Multiple and Variable Capacitors
Similarly, the arrangement in figure given below has four times the capacitance of single capacitor.
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If one set of plates is fixed and the other is capable of rotation, then capacitance of such a multi-plate capacitor can be varied. Such variable capacitance air capacitors are widely used in radio work (Figure given below). The set of fixed plates F is insulated from the other set R which can be rotated by turning the knob K. The common area between the two sets is varied by rotating K, hence the capacitance between the two is altered. Minimum capacitance is obtained when R is completely rotated out of F and maximum when R is completely rotated in i.e. when the two sets of plates completely overlap each other.
The capacitance of such a capacitor is
where n is the number of plates which means that (n − 1) is the number of capacitors.
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