# Impedance of a parallel rc circuit

#### ryanmoser

Joined Nov 5, 2015
3
1. If a circuit consisted of a resistor R parallel with a capacitor C is given an external current excitation with frequency of ω, let the current equals to I0*exp(jωt), what is the response of voltage E? Would the impedance equals to R(1-ωCRj)/(1+ω^2*C^2*R^2), the same as the case when the circuit is excited by AC voltage E0*exp(jωt)?

#### crutschow

Joined Mar 14, 2008
27,890
The impedance of a parallel RC circuit is inversely proportional to frequency.

#### alfacliff

Joined Dec 13, 2013
2,458
there is no resonant point for rc circuits like there is on lc circuits. the impedance, actually reactance, changes with frequency in a fairly linear manner.

#### BillB3857

Joined Feb 28, 2009
2,546
I seem to remember something about phase angle and ELI the ICE man coming into play. Something about a vector???

#### alfacliff

Joined Dec 13, 2013
2,458
eli the ice man is for lcr circuits. phase lead and lag with capacitive and inductive circuits. for rc, its just a ratio of capaccitive reactance and resistance gives phase shift.

#### BillB3857

Joined Feb 28, 2009
2,546
My post (#4) was intended to be a hint to the OP. Please correct me if I'm wrong, but when calculating effective impedance, isn't the answer a vector value of Xc and R? It has been a long time since exposure to those details.

#### jjw

Joined Dec 24, 2013
718
Do you really need to derive these equations or can you just use known impedances of R and C and then the parallel impedance of these ?

The term I0/(s-jw) must be wrong.

#### BillB3857

Joined Feb 28, 2009
2,546
It took some digging, but I found support to what I seemed to have remembered from so long ago....

You probably don't have a navigation computer, but a calculator will work.

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