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Open Circuit

  1. Aug 30, 2014 #1
    Hello,

    Suppose we have an electric circuit consists of a battery and 2 wires connected to both ends of the battery. When the two wires are not connected together (open circuit) do the surface charges of both conductors accumulate? if yes, can we say at the end of both wires separated by the air,do they act like capacitors?

    Thank you
     
  2. jcsd
  3. Aug 30, 2014 #2

    ehild

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    Yes, but that is a very little capacitor!
    Think the diameter of the wires is 2 mm and they are 1 mm apart, what is the capacitance then?

    ehild
     
  4. Aug 30, 2014 #3

    CWatters

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    Yes. There is also capacitance between the wires along their length. eg not just at the ends of the wires.

    There is also capacitance between the two terminals of the battery. You can divide that capacitance into two components... The small capacitance between the two physical terminals and the capacitance between the plates inside the battery.

    Also capacitance between any of the above conductors and the earth or your body. In short.. there is usually capacitance between any two conductors you care to identify although much of the time it is small enough that it can be ignored.
     
  5. Aug 30, 2014 #4
    Something that might help is thinking of it as electrical pressure. The battery increases the electrical pressure (potential) so those electrons will want to spread out as much as possible. This is why you will still probably get shocked if you stuck a paperclip in one of the live pins in a wall socket and simultaneously jumped in the air. Since you're not touching the ground or the other live pin, there is no closed circuit, but your skin and body are still at a lower electrical pressure than the outlet so there will be a current for a small amount of time until your body and the outlet are in equillibrium. (actually, an alternating current would complicate it, but that's the general idea)
     
  6. Aug 31, 2014 #5

    sophiecentaur

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    Why use the term "pressure", which stands for something entirely different? Pressure is Force per Unit area. Which particular area would you be using, to apply this model? It's bad enough when we use the term Electro Motive Force (emf) but this is an acknowledged exception and is well known not to be a Force.

    I realise that the 'water model' is there inside your explanation. That model is so full of holes that you risk getting very confused when you actually try to use it to make predictions.
     
  7. Aug 31, 2014 #6
    Thank You all. Fully agree with Sophie.
     
  8. Aug 31, 2014 #7

    sophiecentaur

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    As a ball park - rule of thumb guide to typical capacitance, you can expect around 50pF per metre of wire, next to an Earth. AS for two pieces of wire, it would depend on the layout.
     
  9. Aug 31, 2014 #8
    Ok, maybe pressure isn't an appropriate word, but wouldn't the notion of a circuit reaching equilibrium still apply without the water model? It's always helped me, but if it has a blatant disadvantage or inconsistency that I'm looking past, I'd love to know.
     
  10. Aug 31, 2014 #9

    sophiecentaur

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    Is there any good reason for not wanting to see the situation 'as it is', rather than trying to apply a risky metaphor? If you get used to the conventional terminology and models, they will get to be just as cuddly as other attractive (at first) alternatives.

    The way it is normally described is that the charge flow continues until the PD equals that of the source. This is, as you say, an equilibrium condition. (Q=CV, where C is the capacitance)

    Btw, by "as it is", I mean to say as it is currently modelled by established Science. I don't actually hold with any idea of an 'ultimate' truth - just good working models.
     
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