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## Physics - Capacitors

These plates that we have been talking about are called capacitors. Capacitors store energy. That is because between oppositely charged conductive plates there is an electrostatic potential which represents potential energy. Now, the amount of energy a capacitor can store depends on the magnitude of the charge on each plate and the electrostatic potential between the plates.

Rule number 31:
Energy stored by a capacitor equals magnitude of charge times the electrostatic potential between the plates divided by two or U = 1/2QV.

Capacitance is a capacitors ability to hold a relatively large charge on its plates without producing too much of an electrostatic potential between its plates. If a capacitor can hold a lot of charge and produce only a low electrostatic potential between its plates then it has a high capacitance. If a capacitor produces a high electrostatic potential between its plates even when it is holding only a small charge, then it has a low capacitance.

Rule number 32:
Capacitance equals charge divided by potential difference or C = Q/V.

Capacitance is measured in farads. There are two features that determine the capacitance of a capacitor. One, the geometry of the plates, increasing the size of the plates will increase capacitance. And, increasing the distance between the plates will decrease capacitance. Two, the material between the plates, any material placed between the plates of a capacitor must be an insulator. As an insulator this material is able to resist the permittivity of the electric field caused by the charged plates. Some materials are better insulators than others. This quality is called, the dielectric constant of a material and is denoted by a K. The better an insulator a material is, the higher the dielectric constant. Empty space has a dielectric constant of one and air has a dielectric constant very close to one.

Sometimes, capacitors are connected to one another and you may have to find their total capacitance. When separate capacitors are connected in series then one, the potential difference across the resulting system is the sum of all the potential differences across the individual capacitors. And two, the reciprocal of the total capacitance of the system is equal to the sum of the reciprocals of the capacitance of each individual capacitor. When separate capacitors are connected in parallel then one, the charge across the resulting system is the sum of all of the charges across the individual capacitors. And two, the total capacitance of the system is equal to the sum of the capacitances of each individual capacitor.

Rule number 33:
For capacitors connected in series, 1/Ctotal = 1/C1+1/C2+1/C3 and etc. For capacitor connected in parallel, Ctotal = C1+C2+C3 and etc. Capacitors have limits too. If you kept raising the electrostatic force between two plates eventually the field would become so strong that electrons would move from the negatively charged plate to the positively charged plate causing the plates to become neutral. When this happens, the capacitor is said to discharge.