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Zero voltage, non-zero current?

  1. Sep 18, 2012 #1
    Reading about solar panels i came across a peculiar description - "When the panel is short circuited, there is current but no voltage, so no power is being generated."

    How does that work?
    What does the current represent in that case, and what moves the needle of an ammeter?

    Another instance of the same problem happens when charging a big capacitor.
    At first, there is essentially no voltage across it, but several dozens of amperes of current is going into it. Once again power, 0*I, is about zero, and yet the power is being consumed into the capacitor.

    What am i missing here?
  2. jcsd
  3. Sep 18, 2012 #2
    When the solar cell is shorted, there is no external load and so no place external to the cell for there to be an IR drop. Hence, no power is externally delivered. Now in the case of an ammeter, an ideal ammeter is a device that draws no voltage (i.e. suffers no IR drop) but yet (somehow) measures current. In practice, a real ammeter draws "negligible" voltage and uses that negligible voltage to sense current. And so, a solar cell actually delivers power, albeit "negligible", to a real ammeter.

    Thank about your capacitor question again. From a circuits perspective, no current flows "through" an ideal capacitor; it merely stores energy but can never dissipate power. Furthermore, it is the current that may changed instantaneously, but not the voltage.
  4. Sep 19, 2012 #3
    There is a voltage and there is current and power dissipated. Just not much of it is being generated external to the device. None if you have a perfect conductor.

    All devices no matter how good have internal resistance(and inductance and capacitance)

    So when you place a short circuit across the output of your solar panels (or capacitors or battery etc) the voltage is developed across the internal components and the power is dissipated there.

    Of course this is why placing a short circuit across any power source can be quite destructive. The internal resistance is usually designed to be as low as possible. A lot of power could be dissipated, resulting in sometimes violent reactions.

    An ammeter does not represent a short circuit. It has an internal calibrated resistance. often in the form of a thick piece of copper.

    Placing an ammeter across the pv panels or capacitor does provide a load and power is dissipated within the ammeter. Just not very much. In the case of a moving needle ammeter very little power is required to move the needle full scale, so the load is very small. In a digital ammeter the load is even smaller as they are much more sensitive.
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