Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Capacitor charging/discharging for R=0

  1. Jun 10, 2010 #1
    Consider an ideal capacitor its given to a switch and at t=t0 switch closes and Voltage =V is applied ..Consider R=0 in the circuit..how will the graph of i(current) v/s time(t) and Voltage (V) v/s time look like ??????in all waveforms i have seen R is never zero..:confused:
     
  2. jcsd
  3. Jun 10, 2010 #2
    R cannot be zero.
    In the theoretical case where it is, the current would be infinitely large for an infinitely short time.
    I don't think the graph would tell you very much! (Even if it were possible to draw it)
     
  4. Jun 10, 2010 #3
    Why would the current be infinitely large? There are a fixed number of electrons in the capacitor and they must move some distance to discharge. Their speed is bound by the speed of light, so some time must elapse. So the current should be REALLY BIG, but not infinite.

    Now saying I'm right, just wondering...

    -David
     
  5. Jun 10, 2010 #4
    the graph of i vs t will be the delta function
    (delta func;f(x); is zero at all values except at 0, where it is infinitely large such that the integral of f(x) from -infinity to +infinity is equal to some finite value[here, CV])
    physically what happens in this ideal situation is that all the requisite charge is transferred to the capacitor plates in infinitesimal time. therefore, i~q/t;t tends to 0, so i tends to infinity.
     
  6. Jun 10, 2010 #5
    That's what I said!
    The resistance cannot be zero in practice. There would be a very large current for a very short time. The fact that we can talk about a current and a time means that there are numbers that can be assigned to them.
    I stated that in the theoretical case when R=0 (which is impossible in practice in this situation) the current would be (theoretically) infinitely large. In the real world things don't go to infinity. In this case because the resistance can't go to zero.
    Hope that makes it clearer.
     
  7. Jun 10, 2010 #6

    rcgldr

    User Avatar
    Homework Helper

    I think what the OP is asking is if the equation for current versus time (or time versus current) can be determined from the limit as R -> infinity.
     
  8. Jun 10, 2010 #7
    i got the point ...also will the voltage across capacitor increase to supplied voltage instantaneously which i presume should happen if i is infinite (theoretically)...however,we study that voltage cannot increase instantaneously in a capacitor
     
  9. Jun 12, 2010 #8
    You are struggling with the difference between fields and circuits. There are no electrons in circuit theory just continous current. A real capacitor has inductances that limit inrush current for example. Circuit answer is infinite current. Not physical answer.
     
  10. Jun 12, 2010 #9
    Superconductors have zero resistance they are used in MRI's
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook