1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Voltage source vs capacitor

  1. Jun 7, 2013 #1
    If we have a circuit (see attached)
    will our voltage source's supplied voltage be delayed by the capacitor in anyway?
    (as the voltage across a capacitor lags the current through a capacitor by 90 degrees)
    then, as the resistor,voltage source, and capacitor are all in parallel does this
    cause the voltage source to end up supplying a 'lagging' voltage (due to the capacitor's characteristics)?

    OR is the capacitor forced to experience whatever the voltage source is supplying?
    (therefore causing the inductor current to lead the capacitor by 90 deg)
     

    Attached Files:

  2. jcsd
  3. Jun 7, 2013 #2

    BruceW

    User Avatar
    Homework Helper

    That's it. We assume that the voltage source is able to do what it is supposed to, regardless of what is going on in the circuit. And you are right, the current will lead the voltage by 90 degrees.
     
  4. Jun 7, 2013 #3

    NascentOxygen

    User Avatar

    Staff: Mentor

    Inductor? Resistor? What is it?

    Resistor current lags the capacitor current by 90°.
     
  5. Jun 7, 2013 #4

    rude man

    User Avatar
    Homework Helper
    Gold Member

    Right.
    What inductor current? What inductor?

    EDIT:

    If the voltage source is dc: for the capacitor, i = C dV(t)/dt. Since dV(t)/dt is infinitely large (going from 0 to V in zero time), the current is infinite for an infinitely short time.

    Mathematically, V(t) = V U(t), the unit step function, and i(t) = CV δ(t) where δ(t) is the Dirac delta function with dimension T-1.

    If the source is a sinusoid, which it just dawned on me it probably is, then yes, the current will lead the voltage by 90 deg. and its magnitude is wCV, w = 2 pi f.
     
    Last edited: Jun 7, 2013
  6. Jun 7, 2013 #5

    BruceW

    User Avatar
    Homework Helper

    @NascentOxygen agreed. and when he says "inductor current", I was assuming he just meant current through the voltage source. Although "inductor current" is a term I have not heard before in this context...
     
  7. Jun 7, 2013 #6

    BruceW

    User Avatar
    Homework Helper

    @rude man - From the OP, I would guess that he/she is supposed to assume sinusoidal voltage
     
  8. Jun 7, 2013 #7

    rude man

    User Avatar
    Homework Helper
    Gold Member

    Yesd, that just dawned on me. Thanks.
     
  9. Jun 7, 2013 #8
    sorry! i meant the capacitor current when i said 'inductor current' - got mixed up reading my notes over and over, thanks guys!
     
  10. Jun 7, 2013 #9
    So in this case, the current through the resistor should be in phase with the voltage supply, while the current through the capacitor leads the capacitor voltage by 90 deg am I correct?
     
    Last edited: Jun 7, 2013
  11. Jun 8, 2013 #10

    NascentOxygen

    User Avatar

    Staff: Mentor

    You now realise you are really stating nothing new. "The current through the resistor is always in phase with the voltage across that resistor", while "the current through the capacitor always leads the capacitor voltage by 90 deg". :smile:
     
  12. Jun 8, 2013 #11

    BruceW

    User Avatar
    Homework Helper

    yep. you got it.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Voltage source vs capacitor
  1. Voltage on capacitor (Replies: 3)

Loading...