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Capacitance is the ability of a body to store an electrical charge. Any object that can be electrically charged exhibits capacitance. A common form is a parallel-plate capacitor, which consists of two conductive plates insulated from each other, usually sandwiching a dielectric material. In a parallel plate capacitor, capacitance is directly proportional to the surface area of the conductor plates and inversely proportional to the separation distance between the plates. If the charges on the plates are +''q'' and −''q'' respectively, and ''V'' gives the voltage between the plates, then the capacitance ''C'' is given by : which gives the voltage/current relationship : The capacitance is a function only of the geometry of the design (area of the plates and the distance between them) and the permittivity of the dielectric material between the plates of the capacitor. For many dielectric materials, the permittivity and thus the capacitance, is independent of the potential difference between the conductors and the total charge on them. The SI unit of capacitance is the farad (symbol: F), named after the English physicist Michael Faraday. A 1 farad capacitor, when charged with 1 coulomb of electrical charge, has a potential difference of 1 volt between its plates.〔http://www.collinsdictionary.com/dictionary/english/farad〕 Historically, a farad was regarded as an inconveniently large unit, both electrically and physically. Its subdivisions were invariably used, namely the microfarad, nanofarad and picofarad. More recently, technology has advanced such that capacitors of 1 farad and greater (so-called 'supercapacitors') can be constructed in a structure little larger than a coin battery. Such capacitors are principally used for energy storage replacing more traditional batteries. The energy stored in a capacitor is found by integrating the work ''W'': : ==Capacitors== (詳細はfarad. The most common subunits of capacitance in use today are the microfarad (µF), nanofarad (nF), picofarad (pF), and, in microcircuits, femtofarad (fF). However, specially made supercapacitors can be much larger (as much as hundreds of farads), and parasitic capacitive elements can be less than a femtofarad. Capacitance can be calculated if the geometry of the conductors and the dielectric properties of the insulator between the conductors are known. A qualitative explanation for this can be given as follows. Once a positive charge is put unto a conductor, this charge creates an electrical field, repelling any other positive charge to be moved onto the conductor. I.e. increasing the necessary voltage. But if nearby there is another conductor with a negative charge on it, the electrical field of the positive conductor repelling the second positive charge is weakened (the second positive charge also feels the attracting force of the negative charge). So due to the second conductor with a negative charge, it becomes easier to put a positive charge on the already positive charged first conductor, and vice versa. I.e. the necessary voltage is lowered. As a quantitative example consider the capacitance of a capacitor constructed of two parallel plates both of area ''A'' separated by a distance ''d'': : where :''C'' is the capacitance, in Farads; :''A'' is the area of overlap of the two plates, in square meters; :''ε''r is the relative static permittivity (sometimes called the dielectric constant) of the material between the plates (for a vacuum, ); :''ε''0 is the electric constant (''ε''0 ≈ ); and :''d'' is the separation between the plates, in meters; Capacitance is proportional to the area of overlap and inversely proportional to the separation between conducting sheets. The closer the sheets are to each other, the greater the capacitance. The equation is a good approximation if ''d'' is small compared to the other dimensions of the plates so that the electric field in the capacitor area is uniform, and the so-called ''fringing field'' around the periphery provides only a small contribution to the capacitance. In CGS units the equation has the form:〔''The Physics Problem Solver'', 1986, (Google books link )〕 : where ''C'' in this case has the units of length. Combining the SI equation for capacitance with the above equation for the energy stored in a capacitance, for a flat-plate capacitor the energy stored is: : where ''W'' is the energy, in joules; ''C'' is the capacitance, in farads; and ''V'' is the voltage, in volts. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「capacitance」の詳細全文を読む スポンサード リンク
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