# Basic question

Discussion in 'General Electronics Chat' started by Battousai, Nov 14, 2003.

1. ### Battousai Thread Starter Senior Member

Nov 14, 2003
141
44
Hi,

I'm new here but I'm kind of familiar with circuits. I'm taking some analog design courses in college. However I have a basic question that was never answered. How do electrons in a wire know where to go? If you have a junction and one end of the junction (let's say the left side) is broken (open), and the other side (right side) is free to conduct then how do the electrons know to go down the right side instead of the left side? In reality are some of the electrons going into the left side, hitting a dead end and heading towards the right side?

Thanks.

2. ### Dave Retired Moderator

Nov 17, 2003
6,960
145
The electrons would cease to flow down the left side, and current flow would be channels solely down the right side.

Think of it in terms of Kirchoff's Current Law I(1)=I(2)+I(3)

Where I(1) is the current into the node, I(2) is the current down the left side and I(3) is the current down the right side. If the left side is broken (open as you say it) then current won't flow down there because the potential across it has gone. The potential will remain the same across the right-side but by using KCL above, you can see that I(1)=I(2), i.e. all the current flows down the right side, and current is merely electron flow.

It isn't the electrons "knowing" were to flow, its that they are attracted to flow down the right hand side, and electrons in the left side will fall back into the bonds with the atoms in the wires on that side as they don't have the potential to move.

Sorry if that made little sense, I'm in a bit of a rush, but will check back later if you wish to reply.

3. ### Battousai Thread Starter Senior Member

Nov 14, 2003
141
44
It does help some, I guess I'm interested in the dynamics. If I have two wires running in parallel, and I cut one wire, what happens?

I guess the potential in the cut wire should build up so that there is no current flow down that wire... but how does it happen concenptually? It sounds like a capacitor... :blink:

4. ### Dave Retired Moderator

Nov 17, 2003
6,960
145
Remember that a capacitor still has a voltage across it, hence thats why the charge builds up on it. Two wires in parallel and you cut one then the voltage is solely across the wire that isn't cut.

If your interested in the physical dynamics of the electrons, then you will have to think about it interms of an energy band diagram.

When the voltage is across the wire the electrons will possess energy and go into the conduction band of the material, where they are free to move around as current. When you remove the load the electrons will lose energy and will fall back into the valence band where they will be bound by the strong bonds with the atoms there, hence the current will cease to flow.

Don't forget some electrons will remain in the conduction band due to thermal energy and what not, but there amount will be tiny and an ammeter would fail to pick up there effect. But also because there is no load there movement in the conduction band will be completely random.

5. ### Battousai Thread Starter Senior Member

Nov 14, 2003
141
44
The energy band explanation makes sense. I never thought of it like that (probably because I don't like thinking in terms of bands... ). Thanks for all your help. B)