Electrical Engineering ⇒ Topic : Multiplate Capacitor
The most *convenient way of achieving large capacitance is by using large plate area. Increasing the plate area may increase the physical size of the capacitor enormously. In order to obtain a large area of plate surface without using too bulky a capacitor, multiplate construction is employed. In this construction, the capacitor is built up of alternate sheets of metal foil (i.e. plates) and thin sheets of dielectric. The odd numbered metal sheets are connected together to form one terminal T1 and even-numbered metal sheets are connected together to form the second terminal T2.
Fig. (a) shows a multiplate capacitor with seven plates. A little reflection shows that this arrangement is equivalent to 6 capacitors in parallel. The total capacitance will, therefore, be 6 times the capacitance of a single capacitor (formed by say plates 1 and 2). If there are n plates, each of are A, then (n - 1) capacitors will be in parallel.
Capacitance of n plate capacitor is
where d is the distance between any two adjacent plates and Er is the relative permittivity of the medium. It may be seen that plate area increased from A to A (n - 1).
Variable Air capacitor. It is a multiplate (air capacitor whose capacitance can be varied by changing the plate area. Fig. (a) shows a variable air capacitor commonly used to "tune in" radio stations in the radio receiver. It consists of a set of stationary metal plates Y fixed to the frame and another set of movable metal plates X fixed to the central shaft.
The two sets of plates are electrically insulated from each other. Rotation of the shaft moves the plates X into the spaces between plates Y, thus changing the *common (or effective) plate area and hence the capacitance. The capacitance of such a capacitor is given by
When the movable plates X are completely rotated in (i. e. the two sets of plates completely overlap each other), the common plate area 'A' is maximum and so is the capacitance of the capacitor.Minimum capacitance is obtained when the movable plates X are completely rotated out of stationary plates Y. The capacitance of such variable capacitors is from zero to about 4000 pF.
Note. In all the formulas derived for capacitance, capacitance will be in farad if area is in m2 and the distance between plates is in m.
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