# Does a current instantaneously disappear?

1. Sep 11, 2011

### perplexabot

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. Sep 11, 2011

### 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.

3. Sep 11, 2011

### phyzguy

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.

4. Sep 11, 2011

### perplexabot

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.