Capacitive reactance and capacitance

Discussion in 'General Electronics Chat' started by Unicorn Notreal, Feb 1, 2016.

  1. Unicorn Notreal

    Thread Starter New Member

    Dec 24, 2015
    5
    0
    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 ?
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,023
    3,236
    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 likes this.
  3. Unicorn Notreal

    Thread Starter New Member

    Dec 24, 2015
    5
    0
    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?
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,023
    3,236
    Read my second paragraph again.
    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 like this.
  5. MrChips

    Moderator

    Oct 2, 2009
    12,446
    3,362
    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.
     
    Unicorn Notreal and cmartinez like this.
  6. cmartinez

    AAC Fanatic!

    Jan 17, 2007
    3,574
    2,543
    Sorry for the dumb question, but what is "the j bit"?
     
  7. Papabravo

    Expert

    Feb 24, 2006
    10,148
    1,791
    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 likes this.
Loading...