My understanding of voltage is broken again

[sentry7]

So after eight years I though it made sense, but I guess not.

What I think I understand:



1) Suppose I had a standard AA battery -- 1.5V, right? Then the electric potential difference from A to B is +1.5 volts (ideally). The absolute electrical potentials at these points are irrelevant, the difference in electric potential is what we care about. Let's just say that B is 0V, so then A is 1.5V.

2) Assuming no internal resistance and no leakage current or other madness, then if I connect a resistance across A and B, then 1.5 volts will show up across that resistance.

What I do not understand:



3) Suppose I had another exactly identical AA battery (which I know is impossible, but for the sake of argument...). If we connected A to D, then wouldn't it be as if we connected A to B? Why doesn't current flow from A to D by the merit that there is a potential difference between them? Maybe I also don't understand how batteries work either.

[KD0CAC John]

My guess is that current only flows in one direction , maybe the drawing should look more like a diode and or a combination of , there is a lack of circuit / load between either end of the battery / bank - adding batteries in either series or parallel - does not complete a circuit . 

[Ian.M]

Its nothing to do with direction of current flow or diodes.

As the two cells have separate electrolytes, insulated from each other, when a single terminal of each cell is connected, there is no complete circuit so no current can flow^#.   There may be an initial potential difference between the unconnected cells e.g. due to static charges, but as soon as you connect a voltmeter with less than infinite input impedance it starts to discharge any static charge and as the maximum possible capacitance between two cylinders the size of AA 'batteries'^* is very small indeed, any practical voltmeter will discharge any potential difference between the otherwise unconnected cells faster than you can read it.

# That is: no sustained current. A brief transient current may flow due to static charges - see next sentence.

* I'm being slightly pedantic here - a battery is an assembly of individual cells.  Common types of 1.5V 'battery' only contain one cell so don't meet the technical definition of a battery. 

[KD0CAC John]

And as long as I am at it , asking the question again [ asked many times in the past and never got an answer ] .
When ever I click on the notify tab - to keep up with a thread .
Every time I start getting random threads sent to my email , sometimes multiples , and sometimes not any from thread I linked to .

[Ahrenp]

When the batteries are not connected, the voltages are floating. The only thing that is important is the relative voltage between points A and B (or between D and C). There is no absolute voltage (i.e. no common 'ground' or 0V reference), and by connecting point D and point A all you are doing is referencing the relative '0V' of point D to the '1.5V' of point A. By doing this, you are putting point D 1.5V above point B, and point C 3.0V above B.

Unless you physically connect points B and D, they are not at the same potential.

[KD0CAC John]

Another thing , if you put the batteries in some type of circuit - load doing work , charging , the battery bank / cells will equalize between cells / batteries .

[suicidaleggroll]

This is the difference between floating/isolated voltage supplies and non-isolated voltage supplies.

A floating/isolated supply provides a potential difference between two points, and neither of those two points is fixed to any absolute voltage.  They float.  A or B can be any voltage, the only driving force is that they remain 1.5V apart from each other.  You could connect "A" to earth ground, putting "B" at -1.5V, you could connect "B" to earth ground, putting "A" at +1.5V, or you could connect them to any other arbitrary point in a circuit.

A non-isolated supply provides a potential difference, but one of those outputs (either the + or -) is also hard-wired to one of the inputs.  Its output is not floating, and cannot be arbitrarily connected wherever you want, or you might short something out.

All batteries are floating until you connect them to something.

[AG6QR]

What I do not understand:



3) Suppose I had another exactly identical AA battery (which I know is impossible, but for the sake of argument...). If we connected A to D, then wouldn't it be as if we connected A to B? Why doesn't current flow from A to D by the merit that there is a potential difference between them? Maybe I also don't understand how batteries work either.

Electricity flows in circuits.  A circuit is a complete path (from the same Latin root as circle).

A and B are connected by the internals of the battery, such that the internals of the battery do whatever they need to do (often that means supplying current) to make A be 1.5V higher in potential than B.  So when you connect them with a wire, current flows through the wire and through the battery, in a circuit.

When you connect A to D, you equalize their potential.  But there is no path for current to flow in a complete circuit.  Perhaps at the moment they touch, some static charge equalizes, but that effect is very temporary and usually negligible.  This is similar to what happens when you touch two coins together -- if they had been at different static potentials before they touch, they would equalize, and a few electrons might move from one to the other, but it is not a sustained current flow, and it is usually negligible. 

Back to the batteries:  after they have touched, then the first battery guarantees that A is 1.5V higher than B, and the second battery guarantees that C is 1.5V higher than D, so a little math shows you that C is 3V higher than A.  If there were a current path for electrons to flow between C and A, there would be a circuit, and current would flow.

[Vtile]

Interesting thing is because voltage sources are sums of each when in series connected means that if you connect A and C and measure voltage between B and D it shows zero (or the difference of the battery potentials). 

