# Path of an electron through an electric circuit

When a potential difference is applied across a conductor, and if an electron moves from the negative terminal of the battery and reaches the positive terminal, then I want to know if the electron will remain at the positive terminal or will it again move toward the negative terminal through the battery?

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How do we define "battery"? If we have a black box with a capacitor inside is this a battery? How about a fuel cell? Or inside the black box we have a Van der Graaf generator driven by some chemicals that we can continuously add/replace to keep it going indefinitely? – Maxim Umansky Jul 14 '13 at 3:07

## 3 Answers

Electrons that reach the positive terminal indeed remain there. The potential difference between the two terminals pushes electrons from the negative anode toward the positive cathode. When an electron reaches the cathode, it stays there to equalize the original charge imbalance between the two nodes. When electrochemical redox reaction sustaining the electron movement equilibrates, the motion will stop and the battery will "die."

As the diagram shows, the two terminals are connected by a "salt bridge." But the salt bridge is specifically designed to prevent electrons from flowing directly from the anode to the cathode. So the electrons can only flow through the circuit.

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This really helped me to understand what I wasn't able to understand for many days. But, How can electrons from the positive terminal go any where if they are attracted by the positive terminal? – Samama Fahim Mar 21 '13 at 20:44
@SamamaFahim I am trying to make a cartoon model of the battery for a nephew, and this is a good question. In my cartoon, the electrons in the battery must hop a ride on 'ion cars' to get to the other side. I have them filling empty box-cars when they arrive at their terminal when the battery is discharging, and the filled box-cars (ions) pumped to the other side when the battery is charged. This may be way off base. Any help from the experts? – Bobbi Bennett Mar 22 '13 at 0:14
Samama, you are correct--the electrons that reach the positive terminal do stay there in an attempt to equilibrate the charge imbalance between the two terminals. – dannygoldstein Mar 22 '13 at 0:52
@willie.holdman - if electrons just stop at the cathode and end their joruney there, so no electron can pass to the anode because of the salt bridge, then where does the current inside the battery come from? I mean, battery heats up, and there's a current there and we can calculate it with Ohm's law, but why is there any current inside the battery if no electrons can jump between anode and cathode? – www Jul 12 '13 at 22:12
@WalterWhite, I have explained it in my answer. – Val Jul 14 '13 at 16:53

Battery is an electrolyte and positive ions can move in it rather than electrons. Similarly to the current of holes in semiconductor, directed flow of such ions complements the flow of electrons in the wire.

Here, electron-cation pairs are created at one electronde of the battary and recombine at the other (you may see that one electrode is destroyed while some material is deposed at the other by simply passing a current through the salty water).

Opposite flow of positive charges in battery/semiconductor is exactly equal to the direct current of the electrons in the conductor. You can think of positive charge moving in one direction as the current of negative charges moving in opposite direction.

Other answers just tell you that (a tiny pulse of) ion current is made first in the battery. It creates the voltage difference between anode and cathode. This causes electron current in the wire. The electrons stop at the positive electronde, thus, reducing the voltage. The battery restores the voltage by pushing more positive ions to that electrode. The flow of these ions is the current that you miss.

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In an ideal electrochemical cell case, it should remain at the positive terminal. The salt bridge should only allow for the ions to flow.

However, practical batteries have other physical phenomenon that restrict the flow of current, and hence introduce this 'internal resistance'. There could be a wide variety of reasons depending on the type of cell used. I believe one of the most common reason is polarisation. But that too, shouldn't cause the electrons to flow through the cell.

To quote a good explanation from University of Illinois Physics Dept Q&A page:

You might wonder why the electrons don't just flow back through the battery, until the charge changes enough to make the voltage zero. The reason is that an electron can't move from one side to the other inside the battery without a chemical reaction occurring. In other words, inside the battery plain electrons can’t travel around because it takes too much energy to put a plain electron in solution. Electrons can only travel inside the battery via charged chemicals, ions, which can dissolve off the electrodes. The chemical reaction is what pushes the electrons inside toward the negative end, because the electrodes at the two ends are made of different materials, which have different chemical stabilities. So overall, electrons flow AROUND the circuit, toward the negative end inside the battery, pushed by the chemical reaction, and toward the positive end in the outside circuit, pushed by the electrical voltage.

Electrical current can flow in the other way in the battery too, if the battery is hooked up to something with a bigger voltage difference (a battery charger, for example).

EDIT: As to why there is current flow inside the battery: Electrons are not necessary for current to flow. The flow of ions does happen inside the battery.

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