Capacitive Reactance of a Capacitor Simply Explained
Important Formula
What do the formula symbols mean?
Capacitive reactance
$$ \class{purple}{X_{\text C}} $$ Unit $$ \mathrm{\Omega} $$In the case of a DC circuit, the voltage frequency is \( f = 0 \). In this case, the capacitor has an infinite reactance and thus the capacitor does not conduct current.
Frequency
$$ f $$ Unit $$ \mathrm{Hz} = \frac{ 1 }{ \mathrm{s} } $$The alternating current flowing through the capacitor also changes direction at this frequency.
With the angular frequency \(\omega ~=~ 2\pi \, f\) the capacitive reactance can be written as: $$ X_{\text C} ~=~ -\frac{1}{\omega \, C} $$
Capacitance
$$ C $$ Unit $$ \mathrm{F} = \frac{ \mathrm{C} }{ \mathrm{V} } $$If we apply an AC voltage \( U_{\text C}(t) \) to a capacitor of capacitance \(C\), then an AC current \( I_{\text C}(t) \) flows through the capacitor. The alternating voltage changes polarity with the frequency \( f \). With this frequency, the alternating current also changes its direction.
A capacitor to which an alternating voltage is applied has a complex non-ohmic resistance which is called capacitive reactance \( X_{\text C}(t) \).
Du kannst den kapazitiven Blindwiderstand ganz einfach berechnen. Du brauchst dafür lediglich die Wechselspannungsfrequenz \(f\) und die Kondensator-Kapazität \(C\):
\(\pi\) is a mathematical constant with the value \( \pi = 3.14 \). The minus sign in the formula states, that the alternating voltage at a capacitor lags behind the alternating current.
The unit of capacitive reactance is Ohm:
By the way, the factor \( 2 \, \pi \, f \) in Eq. 1
is often combined to angular frequency \( \omega \):
-
If you use a very large AC frequency, then the capacitive reactance becomes very small and the capacitor easily lets the current through.
-
If, on the other hand, the AC voltage frequency is very low or even zero, i.e. if a DC voltage is applied, then the capacitive reactance becomes infinitely large. The capacitor does not allow any current to pass.
As you can see from Eq. 1
or 2
, you can also use the capacitance \(C\) to adjust the reactance \( X_{\text C} \) of the capacitor.
When we work with rms values of voltage and current, we are usually only interested in the magnitude \( |X_{\text C}| \) of the capacitive reactance: