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RC circuit question

Discussion in 'Electrical Engineering' started by CD, Jan 17, 2004.

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  1. CD

    CD Guest

    Hi all,

    I'm reading The Art of Electronics and they have a circuit that shows an
    arbitrary resistor and a capacitor connected in parallel (no source
    connections). They also stated that according to the capacitor rules, we
    get:

    C*(dV/dt) = I = - (V/R)

    I can't seem to comprehend the - (minus sign) in front of (V/R). Can anyon
    explain this? Show it mathematically?

    Thanks!
     
  2. Airy R. Bean

    Airy R. Bean Guest

    Comes from Kirchoff's laws - the sum of any voltages round
    a closed loop is zero. The voltage going up the capacitor
    is positive, that coming down the resistor as you go on round
    the loop, is negative.

    Lenz's is the only know breaker of Kirchoff!
     
  3. Brian Reay

    Brian Reay Guest

    If I may, avoid the maths at first and think physics.

    Assume that the capacitor is charged at time 0 to a pd V, and has the
    resistor connected across it. The capacitor will discharge as electrons
    flow through the resistor and, as this happens the pd across the capacitor
    will decrease, ie dV/dt is -ve. , pd is decreasing as time increases.

    The rate of discharge is a function of V, R, and C, related as:

    |dV/dt| = V/(RC) (note the | | ) , RC being the normal time constant.

    From the above, we know dV/dt is -ve so, dropping the | | :


    dV/dt = -V/(RC)

    A simple transposition gives:

    C dV/dt = -V/R


    --
    73
    Brian
    G8OSN
    www.g8osn.org.uk
    www.amateurradiotraining.org.uk for FREE training material for all UK
    amateur radio licences
    www.phoenixradioclub.org.uk - a RADIO club specifically for those wishing
    to learn more about amateur radio
     
  4. Brian Reay

    Brian Reay Guest

    That is a poor explantion of Kirchoff's Law.

    Better to think in terms of EMF and Potential Differences with KVL and
    treat the energy source, in this case the capacitor, as having the EMF and
    the resistor as having the PD across it then:

    Vemf +Vpd =0

    Having said that, applying KVL does't really answer the question asked, for
    which you need to think in terms of dV/dt, for which I refer you to my
    other posting.
    Care to expand on that?


    Brian
     
  5. Airy R. Bean

    Airy R. Bean Guest

    Mr.Reay would appear to be cyber-stalking once again...

    It's not an "explantion" of anything.

    It is, however, a good _explanation_ of Kirchoff's Law,
    especially to someone who appeared to be not well-versed
    in technical matters and to whom an overly technical
    approach would not have helped.
    ..
    If Mr.Reay wants to score points, then he'd better not
    make a fool of himself (once again!)!

    It's noticeable that you did not respond to the questioner in
    the first instance, but only did so to satisfy your paranoid
    obsession that you have with me. For example, what was the
    purpose in your campaign about bestiality with sheep? QED.
     
  6. EEng

    EEng Guest

    The sum of the voltages around a circuit = Vt.
    The subtraction of the voltages around a circuit = Common potential.
     
  7. Airy R. Bean

    Airy R. Bean Guest

    I disagree. It is zero.
    (Save for Lenz's Law)

     
  8. The discharge current is in the opposite direction to the charging current.
    If the charging current is considered to be in a positive direction then the
    discharge current will be in a negative direction. Need more info on the
    type of circuit you are looking at.
     
  9. Chimera

    Chimera Guest

    You really don't have a clue, do you?

    Do you know the difference between potential difference, EMF, and current.

    I'm not surprised the idea of frequency is to much for you.

    Chimera
     
  10. Airy R. Bean

    Airy R. Bean Guest

    If you have something of a technical nature to offer
    to the discussion, then do so.

    Why not, for example, contribute to the discussion
    in an adult manner by replying to the OP and offering
    your own explanation in reply to her expressed difficulty?

    If all you can come up with is rather silly and infantile
    comments as you do below then you do yourself and
    your reputation no favours.
     
  11. Chimera

    Chimera Guest

    WTF are you talking about? Read the guys posts, you might learn something.
    Yours Airy, is a very poor explanation of Kirchoff's Law. In fact, one of
    the worst I've ever seen.

    Actually, Mr Reay appears to be very knowledgeable and to be able to explain
    things very well.

    You should try reading his posts.

    Chimera
     
  12. Chimera

    Chimera Guest

    Do you even know what Lenz's law is?

    I fancy not.

    Chimera
     
  13. Airy R. Bean

    Airy R. Bean Guest

    If you have something of a technical nature to offer
    to the discussion, then do so.

    Why not, for example, contribute to the discussion
    in an adult manner by replying to the OP and offering
    your own explanation in reply to her expressed difficulty?

    If all you can come up with is rather silly and infantile
    comments as you do below then you do yourself and
    your reputation no favours.
     
  14. Airy R. Bean

    Airy R. Bean Guest

    If you have something of a technical nature to offer
    to the discussion, then do so.

    Why not, for example, contribute to the discussion
    in an adult manner by replying to the OP and offering
    your own explanation in reply to her expressed difficulty?

    If all you can come up with is rather silly and infantile
    comments as you do below then you do yourself and
    your reputation no favours.
     
  15. Airy R. Bean

    Airy R. Bean Guest

    If you have something of a technical nature to offer
    to the discussion, then do so.

    Why not, for example, contribute to the discussion
    in an adult manner by replying to the OP and offering
    your own explanation in reply to her expressed difficulty?

    If all you can come up with is rather silly and infantile
    comments as you do below then you do yourself and
    your reputation no favours.
     
  16. Chimera

    Chimera Guest

    Mr Raey has already given a very good explanation which I notice you haven't
    dared to challenge.

    You should read it, it is very good. Unlike you, Mr Raey understands the
    topic and can explain things.

    Now, tell me about Lenz's law.

    Chimera
     
  17. Airy R. Bean

    Airy R. Bean Guest

    Oops! Faraday's Law. Close, though!
     
  18. PCK

    PCK Guest

    its your reputation that is in question airy
     
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