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|Lessons In Electric Circuits -- Volume I (DC) - Chapter 13 lessons electric circuits volume capacitors fields capacitance calcu...
www.ibiblio.org kuphaldt electricCircuits DC DC 13.html
Lessons In Electric Circuits Volume I DC Chapter 13
... Lessons In Electric Circuits Volume I DC Chapter 13 Lessons In Electric Circuits Volume I Chapter 13 CAPACITORS Electric fields and CAPACITAnce Capacitors and calculus Factors affecting CAPACITAnce Series and parallel capacitors Practical considerations Contributors Electric fields and capacit ...
... an insulating material will suffice.
For larger capacitor values, the plates may be strips of metal foil, sandwiched around a flexible insulating medium and rolled up for compactness.
The highest CAPACITAnce values are obtained by using a microscopic thickness layer of insulating oxide separating two conductive surfaces.
In any case, though, the general idea is the same: two conductors, separ ...
... he formation of electric field flux is called the permittivity of the dielectric.
The measure of a capacitor's ability to store energy for a given amount of voltage drop is called CAPACITAnce Not surprisingly, CAPACITAnce is also a measure of the intensity of opposition to changes in voltage exactly how much current it will produce for a given rate of change in voltage C ...
... ally denoted with a capital C, and is measured in the unit of the Farad, abbreviated as F.
Convention, for some odd reason, has favored the metric prefix micro in the measurement of large CAPACITAnces, and so many capacitors are rated in terms of confusingly large microFarad values: for example, one large capacitor I have seen was rated 330,000 microFarads!! Why not state it as 330 mi ...
... ut the negative side and in the positive side, like a battery The ability of a capacitor to store energy in the form of an electric field and consequently to oppose changes in voltage is called CAPACITAnce It is measured in the unit of the Farad F Capacitors used to be commonly known by another term: condenser alternatively spelled condensor Capacitors and ca ...
... voltage increasing at a constant rate of 2 volts per second , the dv/dt term of the formula will be a fixed value.
According to the equation, this fixed value of dv/dt, multiplied by the capacitor's CAPACITAnce in Farads also fixed , results in a fixed current of some magnitude.
From a physical perspective, an increasing voltage across the capacitor demands that there be an increasing charge di ...
... i = C dv/dt , this will result in a current figure i that is likewise negative in sign, indicating a direction of flow corresponding to discharge of the capacitor.
Factors affecting CAPACITAnce There are three basic factors of capacitor construction determining the amount of CAPACITAnce created.
These factors all dictate CAPACITAnce by affecting how much electric field flu ...
... ctrons between plates will develop for a given amount of electric field force voltage between the two plates : PLATE AREA: All other factors being equal, greater plate area gives greater CAPACITAnce; less plate area gives less CAPACITAnce.
Explanation: Larger plate area results in more field flux charge collected on the plates for a given field force voltage across the plate ...
... more field flux charge collected on the plates for a given field force voltage across the plates PLATE SPACING: All other factors being equal, further plate spacing gives less CAPACITAnce; closer plate spacing gives greater CAPACITAnce.
Explanation: Closer spacing results in a greater field force voltage across the capacitor divided by the distance between the plate ...
... ollected on the plates for any given voltage applied across the plates.
DIELECTRIC MATERIAL: All other factors being equal, greater permittivity of the dielectric gives greater CAPACITAnce; less permittivity of the dielectric gives less CAPACITAnce.
Explanation: Although its complicated to explain, some materials offer less opposition to field flux for a given amount ...
0 to 10.
0 Distilled water 80.
0 Barium strontium titanite 7500 An approximation of CAPACITAnce for any pair of separated conductors can be found with this formula: A capacitor can be made variable rather than fixed in value by varying any of the physical factors d ...
... e for any pair of separated conductors can be found with this formula: A capacitor can be made variable rather than fixed in value by varying any of the physical factors determining CAPACITAnce.
One relatively easy factor to vary in capacitor construction is that of plate area, or more properly, the amount of plate overlap.
The following photograph shows an example of a var ...
... ree to which the sets of plates overlap each other will vary, changing the effective area of the plates between which a concentrated electric field can be established.
This particular capacitor has a CAPACITAnce in the picofarad range, and finds use in radio circuitry.
Series and parallel capacitors When capacitors are connected in series, the total CAPACITAnce is less than ...
... use in radio circuitry.
Series and parallel capacitors When capacitors are connected in series, the total CAPACITAnce is less than any one of the series capacitors' individual CAPACITAnces.
If two or more capacitors are connected in series, the overall effect is that of a single equivalent capacitor having the sum total of the plate spacings of the individual capacitors.
... equivalent capacitor having the sum total of the plate spacings of the individual capacitors.
As we've just seen, an increase in plate spacing, with all other factors unchanged, results in decreased CAPACITAnce.
Thus, the total CAPACITAnce is less than any one of the individual capacitors' CAPACITAnces.
The formula for calculating the series total CAPACITAnce is the same form as for ...
... tors' CAPACITAnces.
The formula for calculating the series total CAPACITAnce is the same form as for calculating parallel resistances: When capacitors are connected in parallel, the total CAPACITAnce is the sum of the individual capacitors' CAPACITAnces.
If two or more capacitors are connected in parallel, the overall effect is that of a single equivalent capacitor having the sum tota ...
... single equivalent capacitor having the sum total of the plate areas of the individual capacitors.
As we've just seen, an increase in plate area, with all other factors unchanged, results in increased CAPACITAnce.
Thus, the total CAPACITAnce is more than any one of the individual capacitors' CAPACITAnces.
The formula for calculating the parallel total CAPACITAnce is the same form as fo ...
... lel connections result in diminished values.
With capacitors, its the reverse: parallel connections result in additive values while series connections result in diminished values.
REVIEW: CAPACITAnces diminish in series.
CAPACITAnces add in parallel.
Practical considerations Capacitors, like all electrical components, have limitations which must be respected fo ...
... rity to an electrolytic capacitor may result in the destruction of that super thin dielectric layer, thus ruining the device.
However, the thinness of that dielectric permits extremely high values of CAPACITAnce in a relatively small package size.
For the same reason, electrolytic capacitors tend to be low in voltage rating as compared with other types of capacitor construction.
... , the more circuitry can be built into a smaller package, and usually weight is saved as well.
With capacitors, there are two major limiting factors to the minimum size of a unit: working voltage and CAPACITAnce.
And these two factors tend to be in opposition to each other.
For any given choice in dielectric materials, the only way to increase the voltage rating of a capacitor is to increase the ...
... choice in dielectric materials, the only way to increase the voltage rating of a capacitor is to increase the thickness of the dielectric.
However, as we have seen, this has the effect of decreasing CAPACITAnce.
CAPACITAnce can be brought back up by increasing plate area.
but this makes for a larger unit.
This is why you cannot judge a capacitor's rating in Farads simply by size.
A capacitor o ...
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