Figure given below shows circuit containing alternating voltage source
V=V_{0}sinωt
connected to a capacitor of capacitance C
Suppose at any time t,q be the charge on the capacitor and i be the current in the circuit
Since there is no resistance in the circuit, so the instantaneous potential drop q/C across the capacitor must be equal to applied alternating voltage
so
q/C=V_{0}sinωt
Since i=dq/dt is the instantaneous current in the circuit so
is the peak value of current
Comparing equation (13) with V=V_{0}sinωt ,we see that in a perfect capacitor current leads emf by a phase angle of π/2
This phase relationship is graphically shown below in the figure
Again comparing peak value of current with ohm's law ,we find that quantity 1/ωC has the dimension of the resistance
Thus the quantity
X_{C}=1/ωC =1/2πfC ---(14)
is known as capacitive reactance
From equation (14) we see that capacitive reactance decreases with increasing frequency of current and in infinite for direct current for which frequency f=0
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