Physics Forums

## batteries

What does it mean when it's said that the positive terminal of a battery is at a higher potential than the negative end? How does electric potential (voltage) relate to electric current/electric fields?[o)]

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Recognitions:        Science  Advisor  Batteries (voltaic cells if you want to google it) work by having 2 chemical half reactions happen at the same time. Have a look at this table: reduction table The reactions can go either left or right, the voltage is reversed if the reaction goes right to left. Reactions that go right to left happen at the negative terminal. Reactions that go left to right happen at the positive terminal. The voltage of the cell is the sum of the voltages for each of the half reactions.    Now as to why the positive can have higher voltage, suppose you had a cell made of perchloric acid (fifth from the top on the left side) and iron (15 from the bottom on the right side). The perchloric acid reaction looks like this:  $$ClO_4^- + 8H^+ + 8e^- ---> Cl^- + 4H_2O$$ (1.39 volts)  The iron reaction looks like this  $$Fe ---> Fe^{2+} + 2e^-$$ (0.45 volts)  You can see that the positive terminal (the acid) has a higher potential than the negative terminal.   Voltage is sort of like the force trying to make electrons flow. Higher voltage means higher current.

You sound like a chemist…god bless you!!!  What do you mean when you say “2 chemical half reactions”?  Do you mean that there is a chemical reaction in each terminal?  How do the electrons know which is the positive and negative terminals?    Also, how can the reactions be reversible?  I thought that was only the case for rechargeable batteries?  Can you tell I’m not a chemist?  Keep it simple for the aspiring physicist over here...  [;)]

Recognitions:
Science Advisor

## batteries

What do you mean when you say “2 chemical half reactions”? Do you mean that there is a chemical reaction in each terminal?
That's exactly what it means. The cell is separate into 2 sides. For the perchloric acid and iron cell I mentioned, the perchloric acid is only on the positive side, and the iron is only on the negative side.
The reaction that happens at each terminal is called a half-reaction because 2 half-reactions are needed before anything happens. The perchloric acid and iron cannot just break down on their own. The acid needs the iron before it can react and the iron needs the acid before it can react.

How do the electrons know which is the positive and negative terminals?
It's based on how spontaneous the reaction is. Look at that table again, Fe 2+ is listed twice.

$$Fe^{2+} + 2e^- ---> Fe$$ Is lower on the table

$$Fe^{3+} + e^- ---> Fe^{2+}$$ is higher on the table.

In one half-reaction, it's being oxidized, in the other reaction, it's being reduced. The terminals on the cell are positive or negative because one of the half-reactions is a strong oxidation reaction and one is a strong reduction half-reaction. In the perchloric acid and iron cell, the acid is able to pull the electrons off the iron. If iron was able to pull electrons away from the acid, the iron would be the positive terminal and the acid would be the negative terminal. That doesn't happen because the acid has a stronger affinity for electrons than the iron.

Also, how can the reactions be reversible? I thought that was only the case for rechargeable batteries?
The reactions can be reversed if you supply a voltage that is higher than the voltage the cell outputs. If a cell supplies 1 volt, you can reverse the reaction by supplying more than 1 volt to the cell.
To the reverse the reaction, positive on the cell should be connect to the positive on the power supply. Negative on the cell should be connected to the negative on the power supply. If you hook it up the other way, the cell will be drained of all energy very quickly.

thank you so much for clearing that up!!!   ps i love undergrads  =p

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