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Does a current instantaneously disappear?

  1. Sep 11, 2011 #1


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    Gold Member

    Hi all. I was working on a circuit question with inductors and I then had a question. Lets say we have a simple circuit with only a resistor, an emf source and a switch. We first close the switch and the current goes though. Now after some time we open the switch.

    My question is: Just after we open the switch does the current instantly go to zero? Or will the resistor have a current running through it at that instant and then decay?

    Thank you
  2. jcsd
  3. Sep 11, 2011 #2


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    Staff: Mentor

    I believe the EMF takes a finite time to propagate, so when you open the switch not only is there a small amount of time that it takes for the switch to completely cut the voltage and current, the rest of the circuit should take a small amount of time to dissipate the remaining current.
  4. Sep 11, 2011 #3


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    If the inductance in the circuit were truly zero, the current would vanish instantly. However, all real circuits have some inductance, so the current will decay with a time constant given by L/R.
  5. Sep 11, 2011 #4


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    Alright, so assuming the circuit has zero inductance (which is what I think my physics book is doing) then the current will immediately stop after the switch is opened. Thanks.
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