Study the circuits (a) and (b) shown in Fig 7.2 and answer the following questions.


(a) Under which conditions would the rms currents in the two circuits be the same?


(b) Can the rms current in circuit (b) be larger than that in (a)?


The rms current in (a) is given by:



Now, the rms current in (b) is given by:



Now, if the source voltage is the same in both cases, then .


Now, for the currents to be equal,





Using, and where ω is angular frequency, L is inductance and C is capacitance,



Using where f is the frequency,



This is called resonance and the last equation is the condition for resonance. Hence, the rms current is the same when there is resonance in (b) and the source rms voltage is the same.


(b) Note that



where Z is impedance


and



Now,



Hence,


No, the rms current in b can never be larger than in (a) (assuming that the rms voltage is the same).


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