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Could you please explain why Q=C*V?

Discussion in 'Electronic Basics' started by Boki, Jul 24, 2003.

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  1. Boki

    Boki Guest

    Hi, All:

    Could you please explain why Q=C*V?


  2. Don Kelly

    Don Kelly Guest

  3. grahamk

    grahamk Guest

    The larger the Capacitor is, and the higher the Voltage is, then the larger
    the charge( Q) is.
  4. That's a dimensional analysis way of looking at it. For a given
    capacitor, the higher the voltage the more charge you expect,

    q proportional to V

    and for a given voltage, the bigger the capacitor is the more
    charge you expect, so:

    q proportional to c

    There don't appear to be other obvious factors at this point,

    q proportional to c*v


    q = k1 * c * v

    with 'k1' being yet to be determined.

    Or, rearranged, C = k2*Q/V.

    Turns out, C is *defined* so that k2 is just 1.000 when Q is in
    Coulombs and V is in Joules per Coulomb. Capacitance is in

    Add to this, an Ohm is a Joule-sec/Coulomb^2 (a Joule-sec is a
    measure of angular momentum, for example, and is the units of
    Planck's constant -- it's also just kg*(m/s)*m.)

    Multiply units of R*C:

    (Joule-sec/Coulomb^2) * (Coulomb^2/Joule)

    and you get seconds! Good thing.

    Since a Henry is just a Joule-sec^2/Coulomb^2 (a kind of
    Ohm-second or Joule per amp^2), the SQRT(L*C) also works out to
    seconds. Also a good thing. And L/R is also in seconds.
    Another good thing.

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