Circuits and YOU!

The capacitor is another fundamental component. The prime role for the capacitor is to hold charge and store energy for later use. This property makes a capacitor very useful for filtering and storing power. The capacitor works by electrostatic attraction, when you have a potential across any two points the opposite charge attracts each other. As long as there is a potential across the capacitor charge will keep building up until the potential across the capacitor equals the potential across the circuit. Once this happens current ceases to flow. You can think of a capacitor as a break in a circuit, once the electrostatic force between the two points is at an equilibrium with the source, there is no force to drive this attraction of charge. No flow of charge means no current; because of this a capacitor is known to block DC current. A capacitor in series once charged will stop current flow. This also give a capacitor a complex impedance, the more a capacitor charge the more the electrostatic force resists flow and increases resistance across the capacitor. A capacitor in series will allow AC to pass through the capacitor since the capacitor is constantly charging and discharging.

The capacitance is a unit of how much a capacitor can "hold" which is a product of permittivity and area by the distance of the plates. Thus as area increases capacitance increases and as distance between the plates increases, capacitance decreases. Capacitors are measured in farads with one farad being 1 coulomb of charge across a capacitor with a potential of 1 volt across it. A coulomb is a large amount of charge so capacitors are usually measured in microfarads

The permittivity is a product of the material between the plates, in simple cases it is air, other cases it can be a sophisticated dielectric.

Now capacitance is a product of any two points in a circuit with a potential across them, separated by any distance. Although when d is a large value, capacitance is low, this still has an effect on a circuit. It is an important consideration how capacitance will affect your circuit since everything is a capacitor. Besides causing some unwanted affects, a capacitor can be a useful component.

Conservation of charge:
A Capacitor holds charge, unlike potential which is the product of an electrostatic force, charge is a "physical" thing, you cannot lose charge. This is very important since charge is a principle part of a capacitor.

Capacitor in Series:

Capacitors in series the capacitance add as the inverse so:

One advantage to using capacitors in series is that you can pass higher voltages across lower voltage capacitors (I.e you can pass 200V through 2 100V capacitors in series)

Capacitors in Parallel:

Capacitors in parallel the capacitance adds across the capacitors.

Capacitance is determined by the area of the capacitor, naturally more capacitors in parallel will have more area, which is why capacitance adds.

Work and Energy:

Capacitors can hold a charge, which means they can store energy. The energy associated with a capacitor is defined as:

This is useful in finding how much energy is stored in a capacitor, Power is the rate consumption of energy, thus by finding the energy stored in a capacitor you can find how much power it has.

Timing:

The largest use of a capacitor is in timing. It takes time to charge the capacitor, which can be used to your advantage as a timing source. The most simple resonator is a RC circuit. Capacitors are used in oscillating circuits which are used to produce specific frequencies. Also capacitors can modulate frequency due to the fact that charging a capacitor is time dependent.

This is a basic intro to the capacitor, I hope to go over more in the future.

Formula Sheet:

Capacitance:

Capacitor in series:

Capacitor in parallel:

Potential energy of a capacitor: