Resonance Question!

Thread Starter


Joined Sep 18, 2006
Hi all,

I have a series resonant circuit that has a Capacitor, Resistor and Inductor.

The first question was to find the Resonant frequency which is no problem. Question 2 is to find the current at resonance. Therefore using the fact that XL=XC at resonance then Z must be equal to R. So I then just used I= Vin/R.
The third question was to vind the voltage across the capacitor. I used that
Q= XL/R = VC/V Therefore VC = XL x V / R. AM I along the right lines with those? Each question was worth 4 marks(Mock Exam)

The final part of the question for 8 marks asks that if a capacitor of 1nF is added to the circuit in parallel, how does the circuit operation change? (No calculations are required)

Now I answered that the value of XC would change therefore XC would no longer be equal to XL so the circuit would no longer be at resonance. To work out the new resonance then a new formula for paralell resonance will have to be used.

Now this question is worth 8 marks so I really don't think I have answered it fully enough?

Anyone got anything to add?

hi paul
the first thing is that you don't have to have such an intricate algorithm for calculating Vc, as Vc=Vl=IXc=IXl
Secondly, applying a capacitor in shunt to the series RLC circuit wont disturb the resonance in the RLC branch. Even now the RLC branch behaves like a pure resisitor and thus whole of the circuit can be treated as a parallel R-C circuit. This circuit will have leading vars as current will be leading the voltage. Effectively there is no inductance involved in the circuit thus the circuit would never behave like a pure resistor i.e resonance won't be there.
Apply the formulas of parallel R-C circiut
Last edited:


Joined Aug 10, 2008
What neeraj posted is correct, if what you typed is correct.

If what you meant is the capacitor in parrallel with the whole circuit, then his answer is correct.

But if you meant to write a capacitor in parrallel with the original capacitor, in series with inductance, resistance, then you would have to treat the parrallel capacitors as the new capacitance and resolve for XC.