Multiple battery common ground question.....

[Conscripted]

Good morning all. I have a question about multiple power sources. I know that if you are running two or more batteries that you need to connect the grounds together. My question is why don't the batteries interact with each other? For instance if I take a 6vdc battery and a 9vdc battery (per the $50 robot) why do I still have 6 and 9vdc instead of something like 3vdc.

I'm not asking if it works. I know that it does. I'm asking why they don't interfere or interact with eachother.

Thanks
Conscripted

[mstacho]

Ground is just a reference voltage.  Ultimately it would mean that the negative terminal of both batteries are at the SAME potential.  Since current only flows where there is a difference in potential, the batteries don't interact.

Now, if you connected both of their negative terminals AND their positive terminals in one circuit, there would be a difference in potential, and interaction would take place (this is exactly what a series circuit is)

MIKE

[Billy]

why do I still have 6 and 9vdc instead of something like 3vdc.

It's all relative.

If you have a volt meter, connect it the positive terminals of the 6 and 9 volt sources. You WILL measure 3 volts.
This is an important concept. Voltage is a measure potential difference. By connecting the negative terminals together, you have brought the potential of both negative terminals to the same potential. Given one is 6V and the other 9, you will find 3V difference.
If you disconnect the negative terminals from each other, you'll only measure noise and find there isn't really any potential difference (ignoring static charges that may be present).

[knossos]

I use Legos to explain it to my cub scouts.  I use the brick height to represent voltage.  The bottom of the brick represents ground, the top the + voltage.  One brick high is 1 volt, 3 bricks high is 3v, etc.  I use the table top as the common ground connection. 

With a 3 stack (3v) standing on the table, and a 5 stack (5v) on the table next to it.  If I take a second stack of 3v and stack it on top of the first 3v stack, I end up with a 6v stack standing next to a 5v stack.  This is like taking batteries and connecting them in series. 

Time for work and I don't really have the time to continue the analogy right now, but I think you can see where I'm going with this.

[VegaObscura]

GND is more than just a reference, its a place for your electrons to go to complete the circuit.  If GND was just a reference, you could make a circuit using the positive side of one battery and the negative side of another battery without connecting the other two battery terminals, yet we all know this isn't possible.  Electrons have to flow from + to GND (or from GND to +, depending on your preferred notation), therefore GND is a path as well as it is a reference.

If the three former posts answered your question, then ignore mine.  But if you were asking "why don't the electrons from one battery flow into the GND of another battery", then you have the same question I used to have.  According to my physics teacher, the electrons do flow from one battery to another, but since a battery cannot accept more electrons than its pumping out, they always trade a perfectly even amount, therefore preventing any "outside current" from flowing into either battery.

This was a very difficult concept for me to grasp.

[Conscripted]

GND is more than just a reference, its a place for your electrons to go to complete the circuit.  If GND was just a reference, you could make a circuit using the positive side of one battery and the negative side of another battery without connecting the other two battery terminals, yet we all know this isn't possible.  Electrons have to flow from + to GND (or from GND to +, depending on your preferred notation), therefore GND is a path as well as it is a reference.

If the three former posts answered your question, then ignore mine.  But if you were asking "why don't the electrons from one battery flow into the GND of another battery", then you have the same question I used to have.  According to my physics teacher, the electrons do flow from one battery to another, but since a battery cannot accept more electrons than its pumping out, they always trade a perfectly even amount, therefore preventing any "outside current" from flowing into either battery.

This was a very difficult concept for me to grasp.

Thank you Vega. That was more what I was looking for.