[sentry7]

Ok, so I understand the bit about the discharge of static potential.

I'm still fuzzy on what it means for something to float. So all batteries are floating until you connect them to something...you mean until you actually connect it to something, you can't know what it's voltage is? When you load the battery, the potential difference just "appears"?

[suicidaleggroll]

I'm still fuzzy on what it means for something to float. So all batteries are floating until you connect them to something...you mean until you actually connect it to something, you can't know what it's voltage is? When you load the battery, the potential difference just "appears"?

All voltages are relative.  When you say "you can't know what its voltage is?", voltage relative to what?  The voltage of the "B" terminal relative to the "A" terminal will be 1.5V, but the voltage of either of those relative to your foot, wall outlet, or another battery sitting on the other side of the room is undefined.  There will be a voltage difference, but you can't really know what it is, it's floating (even hooking up a DMM would change it, even moving your arm to grab the DMM lead would likely change it).

[AG6QR]

Ok, so I understand the bit about the discharge of static potential.

I'm still fuzzy on what it means for something to float. So all batteries are floating until you connect them to something...you mean until you actually connect it to something, you can't know what it's voltage is? When you load the battery, the potential difference just "appears"?

Which potential difference?  The voltage across the battery, or the voltage between the battery and something else?  The voltage across the battery does not appear and disappear when a load is connected and disconnected.

Back to your example, when the batteries are not connected to anything, you still know that A is always 1.5V higher than B.  And you know that C is 1.5V higher than D.   But you can't know anything for sure about the relative voltages between the two batteries, nor between any of these points and ground.

The relative voltage between the two different terminals of a single battery is well-defined.  But the absolute voltage between a terminal of the battery and some external point (such as ground) is not well-defined when the battery is not connected to anything.  That's what it means for a voltage source to be "floating". 

[Vtile]

Yes. It does have stored energy that will create current and voltage according law of ohm when you close the circuit and allow current (electrons) to flow.

V=R*I or with another typing style U=R*I, where R = resistance, I = current and V = voltage.

The Float this or that, is one of the favoured terms in EE and well liked and looked after in many cases since it allows us more liberties how we set up our floating sources to make a power system. It also gives us (sometimes false) safety while working with circuits because of the properties of circuitry that you need to always have a loop to allow the current to flow.

If something is floating it means it does not have any electric connection between it and the another part of the world where it is "floating from". Usually ie. Lab PSU is floating from the mains earth, so that means that it is not connected to it in any way (traditionally said to be "galvanically isolated").

If you measure voltage between the floating circuit A and floating circuit B, your meter is creating a connection between the two circuits, which are now partially connected to each other, but as a 2 circuit system they are still floating from the rest of the world. Also they are only connected to only one node there is still no full circuit between the two, if you now take a wire and connect any other two places of these two circuit together, you start to see some readings in your meter, since you now have a loop of current.

[bson]

The proper technical term for "voltage" is either potential energy as an EE term, or as a physics term electromotive force (EMF).  It's a force that wants to push charge.  Like any force it's a vector relative to a reference and when you stack two batteries you add the two vectors.  The force magnitude doubles, but the impedance between the terminals remains near-infinite and it's not until you form a close circuit that a path of reduced impedance is created for the force to push a charge.  Electrically an open circuit is comparable to a spring-loaded rod that wants to push things sitting embedded in a block of concrete - it's staying put with nowhere to go.  With two stacked batteries and no closed circuit you have two such rods back-to-back embedded in concrete.  Twice the force yet nowhere to go. Electrically, vacuum, air, or other insulators is a near-complete blocker of movement of charge (this is the exact opposite of how we learn relate to physical reality through our senses).

[sentry7]

I understand that connecting A and D doesn't make a complete circuit, but then technically, how does connecting A and B make a complete circuit? Do electrons physically go from B to A, through the battery, and back to B? I thought there was some sort of chemical wall that stops electrons from going through the battery? If charges actually move through the cell, then it makes sense that no current can go from A to D.

[KD0CAC John]

A-B would be a circuit - a short circuit .
In other posts you never connected A-B = either end of a battery , you asked about connecting 2 cells in series .
Now your getting all confuse , if you connect A-B a short circuited battery , your not sending voltage back to the battery - your making that connection the load - a piece of wire , the wire consumes the available potential by heating up the wire / or doing the work - making heat - depleting the battery of its potential .
There is no chemical wall in the battery , the electrolyte , when charged [ basically the acid in the water goes to the lead plates ] no - to little potential , then when you charge a battery , the acid moves back into solution / electrolyte , one way to measure that is specific gravity - you are actually measuring the weight of the liquid , discharges the fluid is closer to water , charged the fluid is acid , the molecules of what makes up the acid either suspend in water = charged , or move to the plates = discharged . 

