Sometimes while solving problems Kirchoff's Voltage Law, there comes a resistor placed in such a position that in one of the loops the potential across that resistor is actually taken positive (which shows an increase). My question is how can this be possible, not that I doubt it but i need an explanation that explains it on the electron level. For example I understand that electrons normally loose energy due to work against the atoms inside a resistor. I need an explanation of how electrons gain potential (or energy) as they move across a resistor in the opposite direction of normal current flow. A microscopic (electron-scopic) scale explanation would be appreciated.


The one thing you can take for sure is that whichever is the direction of the voltage drop, is the direction of the current. So if a resistor is placed so the voltage goes up, instead of down, it means the current is going to go the other way. It's a lot like elevation and the flow of water in a river. The electrons will be pushed in the direction of downward voltage drop.

  • $\begingroup$ But its quite impossible for water to flow upstream. However in a closed loop you have to take current flow in a specific direction which results in the case described. I don't understand the manner in which this potential increase occurs since resistors primarily reduce voltage. $\endgroup$
    Sep 23 '16 at 17:18
  • $\begingroup$ But if you guess wrongly about the direction of current flow, or you make a sign error in your calculations, then that will resolve your problem. As Ken G says, if you are sure that the potential drops in a certain direction for a resistor, then the conventional current is definitely in the same direction. $\endgroup$
    – Bill N
    Sep 23 '16 at 17:31
  • $\begingroup$ So you're saying that the increase in voltage is merely a theoretical supposition created to satisfy the problem? $\endgroup$
    Sep 23 '16 at 18:20
  • $\begingroup$ Water can only flow uphill if it has a huge momentum already, it's generally not the way rivers work. Electrons do not have an inertia that could flow against a potential rise like that. Instead, imagine that electrons accelerate over a short distance and are stopped by a collision in the resistor, then do it again. When they move too freely, they have a way of cancelling out the field that would otherwise accelerate them. But in a resistor, the field is allowed to build up expressly because of the electron's limited ability to move, and that is why you get a potential drop. $\endgroup$
    – Ken G
    Sep 23 '16 at 22:07

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