Selasa, 07 Juni 2011

Capacitance (C), Capacitance Reactance (XC), and the relationship with the Earth Capacitance

Capacitance (C), Capacitance Reactance (XC), and the relationship with the Earth Capacitance

Capacitance or the capacitance is a measure of the amount of electric charge stored (or separated) to a predetermined electric potential. The most common form of charge storage device is a capacitor two plates / plate / chip. If the charges on the plate / plate / chip is + Q and-Q, and V is the voltage between the plates, then the capacitance is:

SI unit of capacitance is the farad, 1 farad = 1 coulomb per volt.
Capacitance and displacement current
Physicist named James Clerk Maxwell invented the concept of displacement current,, to make Ampère's law consistent with conservation of charge in cases where the charge accumulates, for example in a capacitor. He interprets this as a real movement of cargo, even in a vacuum, where Maxwell guessed that in fact the charge movement associated with the charge dipole movement in the ether. Although this interpretation has been abandoned, Maxwell's correction to Ampere's law remains valid (the electric field produces a changing magnetic field).
Maxwell's equations combining Ampere's law with displacement current concept is formulated as. (By integrating both sides, the integral can be replaced by its integral around a closed contour, thus demonstrating the interconnection with Ampère's formulation.)
Potential coefficient
The discussion above only applies in the case of two conductive plates. Definition C = Q / V is still valid if only one plate is given an electrical charge, provided that the field lines generated by that charge ended as if the plate were at the center of an oppositely charged sphere at infinity.
C = Q / V does not apply when the number plate is charged more than two, or when the net charge on the two plates are non-zero. To handle this case, Maxwell introduced the concept of "coefficients of potential." If three plates are given charges Q1, Q2, Q3, the voltage plate 1 is
V1 = p11Q1 + p12Q2 + p13Q3,
and similarly for the other voltages. Maxwell showed that the potential coefficients is symmetrical, so that p12 = p21, etc..
Duality capacitance / inductance
In mathematical terms, the ideal capacity can be regarded as the opposite of the ideal inductance, because the voltage-current equation can converted diversified two phenomena to each other by exchanging the voltage and current term.
Self capacitance
In electrical circuits, or strings of electrical or electrical circuit, the capacitance term usually stands for mutual capacitance (English: mutual capacitance) between two adjacent conductors, such as two slab a capacitor. There is also the term self-capacitance (English: self-capacitance), which is the amount of electrical charge that must be added to an isolated conductor to raise its electrical potential of 1 volt. The reference point for this potential is a scope / area hollow conduction theory, of infinite radius, centered on the conductor. With this method, the self-capacitance of a conducting sphere of radius R is:

Typical self-capacitance values ​​are:
• to "plate" peak van de Graaf generator, typically a ball 20 cm in radius: 20 pF
• planet Earth: about 710 μF
Capacitance majority condenser or capacitor used in electronic circuits are a number of magnitude smaller than the farad. Several subunits of the capacitance of the most commonly used today is milifarad (mF), microfarads (μF), nanofarad (nF), and pikofarad (pF).
Capacitance can be calculated by knowing the geometry of the conductor and the dielectric properties of insulator between the conductors. For example, the large capacitance of a capacitor "plate-parallel" which is composed of two parallel plates of area A separated by distance d is as follows: is approximately equal to the Following:
(In SI units)
C is capacitance in farads, F
A is the area of ​​each plate, measured in square meters
εr is the dielectric constant (also called a relative electric permittivity) of the material between the plates, (vacuum = 1)
ε0 is the permittivity of vacuum or electric constants where ε0 = 8.854x10-12 F / m
d is the distance between the plates, measured in meters
The equation above is very good to use if d small amount when compared with other dimensions of the plate. In CGS units, the equation form:

where C in this case has a unit of length.
Dielectric constant for a number of changes in the dielectric which is very useful as a function of applied electric field, for example fero elektrisitas materials, so that the capacitance for the various devices is no longer just a function of tool geometry. Capacitor that stores the sinusoidal voltage, dielectric constant, is a function of frequency. Dielectric constant change is called the scattered-frequency dielectric, and regulated by the various dielectric relaxation processes, such as relaxation Debye capacitance.
Charging and Discharging Capacitors

Two things to note on a capacitor is now charging and discharging cargo. To this can be described with the aid of image

When switch S linked to position 1 then current will flow from the source through a resistance R to the capacitor C. voltage on C will rise exponentially in accordance with the following equation:

Vc = voltage on the capacitor (V)
Vs = voltage at the source (V)
t = time of charging the capacitor (s)
R = resistance of resisitor (Ω)
C = capacitance of the capacitor (F)
I will stop flowing currents (I = 0) at the time of voltage capacitor C equal to the voltage source Vs. The process is called charging the capacitor. Then when the switch S is connected to position 2, then the current will flow in opposite directions with the direction of charging. The capacitor will release its stored electrical energy back to the voltage equation:

When the capacitor has empty entire load current flow will stop (I = 0). Figure 2.4. graph shows the capacitor discharging cargo. From the graphs - graphs and equations-equations that (a) t = 0, q = 0 and I = Vs / R and (b) if t ~, q Vc Vs. And I 0, ie, the early flow is Vs / R and finally 0, and first charge on the capacitor plates in the beginning is 0 and finally VcVs

(A) graph of the charging capacitor (b) capacitor discharge graph
The time required for charging and discharging of the capacitor depends on the so-called RC time constant (time constant), namely:
t = R.C
t = time constant (seconds)
R = resistance of the capacitor (Ω)
C = Capacitance of the capacitor (F)
If equation (7) substituted togetherness(5) it will obtain the understanding that after a constant time t go through, voltage across the capacitor C which is filling its cargo will reach 63% of the voltage source.

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