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

Homework Help: RC Circuits

  1. Feb 9, 2008 #1
    Why does a constant current source need to be used in a parallel RC circuit?

    Why does a Constant Voltage source need to be used in a Series RC circuit?
     
  2. jcsd
  3. Feb 9, 2008 #2
    I(t) = C dv/dt
     
  4. Feb 9, 2008 #3
    could you explain?
     
  5. Feb 9, 2008 #4
    I doesn't.
     
  6. Feb 10, 2008 #5
    Does to.

    [tex]I = dQ/dt[/tex]

    [tex]C \equiv Q/V[/tex]

    so

    [tex]I = \frac{d}{dt}(CV)[/tex]

    capacitance is constant

    [tex]I = C \frac{dV}{dt}[/tex]

    to the original poster, could you please show some thoughts towards this problem so I, or someone else, knows where to begin helping?
     
  7. Feb 10, 2008 #6

    cepheid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Mindscrape, I think John's answer was directed to the OP, i.e. he wasn't refuting your claim that I = CdV/dt, he was answering the original question:

    Q1. Why does a constant current source need to be used in a parallel RC circuit?

    Q2. Why does a Constant Voltage source need to be used in a Series RC circuit?

    A. It doesn't (in both cases).

    I tend to agree. I don't see any need to make those restrictions. It seems more likely that circuits of those two forms were introduced to the OP in a specific context???

    I think we do need to know what that specific context was to say anything further.
     
  8. Feb 10, 2008 #7
    Oh, my bad, John. I think that the question is getting at how the two configurations will produce similar differential equations, but we'll have to wait for the OP to elaborate.
     
  9. Feb 10, 2008 #8
    Well, that is because each circuit only has one energy storage device. The number of states in a circuit is equal to the number of energy storage devices. Well at least that is usually true if the energy storage devices are resisters and capacitors.
     
  10. Feb 10, 2008 #9
    On another note, the current source and voltage sources are forcing terms which aren't part of the natural response. A non ideal voltage sources is a voltage source in series with a resister. A non ideal current source is a current source in parallel with a resistor. It can be shown that the two are equivalent.
     
  11. Feb 10, 2008 #10
    One other thing I should mention is the the current though an inductor cannot change instantaneously and the voltage across a capacitor cannot change instantaneously. This is the most obvious restriction on a circuit. Therefore ideal voltage sources cannot be in parralel with a capacitor and ideal current sources cannot be in series with an inductor.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook