# Capacitive reactance and capacitance

#### 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
25,632
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?

• Unicorn Notreal

#### 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?

#### crutschow

Joined Mar 14, 2008
25,632
The larger the capacitance, the easier it is to move charge (Q=CV) through the capacitor, so it's reactance (resistance to charge movement) is inversely proportional to capacitance.

• Unicorn Notreal and cmartinez

#### MrChips

Joined Oct 2, 2009
22,062
$$Reactance = \frac{1}{\omega C}$$

$$Susceptance =\omega C$$

If we ignore the j bit, we can say

$$Admittance =\omega C$$

Does that make you happy?

As C increases we admit more AC signal.

• Unicorn Notreal and cmartinez

#### cmartinez

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

$$Susceptance =\omega C$$

If we ignore the j bit, we can say

$$Admittance =\omega C$$

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
14,648
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.

• cmartinez