Definition of capacitance
A small glass can contain a little water, while a large glass can contain more water. The larger the volume of glass, the more water that can be contained. So each glass has the capacity or size of the ability to contain water. Like glass, capacitors also have the ability to store the electrical charges and the electrical potential energy. Capacitor capacity to store the electrical charge and the electric potential energy is called capacitance.
Factors affect capacitance
The size of the glass’s ability to contain water is determined by the volume of the glass. What about capacitors, what determines the size of the capacitor’s ability to store the electric charge?
The figure on the side shows a simple capacitor consisting of two-conductor plates separated by a certain distance. Before connecting to a voltage source, such as a battery, the two plates are not electrically charged. Then one of the plates is connected to the positive pole of the battery and the other plate is connected to the negative pole of the battery using a cable.
After connected to the battery’s positive pole, the positive charge on the battery draws negatively charged electrons on the plate so that the electron moves to the positive pole of the battery. This causes the plates to lack electrons (negative charge) and excess protons (positive charge) so that the plate becomes positively charged.
After being connected to the battery’s negative pole, the positive charge on the plate attracts negatively charged electrons on the negative pole of the battery so that the electrons move to the plate. This causes the plate to overload the electron so that the plate becomes negatively charged. The process of moving electrons between plates and batteries stops after the potential difference between the two plates is equal to the potential difference between the two battery poles.
How do you increase the electrical charge on both conductor plates? In other words, what should be done to transfer electrons to and from the positive pole of the battery? Electron displacement occurs only when the electric potential difference between the two battery poles is greater than the electric potential difference between the two conductors. In order for the electron to move again so that the electrical charge in each conductor plate increases, the battery used is replaced by another voltage source that has a greater electrical potential difference. The displacement of electrons stops when the potential difference in the voltage source is equal to the potential difference of the capacitor therefore if the potential difference in the voltage source is greater then the capacitor potential difference is also greater.
Based on the above review it can be concluded that the greater the electrical charge stored in each conductor plate, the greater the electrical potential difference between the two conductor plates. So the electric charge (Q) is proportional to the electric potential difference (V). The relationship between an electric charge and an electric potential difference is stated in the following comparison:
Q α V
The above comparisons are converted into equations by adding the constant C:
Q = C V or C = Q / V
Q = electric charge (Coulomb), V = electric potential difference or voltage (Volt), C = constant which is called the capacitor capacitance.
The capacitance value does not depend on the electrical charge and electrical voltage but depends on the shape and size of the conductor plate. Mathematical proof that capacitance depends on the shape and size of the conductor plate is explained in the topic of the parallel plate capacitor. In the topic, it is assumed that between the two conductors there is a vacuum.
The capacitance of a capacitor also depends on the nature of the material that is between the two conductor plates. The material that is between the two conductor plates is called a dielectric. The capacitance of capacitors that have dielectrics is discussed in depth in the topic of dielectric constants.
Unit of capacitance
The unit of electric charge is Coulomb and the unit of the electric potential difference is Volt so that based on the capacitance equation above, the capacitance unit is Coulomb per Volt (C/V), also called Farad (F) which comes from the name of the British scientist Michael Faraday (1791-1867). So 1 Farad = 1 Coulomb/Volt.
Suppose that a capacitor having a value of 2 Farad means that the capacitor stores an electrical charge of +2 Coulomb on one of the conductor plates and -2 Coulomb on the other conductor plate, where the two conductor plates have a potential difference of 1 Volt. If the 12 Volt battery is connected to the capacitor, one of the electrically charged conductor plates is Q = C V = (2) (12 Volts) = +24 Coulomb while the other conductor plate is -24 Coulomb.
Please note that Farad is a very large capacitance unit so that it is usually used as a smaller unit namely microFarad abbreviated μF (10-6 Farad) or picofarad abbreviated as pF (10-12 Farad). Mathematical calculations to show that Farad is a very large unit discussed in the topic of the parallel plate capacitor.