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Confused about short circuit and open circuit

  1. Apr 26, 2015 #1
    i know that short circuit is when current is max while resistance is zero and open circuit is when resistance is infinite and current is zero, but in terms of voltage, if i have a part of circuit and i want to know whether it is short or open circuit and i only know : the voltage and if the current is passing through the circuit or no. is the following true ? : the potential difference between the terminals of a short or open circuit is zero, but if a current is passing through the circuit so it is a short circuit while if there is no current is passing so it is an open circuit.
     
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  3. Apr 26, 2015 #2

    Drakkith

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    Staff: Mentor

    Pretty much.
    The potential difference between two points in the circuit that are shorted together is near zero and the current will be very high.
    The potential difference between two points of an open should be equal to the applied voltage to the circuit (or to the applied voltage of that leg of the circuit), while current is zero.
     
  4. Apr 26, 2015 #3
    so that sums it up in :
    for a short circuit : the potential difference between the terminals is zero so the potential difference between the terminals and the voltage source is maximum so the current between the terminals and the voltage source is maximum and the resistance is minimum
    for an open circuit : the potential difference between the terminals is maximum (equal to the voltage source) so the potential difference between the terminals and the voltage source is zero so the current between the terminals and the voltage source is minimum and the resistance is maximum
    right ??
     
  5. Apr 26, 2015 #4

    davenn

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    no, that's the complete opposite of what Drakkith told you

    reread his post again
     
  6. Apr 27, 2015 #5

    Drakkith

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    The potential difference between the terminals of your voltage source (assuming an ideal voltage source with zero internal resistance) would be equal to the applied voltage. The resistance between nearby points along the short circuited path is non-zero, which is why you have a finite amount of current. So the voltage drop between any two points along this path is also non-zero. The further the distance between the two points, the larger the resistance and the larger the voltage drop between them.

    In basic terms, if you measure the voltage and current between two points of the circuit, and find that the voltage drop is low and the current is high, then yes, you have a short circuit. If you instead find that current is zero and the voltage drop is equal to the applied voltage, then you have an open.

    That's right. In an open, measuring the voltage drop between one terminal of your voltage source and the connecting side of an open would give you zero voltage drop and zero current. Measuring across the open would give you zero current and a voltage drop equal to the applied voltage.
     
  7. Apr 27, 2015 #6
    i meant by terminals the terminals of the circuit and by voltage source the applied voltage, sorry for being unclear
    thanks man !
     
  8. Apr 27, 2015 #7

    Drakkith

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    I can't remember seeing the word 'terminal' used at anything except to refer to the connections of a voltage/current source.
     
  9. Apr 27, 2015 #8
    i meant to say endpoints* :D
     
  10. Apr 27, 2015 #9
    to sum up again with correct words :
    assume constant voltage applied for both cases :
    short circuit : potential difference between endpoints of short circuit is low, resistance is low, current is high
    open circuit : potential difference between endpoints of open circuit is equal to the voltage applied, resistance is high, current is low
    right ?
     
  11. Apr 27, 2015 #10

    Drakkith

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    Staff: Mentor

    That's right.
     
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