Consider the parallel resonant circuit shown in adjacent figure. One branch contains an inductor of inductance L and a small ohmic resistance R, whereas the other branch contains a capacitor of capacitance C. The circuit is fed by a source of alternating emf`E= E_(o) e^(jomegat)=E_(o) sin omegat`
The impedance of inductor branch, `Z (1)= R + jomegaL`
The impendence of capacitor branch, Z(2) = (1//jomegaC)` therefour Net impedance Z of the two parallel branches is given by
`(1)/(Z)=(1)/(Z_(1))+ (1)/)Z_(2)=(1)/(R+jomegaC`
`=(R-jomegaL)/(jomegaL)(R-jomegaL)+jomegaC=(R)/(R^(2)+omega^(2)+L^(2))+jomega[C-(L)/(R^(2)+omegaL^(2))]`
The current flowing in the circuit
`I=(E)/(Z)=(E)[(R)/(R^(2)+omega^(2)L^(2))+jomega(C-(L)/(R^(2)+omega^(2)L^(2)))]`
For resonance to occur, the current must be in phase with the applied emf. For this, the reactive component of current should be zero, l.e.
`omega[C-(L)/(R^(2)+omega^(2)L^(2))]=0`
or`C=(L)/(R^(2)+omega_(r)^(2)L^(2))`(writing `omega_(r)`for `omega`at resonance)
This gives resonant angular frequency
`omega_(r)=sqrt((1)/(LC)-R^(2)/(L^(2)))` At parallel circuit resonance, theimpendence is maximum and current is minimum. Parallel resonant circuit is sometimes called the anti-resonance in order to distinguish from series resonance
QFind the impedance of AC circuit at resonance shown in the adjacent figure

The reactance of the circuit in the adjacent figure is 20 Omega. Find the value of `I_(rms)`

Options

A. 4 Amp
B. 2 Amp
C. 4.5 Amp
D. 13.5 Amp

Answer

A