[AG6QR]

I understand that connecting A and D doesn't make a complete circuit, but then technically, how does connecting A and B make a complete circuit? Do electrons physically go from B to A, through the battery, and back to B? I thought there was some sort of chemical wall that stops electrons from going through the battery? If charges actually move through the cell, then it makes sense that no current can go from A to D.

Charge actually moves through the cell.  The details of how the chemical reaction pushes charge through the cell are best covered in a chemistry class, but the fact that charge is moving through the cell can be verified with a hall effect current clamp.  It responds to the magnetic field produced by moving charge.  The current clamp will show the same current flowing through the cell as through any other conductor in the circuit.

Kirchhoff's current law also tells us that the same current moves through the cell as moves through the rest of the circuit.

[timb]

I understand that connecting A and D doesn't make a complete circuit, but then technically, how does connecting A and B make a complete circuit? Do electrons physically go from B to A, through the battery, and back to B? I thought there was some sort of chemical wall that stops electrons from going through the battery? If charges actually move through the cell, then it makes sense that no current can go from A to D.

If you connect A to B with a wire, it's the same as if you connected A to B with a resistor. The wire has resistance, even if it's very small, which means (a large amount of) current flows.

Now, when you try to connect A to D nothing happens. Why? Because current from A *always* has to flow back to B. If you were to connect A to D and then C to B, that would complete a circuit and current would flow (you'll also end up with 3V since the batteries are in series).

Make sense?

[KD0CAC John]

Not sure where your loosing / confusing , maybe the concept of a capacitor - DC hits a open or wall , but AC jumps the gap ?
For DC the battery is not a wall or a open - its a chemical connection , that chemical connection is potential / charge etc.
Leaving the theory of AC / DC out of it for the moment , the battery can be looked at as being the same as your wall plug - 2 connections that can be applied to a load & do work , no connection until it gets back to the power plant / same as a battery - the electrolyte is the connection between the plates - you can think of the battery as a piece of wire , that has a charge / potential .
The battery is not an open circuit .

[Ian.M]

... or imagine a cell as being a conductor containing a pair of little demons batting electrons towards one end of it.  The demons can only hit them so hard (i.e open circuit voltage) and cant bat more than a certain number per second (short circuit current).

If you want a better understanding you have to delve into electrochemistry, specifically redox reactions.

[suicidaleggroll]

... or imagine a cell as being a conductor containing a pair of little demons batting electrons towards one end of it.  The demons can only hit them so hard (i.e open circuit voltage) and cant bat more than a certain number per second (short circuit current).

And the harder and faster they hit, the more tired they get, until they finally drop dead (depleted battery).

[sentry7]

Haven't been back to read the latest responses in depth, but I just want to thank you guys for being patient. I'm purely self taught in this stuff. It's pretty ridiculous actually but I have experience in analog, digital, and mixed signal systems. I get the complex stuff. I know what overall behavior to expect out of a given system, but when it comes to what's actually happening on the small-scale end of things it gets bumpy. I'm continuously trying to improve.

[timb]

Haven't been back to read the latest responses in depth, but I just want to thank you guys for being patient. I'm purely self taught in this stuff. It's pretty ridiculous actually but I have experience in analog, digital, and mixed signal systems. I get the complex stuff. I know what overall behavior to expect out of a given system, but when it comes to what's actually happening on the small-scale end of things it gets bumpy. I'm continuously trying to improve.

Here's an easy way to explain it for you, with images:

First, let's take a battery and connect it to a 100Ohm resistor:



Now, let's add a second battery in series:



Notice in both of these images the current has to flow in a loop. That is, the circuit has to be complete for current to flow.

Now, let's remove the load from our series battery connection:



Since the circuit isn't complete, current can't flow.

Think about your question in a broader context. If current still flowed even without a load then power switches wouldn't work, right?

Hopefully that answers your question.

[jamie297]

3) Suppose I had another exactly identical AA battery (which I know is impossible, but for the sake of argument...). If we connected A to D, then wouldn't it be as if we connected A to B? Why doesn't current flow from A to D by the merit that there is a potential difference between them? Maybe I also don't understand how batteries work either.

This is the way I visualize it:
Current is the flow of charge which is a property of the electrons.
Think of the electrons in the conductors as a slack chain rather than moving totally freely.
As the charge drives them around from positive around to negative, in order to get electrons onto the negative side they need to be able to move off the positive side.
Hope this helps 

[KD0CAC John]

I understand that connecting A and D doesn't make a complete circuit, but then technically, how does connecting A and B make a complete circuit? Do electrons physically go from B to A, through the battery, and back to B? I thought there was some sort of chemical wall that stops electrons from going through the battery? If charges actually move through the cell, then it makes sense that no current can go from A to D.

Not sure where your at in your mind with this , but reading today your post above , another thought - it seems your thinking of the battery as a complete circuit XXXX  , its only a component - not yet in a circuit , if you put a wire A-B / + to - , that would be a circuit as a short , but not a useful circuit , unless your doing a load test on the battery .