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Homework Help: How can voltage flow in a open circuit

  1. Nov 10, 2014 #1
    1. The problem statement, all variables and given/known data
    Hi Guys,

    I am wondering how voltage can flow in an open circuit as I = 0 so surely V=IR; V = 0*R so V = 0

    I know it does as when a capacitor is fully charged no current flows yet there is a voltage.

    I'm about confused thanks!
    2. Relevant equations
    V = IR

    3. The attempt at a solution
  2. jcsd
  3. Nov 10, 2014 #2


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    Staff: Mentor

    Voltage doesn't flow. Current flows.

    Current flows when there is a potential difference between two points and a conductive path between them. If the potentials at the two points are equal then no current flows.

    A capacitor will accept charge (current will flow) so long as its potential difference is different than the potential difference across where it connects in the circuit if the capacitor happened to be removed (the "open terminal" potential difference).

    If you think about it, a battery that's not connected to anything also maintains a potential difference between its terminals, yet no current flows.
  4. Nov 10, 2014 #3

    Simon Bridge

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    I think you need to be clear about what you mean by "voltage flow".
    In the case of a charged capacitor, would you really say that a voltage is flowing? Where is it going?
    You would usually talk about a current flowing.

    In the capacitor example, while it is charging, a current does flow - even though there is literally a break in the circuit. Can you see how this can be?

    It is technically possible to have a signal represented as a voltage changing with time and position along a wire.
    In this way it is possible to set up a travelling voltage wave in the wire - so, in this sense a voltage can be said to flow.
    It is possible to send such a wave down a wire that is not connected to anything just like you can send pressure waves down a pipe.

    When you start out learning about electric circuits, you are usually learning about the special case of DC steady-state circuits.
    The current and voltage you are no doubt used to are defined with that situation in mind, and, in that situation, no electricity flows when there is an open circuit.
  5. Nov 11, 2014 #4
    V = 0 Volts is a short circuit, and I = 0 Amps is an open circuit.

    In an open circuit, the voltage is not zero because the resistance is infinite. All you know is that voltage = V = IR = 0*infinity. This is a meaningless value, and the equation essentially means the voltage can be any finite number. This is why you can have some voltage at an open circuit and have zero current due to the open circuit being represented by an infinite (or very large) resistance.

    Hope this helped to clear things up!
  6. Nov 11, 2014 #5

    Simon Bridge

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    ... those are usually considered to be the DC consequences of an open or a short circuit - the labels are usually used to describe the physical condition of the circuit.

    An electric circuit is an "open circuit" if it lacks a complete path between the terminals of its power source.
    An electric circuit is a "closed circuit" if it contains a complete path between the positive and negative terminals of its power source.
    A short circuit is a type of malfunction of an electrical circuit.
    ... Wikipedia.
  7. Nov 11, 2014 #6


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    If you have an ideal voltage source connected to a resistor it's a mistake to think that the voltage drop across the resistor is determined by Ohms law. It's better to see it as the ideal voltage source dictating the voltage and then Ohms law dictates the current.

    Same applies to an open circuit. The voltage is determined by the voltage source. The current is determined by Ohms law I = V/R = 0
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