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Question about capacitors

  1. Feb 10, 2005 #1
    Hello All,

    I'm a novice when it comes to electronics and I have what I think is a really basic qoestion concerning capacotors. If you have a capacitor and a battery in a circuit so that the battery is set to charge the capacitor, is the capcacitor charged by the voltage potential in the battery alone or is there an actual movement of electrons from the barttery to the capacitor plate in order for it to charge? I also have a couple of other questions but I'll wait on those for the moment.

    Thanks,
    Jason O
     
  2. jcsd
  3. Feb 10, 2005 #2

    chroot

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    Initially, when the capacitor and battery are not connected, there is no potential across the capacitor, and a non-zero potential across the battery. When you connect them, the difference in potential drives electrons from the negative terminal of the battery onto the negative plate of the capacitor. The potential difference drives a current.

    As this happens, the potential across the capacitor increases, and the current decreases. When the potential across the capacitor matches the potential across the battery, there is no longer any potential difference, and therefore no more current.

    - Warren
     
  4. Feb 10, 2005 #3
    Hmmm.... ok that makes sense. Now, here's my next question. Is it possible to charge a capacitor without electrons actually flowing into the capacitor? Maybe this is more of a physics question i'm not sure, but what is the energy that constitutes the charge coming from? Do the electrons actually have to be squeezed onto the plate of the capacitor for it to be considered "charged" or is there some sort of transfer of potential through the electrons from the battery to the capacitor plate that allowes the cap to be charged?

    The reason I ask this is I've heard some storys from auto mechanics about how sometimes someone would being in a car that would not start for some odd reason, but when they measured the voltage over th battery cables and in all the other electrical systems, their meters showed that the battery was fully charged; the stator motors would even work fine when they were connected again to the same battery using a different set of battery cables. But for whatever reason, the original cables must have been corroded or something and wouldn't let any current pass through to run the stator motor but yet someone connected a capacitor to the cable and was able to continualy charge the capacitor and discharge it over another circuit using the bad cable. Any idea of what might be going on here?

    Thanks,
    Jason O
     
    Last edited: Feb 10, 2005
  5. Feb 10, 2005 #4

    chroot

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    As I've already explained, you cannot charge a capacitor without moving electrons.

    The reason the battery cables didn't work is that the corrosion produces resistance. When there's no current flowing through the resistance, ohm's law says that the voltage drop across it is zero volts. In other words, when there's no current, the voltage will appear to "go right through" the resistance unimpeded. If you actually begin drawing current through that resistance, however, a voltage drop will appear across it, "wasting" the voltage that the starter motor needs to turn.

    You can consider the water-pipe analogy: if you take a garden hose and stuff a sponge into it (analogous to a resistor), then cap off the end, the pressure (analogous to voltage) will eventually equalize everywhere inside the hose. If you just used a pressure meter at the end, you'd never know there was a sponge inside the hose.

    However, as soon as you open the cap and let water flow, you'll see that the flow (analogous to current) is drastically reduced by the sponge. The pressure on the near side of the sponge will also be very low as compared to the pressure elsewhere in the hose. When water is flowing through it, the sponge creates a pressure drop, in the same way that a resistor creates a voltage drop.

    - Warren
     
  6. Feb 10, 2005 #5

    Integral

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    I have experienced this failure in battery cables. Even though they checked fine with my Ohm meter, the car would not start. The reason for this is that the voltage drop is negligible for small currents, as far as continuity goes there is no issue. The major difference it that the current drawn by your starter is >50A at these currents even small resistances can cause problems.

    To use a variation on Warren's hose analogy. Suppose you had a fire hose with a constriction at some point that reduced the inside diameter to that of a garden hose. If you tested this fire hose by connecting it to a garden hose and running water through it from the garden hose. you would not see any reduction in flow, it would appear to be just fine. But if you connected it to a fire hydrant, your flow would still be similar to that of the garden hose. You would see a huge change in expected behavior. This is your bad battery cable, it needs to carry a very large current to start your car, any test that does not match this current load is not viable.
     
    Last edited: Feb 10, 2005
  7. Feb 10, 2005 #6
    Ahhh okay I see. Thanks for the thurough explanations :-).
     
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