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Q & A: series batteries

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Most recent answer: 08/26/2020
Q:
Ok, first thing you will think is that you already answered this question, but i think i will ask it more specifically so that you may focus on the part that confuses and frustrates me. When you stuff lets say two batteries into a flashlight into a series connection, the positive terminal of one of them is immedately touching the negative terminal of the other. It seems to me that this connection of plus and minus terminals (although of two different batteries) creates a short circuit, just like connecting + and - terminals of the SAME battery creates a short circuit and drains all the charge. Yet, this cannot be true because the voltages of batteries add up in series. What’s wrong with my thinking?
- Sretko (age 21)
university of toronto, canada
A:
Actually, this is just the sort of question which many people want to ask.

Think of it this way. The battery is a sort of pump which uses chemical energy to give a push to electrons, pushing them toward the negative terminal. If you connect one negative terminal to the next positive terminal, the second battery will give the electrons a further push in the same direction.
Mike W.

Just putting the ends of two batteries together is insufficient to make a circuit, short or otherwise. In a circuit, current needs to travel around in a loop. You certainly can make a short circuit if you connect a wire (without a light bulb) from the positive end of your two-battery series combination back around to the negative end of the other battery. The voltages of the two batteries add in series, but the wire, since it conducts electricity with little resistance, forces the voltages of the two ends to be the same.

Tom

(published on 10/22/2007)

Follow-Up #1: electron flow in battery

Q:
I wanted to follow this information up because I believe there's a small inaccuracy in the info provided. It's my understanding that electrons do NOT flow from the positive terminal to the negative terminal. Electrons, being negatively charged, flow toward the area of need: the positively charged terminal.Positive charge, on the other hand, is represented by the absence of electrons during that flow, and could be said to be moving from + to -. But the specific reference to electron flow here is inaccurate, as I understand it.
- Bradley Muehlemann (age 35)
Colfax, IA, USA
A:

Outside the battery, you're absolutely right that electrons flow from the neagtve terminal to the positive one, through some sort of circuit.


But think of what that means. Electrons don't just pile up somewhere. Somehow inside the battery something must be pushing them from the positive terminal to the negative terminal. That sounds backwards, since as you say that's where they have higher electrical potential. So how does it work?

The idea is that elctrons flow to where their total electrochemical potential is lowest, not just their plain electrical potential. The electrochemical potential involves the detailed binding of electrons to atoms and molecules, so it depends on chemistry. In any battery, the chemistry is different on the two sides.

You can see an example of the elctrochemical potential in very simple chemisrty, the way table salt dissolves in water. It forms Na+ and Cl- ions. Why does the last electron jon the Cl? Shouldn't it get pulled back to the Na+? It doesn't because of the detailed quantum states available on those two ions. 

Mike W.

 


(published on 08/26/2020)

Follow-Up #2: further remarks on series batteries

Q:
Hello, Recently you answered a question of a reader titled: Series Batteries. If you don’t mind, I add the following: The chemical interaction in a bettery creates an electric field, which results in sucking electrons from the positive end and pushing out electrons to the negative end. These battery ends are made of small metals; they can give or recieve only so much electrons (known as "electron affinity"). Hence, the chemical reaction comes to a halt -- this is similar to a person sucking or blwoing air from/into a bottle which comes to a halt after a certain time, depending on the size of the bottle and the strength of the person. When you connect the battery ends to a capacitor, the plates of the capacitor provide additional pieces of metal from/to which the battery extracts/pushes more electrons -- hence the electron current flows a while and, when the capacity (the elctron affinity) of the capcitor is reached, comes to a stop. This is one way of creating a current -- by increasing the capacity. Another way is by increasing the electric field. This happens -- as the reader had mentioned -- when you connect two batteries in a series, which causes stronger electric field (stronger sucking and pushing force) to suck/push more electrons from/to the FAR ends of the pair of batteries. In both of these methods the circuit is not complete -- the far ends are not connected. And, as explained above, the current comes to an end. A third way, is to connect the ends (i.e., to complete the circuit) through a wire or a resitance such as a light bulb. Now, the sucking from the positive end continues through the wire until it reaches the negative end -- which is glad to give away electrons which were pushed into it. The negative end is re-supplied electrons by the chemical interaction inside the battery which, in turn, receives electrons from the positive end. This symbiotic trade of electrons between the positve and the negative ends, made possible through the channel of wire, continues until all the chemicals in the battery have reacted themselves into inert salts, which we call: dead battery. Hence, (the ends of) a live battery, is in a meta-stable balance: it is waiting for an opportunity for converting its (chemical) potential energy into heat energy -- the law of entropy. Thank you.
- Mehran
Miami
A:
Thanks, Mehran

Mike W.

(published on 10/22/2007)

Follow-up on this answer.