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

Capacitors strikes back

  1. May 13, 2003 #1
    Hello everybody.

    Can't surrender so much easy... Take a look to this:


    You know the proccess: tell where do you see the faults, and then i will understand somethings, but new doubts will come to my mind.

    Thanks for all your support in physics, you're making a good job into my understanding, first on centrifugal-centripetal, then on magnets and finally into electrostatics. Will it be the end?... SURE NOT!
  2. jcsd
  3. May 13, 2003 #2
    Cala said:

    "1 - You have some kind of device that makes 12 V voltage difference between two points, but no current can go through this device, so it never discharges. From the 12 V point you connect a 12 V charged capacitor, then a LOAD, and then two parallel 6V charged capacitors."

    Then your resistor is under 6 v potential, right? Thus, the current I=6v/R is passing via loop - source and all capacitors. This contradicts to your requirement of "no current can go through this device".
  4. May 14, 2003 #3
    What i mean is that the 12 V device force the capacitor to discharge through the load.

    What i want the device to do is add voltage to the capacitor, but avoiding this capacitor to discharge to that part of the circuit. (keep the voltage, but without current allowed to flow).

    I mean, the 12 V capacitor is not allowed to discharge into the device, but the device force the capacitor to see 12 V added to its internal 12 V, so it discharges to the right side, not to the left.

    Is there not a method to put some point into 12 V without allowing any current going back?
  5. May 14, 2003 #4
    How can you have non-zero voltage on a resistor and zero current via it? This simply would mean that the resistance is infinite (no resistor in your circuit, just open circuit (discontinuity) in this place).
  6. May 15, 2003 #5
    1. Charged capacitors in sequence cause voltage on resistor to be sum: 24V
    2. Current flows only when circuit is closed, same current must flow in both of left capacitors. If your leftmost capacitor does not offer current, its internal resistence is infinite, and no current will ever flow. You can't 'discharge' upper capacitor.
    3. There is no source to recharge upper capacitor to 24V potential.
    Add +/- signs to capacitors to better understand what happens.
    4. Capacitors store charge. When you divide it to two capacitors, each will have half of it. Your rightside capacitors will have each half of charge. When you further use one of them to 'recharge' upper cap, it will again have half of that half. You loose charge, whatever you think of voltage games.
    5. If your load does any work, there is no way you could get 12V on rightside capacitors. It will be less by amount of voltage drop over the load resistor.
    You might get this thing to work for 2-3 cicles, but then it would be empty by any measure.

    Finally, what you try is like take few bottles, try shuffling water around them in different ways, and hope to have same amount of water in bottles although draining some of it out to the sink (resistor)..
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook