CAPACITORS
Figure 1.1 illustrates a capacitor. It consists of two insulated conductors a and b. They carry equal and opposite charges q and –q, respectively. All lines of force that originate on a terminate on b. The capacitor is characterized by the following parameters:
● q, the magnitude of the charge on each conductor
● V, the potential difference between the conductors
The parameters q and V are proportional to each other in a capacitor, or q = CV, where C is the constant of proportionality. It is called the capacitance of the capacitor. The capac- itance depends on the following parameters:
● Shape of the conductors
● Relative position of the conductors
● Medium that separates the conductors
The unit of capacitance is the coulomb/volt (C/V) or farad (F). Thus
● To reduce voltage fluctuations in electronic power supplies
● To transmit pulsed signals
● To generate or detect electromagnetic oscillations at radio frequencies
● To provide electronic time delays
Figure 1.2 illustrates a parallel-plate capacitor in which the conductors are two parallel plates of area A separated by a distance d. If each plate is connected momentarily to the ter- minals of the battery, a charge q will appear on one plate and a charge –q on the other. If d is relatively small, the electric field E between the plates will be uniform.
The capacitance of a capacitor increases when a dielectric (insulation) is placed between the plates. The dielectric constant K of a material is the ratio of the capacitance with
dielectric to that without dielectric. Table 1.1 illustrates the dielectric constant and dielectric strength of various materials.
The high dielectric strength of vacuum (¥, infinity) should be noted. It indicates that if
two plates are separated by vacuum, the voltage difference between them can reach infinity without having flashover (arcing) between the plates. This important characteristic of vacuum has led to the development of vacuum circuit breakers, which have proved to have excellent performance in modern industry.