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What is the point in using a dielectric

  1. Oct 21, 2004 #1
    Hi tide.

    question. What is the point in using a dielectric. I was doing a problem in the physics book and it showed that when you use a dielectric, the amount of energy stored in your capictor decreases. Wouldent you want to store the MOST amount of energy in your capacitor? So why bother using it. I guess it is only good when you have a FIXED voltage difference at your power supply. In that case, you dont have to worry about voltage going down as you use the dielectric. Therefore your charge increases, and your total stored energy also increases. Is that correct?
     
    Last edited: Oct 21, 2004
  2. jcsd
  3. Oct 22, 2004 #2

    Tide

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    I don't know what the conditions were for the comparison you made but it's not just energy that one may be interested in. For a given amount of charge on say a pair of conducting plates the potential difference across the plates will be smaller when a dielectric material is present compared to the situation with empty space between the plates. Said differently, the dielectric will permit more charge to be stored for a given voltage.

    This has consequences in terms of things ranging from RF circuits to production of relativistic electron beams. In the latter case, one could charge up a number of capacitors in parallel and discharge them in series providing high voltage.
     
  4. Oct 22, 2004 #3
    TIDE YOUR ONLINE EARLY!!!!! LOL.

    Can you, or anyone please explain how a capacitor works. Im just not getting it.

    I have a battery.
    I hook up ONLY one plate to one terminal of the battery. Now that plate is at the same positive potential as the battery terminal. WHY!? Im doing loops around this all week. If the potental of the wire changes, then charge had to move in the wire?

    Now I do the same to the other plate.

    So these two plates are now at the same potental as the battery.

    Now I move the two plates close together.

    So charge starts to move now?

    What the hells going on, :-(

    So does the electrons move from one plate to another when I bring them close together, or do they move when i hooked it up to the battery, or a combination of both.

    Let me ask it like this also,

    IF each wire goes to the same potential as each terminal on the batter when its hooked up,

    and charge moves from one plate to another when i bring them close,

    then wouldent this imply that there is a different voltage across the plates, because now charge has moved, affecting the voltage across?

    Its hard for me to pose the question clearly, sorry, im in a tangeled web of confusion at this point.
     
  5. Oct 22, 2004 #4

    Tide

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    Hey, Cyrus!

    You are confused - maybe you need more sleep! Just kidding!!

    Seriously, you are playing a bit fast and loose with your interpretations.

    First, the potential of a wire remains unchanged when you connect one or both ends to a battery - remember, being a conductor it remains an equipotential!

    Second, the terminals of the battery themselves form a capacitor. Capacitance varies roughly in proportion to the area of conductors and inversely with the separation between them (I'm ignoring other complications of geometry). The battery has very low capacitance and so the terminals typically hold very little charge.

    Third, in the scenario you described hooking up the plates of the capacitor and then bringing the plates together charge doesn't just start moving at some magical point. You are changing the capacitance as you bring the plates together. Even when they are far separated, there is finite capacitance and they will hold a charge albeit a very small one. In any case, charges do NOT just move from one plate to the other. They are driven to and from the plates by the battery but don't cross the gap!

    It's not simply the potential difference that drives electrons. What drives them is electric field. For a given potential difference between a pair of plates, the electric field will be stronger when the plates are closer together (think of Coulomb's Law here!).
     
  6. Oct 22, 2004 #5
    So if I were to hook a single wire to one end of a battery, it would not go to the same potential as that terminal? I thought if a battery is say, 1.5 volts, then we can think of one terminal as 0 potential, and the other terminal as 1.5V. If i hook it up to the 1.5 end, the wire wont be 1.5V also?
     
  7. Oct 23, 2004 #6

    Tide

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    What, exactly, is the potential of the wire before you connect it to a terminal? What's the potential of the wire when you connect its other end to the second terminal?

    For that matter, what's the potential of the first terminal before you do anything at all? All that you know is that the potential difference between the two terminals is X Volts or whatever.

    Again, it's not the potential per se that moves charge - it is the electric field or gradient of electrical potential. It might help to visualize the terminals as collections of charges. What happens when you move a conducting wire around the room in the presence of those charges? Not much happens - until you get the wire close to a charge! Wave one end of the wire close to one of the charges (terminal) and the charges in the wire redistribute themselves to provide an equipotential over the length of the wire. Move that end of the wire over toward the second charge (terminal) and the same thing happens - the charges in the wire once again redistribute themselves to provide an equipotential.

    What happens when your wire makes contact with a terminal? At that point it becomes an extension of the terminal with charges of both the terminal and the wire redistributing themselves to form a single equipotential entity. It doesn't matter what the original "potential" of the wire was.
     
  8. Oct 23, 2004 #7

    Chronos

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    Capacitance works on the energy potential between charged, conductive plates. The charge is retained because it is attracted, but, cannot efficiently cross the dielectric barrier. It is a hugely useful exploitation of quantum effects. I discovered, firsthand, the enormous power of capacitance when I opened up the power box of an old BW television and touched the metal cap on one of those big tubes. It had been unplugged for weeks, but, my finger insisted it was still hot. For a more authoritative source than my finger, see
    http://micro.magnet.fsu.edu/electromag/electricity/capacitance.html
     
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