# B Please help me understand how a battery works

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1. Dec 24, 2016

### gigie

There is an explanation of how a battery works that says that in a battery circuit, the electrons do a complete loop and, given enough time, they can return to their starting point. This theory says that when the electrons arrive in the positive terminal and they lost all of their potential energy, the battery does work on the electrons to put them back in the negative terminal so they are full of potential energy and reenter the circuit.

But, is this theory true? If it is, could someone explain me how? From what I understand of batteries, there aren't any electrons that move from the positive terminal to the negative terminal of the battery. The electrons of the circuit can come from the molecules of the anode and once they reach the positive terminal there is nothing else happening except that they detach from the positive electrode and make the electrolyte ions neutral. They stop at the electrolyte. They don't return to the negative terminal.

This theory is on the two last paragraphs here : http://www.physicsclassroom.com/class/circuits/Lesson-1/Electric-Potential

2. Dec 24, 2016

### ZapperZ

Staff Emeritus
3. Dec 25, 2016

### SleepDeprived

You may already said in your post but I just want to add that we may want to start by trying to understand how a current flow and what makes a current flow.
Most people have the wrong impression that the electrons actually travel at the speed of light, but it's more like the pulse of energy or the formal term is electric field that travels at the speed of light or close to it. The electron will push its neighbor electron which in turn will push its neighbor ..., which a pulse of energy that travels at or close to the speed of light.
As what makes a current flow, it is caused by an electric field. An electric field will "push" the electrons to make it "flow". The battery uses a chemical process to separate the positive charges and negative charges which in turn gives an electric field that will "push" the electron to make it flow. The same charge that leaves the positive terminal will be replaced by another charge coming back to the negative terminal (although may not be exactly the same electron).
Voltage is caused by an electric field and is defined as Joules per Coulomb.

4. Dec 25, 2016

### gigie

The last equation of the text says that to have a voltage, the battery MUST do work on a charge at some point. Your text says that the battery DOES work on the charge, but not how. I'm still a little bit confused...

5. Dec 25, 2016

### SleepDeprived

I guess you can use the corpuscular approach -- imagine charges like particles. But this is just an analogy and not complete.

6. Dec 25, 2016

### gigie

This is something that I have trouble understanding. The thing that makes electrons flow in a battery circuit is the accumulation of electrons in one side and the lack in the other side. This makes the electrons push each other, there isn't any field involved there? The general movement of electrons because they push each other is not the same thing as saying that the electrons move because of an electric field? The definition of a field is "a region in which each point is affected by a force". The movement in a battery circuit is more caused by an electrical "pressure" than a field...

7. Dec 25, 2016

### SleepDeprived

I guess you could be right but you could be wrong -- I don't know. Voltage is defined as Joules/Coulomb or the amount of work a Coulomb has. Or Voltage is defined as dV = -Eds where ds is the differential distance and E is the electric field. I can understand how you would think the current induced by the battery has nothing to do with field but by "pressure" from a chemical process, but then others would say you cannot argue against the equations. If there exists a voltage, there exists a electrical field. Remember Voltage is a scalar whereas field is a vector.
Maybe someone can reconcile your thinking and what the equation says.

8. Dec 25, 2016

### Staff: Mentor

My understanding is the same as yours. For instance, in a typical copper zinc cell a zinc atom leaves the electrode and enters solution, leaving behind a pair of electrons in the metal. Those electrons travel (slowly) through the circuit to the copper electrode where they neutralize a copper ion in solution which plates on to the electrode surface.

The current in the electrolyte is carried by a flow of sulfate ions, but the sulfate ions don't become charged with the electrons that go to the copper electrode.

9. Dec 25, 2016

### SleepDeprived

I guess this picture explains how you can reconcile the chemical vs. equation point of view. A link is also provided.
For a rigorous treatment, I would recommend the online course from nanohub at Purdue university.
http://www.qrg.northwestern.edu/projects/vss/docs/power/2-how-do-batteries-work.html

The cathode and anode are separated by the electrolyte which is essentially a insulator. The electrons at the anode cannot direct move to the cathode directly.
First the excess electrons at the anode and excess positive charges at the cathode created an electrical field. Hence the V = Eds equation.
When the battery terminals are connected by the wire, the electrons start flowing along the wire to the cathode side. The negative and positive charge will eventually combined and the battery will run out of charge. The chemical reaction at the terminals allow the charges to recombine. I left out the detail of the actual chemical reaction but I guess you could look it up.

10. Dec 25, 2016

### Delta²

I believe you are right that the electrons don't flow through the electrolyte. However I believe that the chemical reactions that happen between the electrolyte and the cathode and between the electrolyte and the anode are such that the electrons are LIKE they flow through the electrolyte. I believe that the chemical reactions are such that for every electron that leaves the electrolyte through the anode, another electron reaches the electrolyte through the cathode. The ratio cant be exactly 1:1 since that would mean that the battery doesn't discharge but it must be close to 1:1.

Unfortunately I haven't studied chemistry a lot since my high school days, someone else has to do the tedious but precious task of presenting us the chemical reactions that happen inside a typical battery in order to fully understand what's happening.

11. Dec 25, 2016

### Staff: Mentor

From hyperphysics[/PLAIN] [Broken] on a zinc-copper electrochemical cell:
The reactions occur because copper has a higher reduction potential than zinc, which basically means that copper is going to react and "pull" electrons from zinc (reduction is the transfer of electrons to an atom/molecule, contrasting with oxidation which is the transfer of electrons from an atom/molecule).

Last edited by a moderator: May 8, 2017
12. Dec 26, 2016

### Delta²

So it is like we have a flow of electrons in the solution, but what we actually have is a flow of negatively charged ions (that do NOT carry the electrons of the oxidation/reduction reactions as Dale pointed out).

I suppose when the battery is fully charged we have much higher concentration of the negative sulfate ions on the copper side than on the zinc side, and what causes the movement of the negative sulfate ions is the phenomenon of osmosis? (because if I got it right the sulfate ions move in an opposite way of what we would expect according to the external macroscopic electric field in the solution...)

Last edited: Dec 26, 2016