# Current in an open circuit

1. Aug 15, 2010

### Noesis

I've always wondered why is it possible for charge to flow in an open circuit, e.g., when a terminal is connected to a grounded pole, a large metal chassis, or say a water pipe.

I've assumed that it's because there is a positive potential difference between where the charge is and the large conductive body, and so its potential energy is dissipated in moving towards the conductive body which thus creates current.

The circuit can remain open, with no return path, since there is no charge buildup as there would be on say a parallel plate capacitor.

Are these thoughts correct? Any further insight into the matter? I noticed that it seems that this is how Leyden Jars were originally charged, the outer foil on the jar had no return path. Please let me know if I'm wrong regarding this as well.

2. Aug 15, 2010

### mgb_phys

Current doesn't flow without a path
It's just that sometimes the path isn't obvious.
There is a path through the earth from a grounded pole, on a Leyden jar there is a path across the glass or through the air.

3. Aug 15, 2010

### Curl

The Layden Jar is a capacitor.

The grounded circuits don't work without a return path. If you are using a battery as a voltage source, the reaction inside the battery cannot occur unless there is charge balance in the electrochemica reaction.

If you are using a change in magnetic flux as your emf, then the wire cannot simply give away charges without getting the charge back. Okay, maybe a few electrons can get out but the coulombic attraction will become large very quickly, and the wire will stay approximatelly neutral.

4. Aug 15, 2010

### Noesis

Well how would we explain say lightning, where is no apparent return loop? Or when a circuit is grounded to earth in case of an overabundance of current, and the charge will indeed travel to the ground. There doesn't seem to be a return loop in either example, and it seems that what makes both events occur is the high potential difference and the presence conductor on the other side.

Regarding a Leyden jar: it is seemingly charged without a return loop, that is the capacitor's plates are not put across a battery. The original jar was connected to an electrostatic generator at the top and then charged by it, hence current temporarily existed (within its RC time constant I imagine) before the capacitor was charged. Again, how can charge ever flow without a return path?

This, IMO, http://www.wonderhowto.com/how-to-build-leyden-jar-and-use-capacitor-272817/" [Broken] regarding capacitors also made me question this. You can watch the video, or fast forward until ~3:50 for the construction of a miniature Leyden Jar, and at ~4:37 he comments that he will connect the red wire to a faucet.

There is no way that charge is returning to the yellow wire in the video as it would have to traverse relatively enormous quantities of air, and arcing would be very visible.

Last edited by a moderator: May 4, 2017