

3) Any insulator, including air, will "break down" if the electric field across that insulator becomes too large. When this happens, the insulator becomes ionized at some small point, and electricity flows across the insulator as a result. Capacitor dielectrics are selected such that they have a somewhat higher breakdown point than air (i.e., a higher dielectric strength), which means that the capacitor can accept a higher voltage across its terminals, and a higher charge as a result.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.