# Capacative Reactance calculation

Discussion in 'Homework Help' started by JDR04, Aug 20, 2012.

1. ### JDR04 Thread Starter Active Member

May 5, 2011
339
4
Hi Folks, any help on this one will be appreciated.

I'm given a circuit that has a 200mF capacitor connected in series with a 2 Mohm resistor.

The question is, calculate the capacitave impedance.

As far as I understand it, I need a frequency to calculate the capacative reactance as Xc is part of the formula??

Z = R squared x Xc squared and then answer squared again.

Am I missing something here.Your help will be appreciated. JDR04

2. ### #12 Expert

Nov 30, 2010
16,705
7,358
Xc=1/(2PiFC)

again, the simple form.

3. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
The impedance, Z = R + jXc, where j is the imaginary unit.

The magnitude of the impedance |Z| = SQRT(R^2 + Xc^2)

The argument of Z = arctan(Xc/R)

4. ### WBahn Moderator

Mar 31, 2012
18,094
4,920
The term "capacitive impedance" is confusing as it implies that you should, or at least could, get a different answer than if asked to find the "impedance".

5. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
The terms "inductive impedance" and capacitative impedance" sound like poor tanslations into English. Le'ts be clear and precise:

Inductive Reactance is a real number equal to ωL
Capacitative Reactance is also a real number equal to 1/ωC

Impedance is a complex number
The magnitude of impedance is a real number
The argument of impedance is a real number

The word impedance needs no qualifiers -- it stands on its own just fine thanks very much.

#12 likes this.
6. ### WBahn Moderator

Mar 31, 2012
18,094
4,920
And, paralleling a discussion in another thread, whether capacitive reactance is (1/ωC) or (-1/ωC) depends on which school of thought you belong to.

I strongly prefer the latter, because it just makes the math and the logic so much cleaner and simpler. But, regardless of which someone prefers, they need to be aware of both and then strive to make sure that they know, from the context, which definition someone is using. It can be tough.

7. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
The sign of the capacitative reactance comes about when the rotation operator -j, aka the imaginary unit, is applied to the quantity 1/ωC

Yes it can be difficult, and even more so without basic math skills which was the subject of another thread about how much math was required to do...