What Is Capacitance?

Capacitance is an electrical parameter which physically is most easy to define, electrically often misconstrued.

Physically, a capacitor is two electrical conductors separated by a non-conducting (or very high resistance) medium between the conductors. Consider the two plates of area (A) in Figure 1. The plates are metallic and they are separated by a distance (D).

Capacitance Fig. 1

If we fill the space between the plates with a conductor (water, acid, etc.) and close the switch (S), we get a current dictated by ohms law, I=v/r (where V is voltage, and R is the resistance between the plates). If the space is a non-conductor, no current will flow but the voltage will exist across the plates. The plus side and the negative side attract, and electrical charges will exist on the plates; thus an electrical field will exist in the space between. Clearly, the larger the plates, the more charges will exist and the closer the plates (D), the stronger the electrical attraction between the plates will be.

If we now reverse the polarity of the battery, the plus plate is now negative and the negative plate plus. For the electrical charge to reverse, ‘electrons’ must have  lowed, reversing their position. Since ¢electron flow¢ is current, we have current flow when we reverse the polarity.

If we now substitute the battery with an alternating current source, the polarity will reverse every 1/2 cycle and we will, hence, get continuous current flow. Therefore, while a capacitor will not allow DC current to flow through, AC current can pass. The amount of current will depend on the supply voltage, the capacity of the plates to hold charges and the distance (D), which determines the leakage (or electrical field) in the space and the material between the plates.

Capacitance Fig. 2

The formula for current flow through a capacitor with an alternating voltage applied is I=V2 FKA/D. K is defined as the dielectric of material or ability to store electrons.

Note: – the higher the area A, the larger the current
– the smaller the D, the larger the current
– K is determined by the material having a high value (eg. water = 80) and non-conducting materials having a lower value (eg. air = 1). Current is increased with
the supply frequency (f), as you are reversing the charges more frequently and current increases with supply voltage.

The plates, needless to say, do not have to be rectangular and can be any shape or size. One plate can be grounded and mostly is, in industrial circuits. While the properties of a capacitor have many useful purposes, capacitance can be an annoyance. Capacitance exists between power transmission lines and ground resulting in additional current, and hence, power loss in the lines. Great effort is invested by utilities to reduce this effect. In capacitance instruments, capacitance is put to a useful purpose. If we keep V and F constant from a regulated circuit, then we have a situation dependent on KA/D. Keeping any two constant, we can measure the third. Here
capacitance instruments can be used to measure area, or distance, or the dielectric constant.

In most applications, A and D are constant and we detect a change of K. In most circumstances, the probe is in a steel tank such as Figure 3. A is the surface area of the probe and D is the distance between the probe and the tank wall or other ground.

Capacitance Fig. 3

If the tank is filled with water (K = 80), it displaces air (K = 1), and we have a dramatic change in capacitance which is utilized in the electronics to activate a relay or transmit a signal proportional to the amount of water in the tank. The electronics allow for tuning of the system so that the operator can activate a relay at any point or measure between a wide choice of spans.

Modern day instruments detect small changes and many insulating products with K factors close to 1 (eg. many petroleum products have a K of approximately 1.5) can be measured.

Most circuits use the electrical ground as one plate, or side of the capacitor, and hence the ‘line’ side (the probe) is ‘looking’ across to ground. Any change in the medium which would change net K factor will disturb the instrument (eg. a person walking by). Therefore, if the probe is not looking to a fixed ground (eg. steel vessel) but rather, in a fiberglass ungrounded tank, then a suitable ground rod must be installed. Another typical application is in dry pump protection, where a ring probe designed to mate between two flanges at the inlet of a pump detects presence or absence of liquid. The probe and ground are inside the ring probe.

The total applications for capacitance is growing continuously and is limited only by the imagination of the industry. Many proximity (variable D) applications are in use, as well as interface applications of dissimilar liquids in all shapes and sizes of vessels and pipelines. Capacitance also has the ability to measure solids (eg. wheat, flour, coal) which widely extends its application.

Capacitance is used for safety such as warning people from entering a dangerous area (the person changes the K). The probe may be railing or machine body, or many other configurations.

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Original Article written by Greg Reeves. Original “What Is Capacitance” Article.