Capacitive reactance and capacitance

Thread Starter

Unicorn Notreal

Joined Dec 24, 2015
5
I don't understand why is the capacitance of a capacitor is inversely proportional to the quantity of charges !
And also why the capacitive reactance is inversely proportional to the capacitance ?
 

crutschow

Joined Mar 14, 2008
25,634
I don't understand why is the capacitance of a capacitor is inversely proportional to the quantity of charges !
And also why the capacitive reactance is inversely proportional to the capacitance ?
You have it wrong. The value of the capacitance is proportional to the amount of charge stored: C = Q/V.

As the above equation shows, the larger the capacitance, the more charge that is moved for a given voltage (Q=CV), thus the "resistance" to moving that charge (the reactance) is reduced as the capacitance is increased.

Make sense?
 

Thread Starter

Unicorn Notreal

Joined Dec 24, 2015
5
Yes, sorry I was probably asleep while reading the equation.
But what about the capacitive reactance (Xc=1/2πυC) why is it inversely proportional to the capacitance?
You have it wrong. The value of the capacitance is proportional to the amount of charge stored: C = Q/V.

As the above equation shows, the larger the capacitance, the more charge that is moved for a given voltage (Q=CV), thus the "resistance" to moving that charge (the reactance) is reduced as the capacitance is increased.

Make sense?
 

MrChips

Joined Oct 2, 2009
22,069
\(Reactance = \frac{1}{\omega C}\)

\(Susceptance =\omega C\)

If we ignore the j bit, we can say

\(Admittance =\omega C\)

Hence admittance increases with C.
Does that make you happy?

As C increases we admit more AC signal.
 

cmartinez

Joined Jan 17, 2007
7,172
\(Reactance = \frac{1}{\omega C}\)

\(Susceptance =\omega C\)

If we ignore the j bit, we can say

\(Admittance =\omega C\)

Hence admittance increases with C.
Does that make you happy?

As C increases we admit more AC signal.
Sorry for the dumb question, but what is "the j bit"?
 

Papabravo

Joined Feb 24, 2006
14,659
The "j bit" is a reference to the imaginary unit. Mathematicians use a lower case i, but Electrical Engineers might confuse that with current, so we use the letter j instead.
Reactance is is a real number, but impedance is a complex number.

Also:

\(\frac{1}{j \omega C}=-j \frac{1}{\omega C}\)

If the square root of -1 gives you a funny feeling, just think of j as a rotation operator.
 
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