Capacitors Flashcards
Why are my questions not a good idea for anyone else?
I’ve merely paraphrased sentences that caught my eye (without bothering to rank them by hierarchy) from the Navy book on Basic Electricity, a very old edition of Basic Electricity by Herman, etc. Plus, I am doing this quickly and there’s no check on accuracy.
What are the dangers of charging a capacitor to a high voltage and handing it to someone as a joke?
This is an extremely dangerous practice since capacitors have the ability to supply an almost infinite amount of current and under some circumstances can have enough energy to cause a person’s heart to go into fibrillation.
What are the three factors that determine capacitance?
- Surface area of the plates
- Distance between the plates
- Type of dielectric
With a DC battery source, how long do electrons move from the positive plate to the negative plate?
Until a voltage equal to the battery voltage is established across the plates, and the cap is now charged.
What is the device in electrical circuits used to store a charge?
The capacitor.
Bound electrons in the air or dielectric between capacitor plates move in what direction in their orbits?
They are attracted toward the positive surface and repelled by the negative. In other words, if there is a difference in charge across the plates, the orbits will be elongated in the direction of the positive charge.
Where is the energy stored in a capacitor?
In the field between the plates, where the energy required to distort the orbits can be recovered when the electon orbits are permitted to return to normal, analogous to a spring.
What is the material between the capacitor plates called?
The dielectric.
What is the significance of the area of the plates relative to the attached conductors?
The cross-sectional area of the plates is tremendously large in comparison to the conductors’ area, which means there is an abundance of free electrons available in each plate.
What happens if the difference in charge between a capacitor’s plates becomes extremely large?
It may cause ionization of the insulating material and cause bound electrons to be freed, which places a limit on the amount of charge that can be stored in the capacitor.
What limit is there on the amount of charge that can be stored on a capacitor?
The limit is a charge so great that it causes ionization of the dielectric and the freeing of bound electrons.
Where does the name “capacitor” come from?
It comes from the idea that there is a definite quantity of electrons to fill up, or charge, a capacitor. Hence, it is said to have a capacity.
What is the “dielectric constant” and what do the numbers indicate?
This is a rating of an insulator or dielectric’s ability to support electric flux. The higher the number, the better dielectric.
What is the standard of reference for a dielectric material and what is the dielectric constant for air?
The standard is a vacuum, which has a dielectric constant of 1.0000, while the constant of air is 1.0006 but considered to be 1.
What is the formula used to compute the value of capacitance?
C = 0.2249 (KA/d), where 0.2249 is a conversion factor from metric to to British; K the dielectric constant (say, 3.5 for parafin paper); A the area of one plate in square inches; and d the distance between the plates in inches. C is capacitance in picofarads (10^-12).
What is a “farad” and what is its basic formula?
Capacitance is equal to 1 farad when a voltage charging at the rate of 1 volt/second causes a charging current of 1 amp to flow.
C = i/(delta e/delta t), where i is the instantaneous current in amps and the numerator is the rate of change of voltage in volts with time in seconds; ie; 1 amp flows when voltage changes 1 volt each second.
What else can the farad be defined in terms of?
Charge and voltage.
When a capacitor has a charge of 1 farad, what will it store in charge when connected across a potential of 1 volt and how is this relationship expressed mathematically?
1 Coulomb of charge. C = Q/E, where Q is in coulombs and E applied voltage in volts. For example, given Q = 0.001 coulomb and E = 200 volts, C = (10 x 10^-4)/(2 x 10^2), = 0.000005 farads, or five millionths of a farad.
If the capacitor is composed of more than two plates, how is the capacitance calculated?
The capacitance is calculated by multiplying the basic formula,
C = 0.225(KA/d) by N-1, where N is the number of plates.
In multi-plate construction, how are the plates arranged?
Interlaced, as in Fig 11-4, p. 192.
What happens if the voltage applied across the plates is too great?
The dielectric will break down and arcing will occur between the plates. The capacitor is then short-circuited and the possible flow of direct current through it can damage other parts of the equipment.
What is the working voltage of a capacitor and what limits it?
The maximum voltage that can be steadily applied without danger of causing an arc-over.
What does the working voltage rating of capacitor depend on?
(1) the type of material used as the dielectric; (2) the thickness of the dielectric; and (3) frequency because the losses and resultant heating effect increase as the frequency increases.
