If voltage is electrical pressure, where does such pressure come from? What's the scientific explanation of such pressure?

 

When a positive charge is separated from a negative charge there is a force of attraction between the two charges very much like gravity.
This is the electrical pressure or potential.

 

When a positive charge is separated from a negative charge there is a force of attraction between the two charges very much like gravity.
This is the electrical pressure or potential.

So does the strength of the pressure positively relate to the number of electrons and positive charges on both sides?
What else affect the strength of such pressure?

 

The distance between them.

Imagine you have some arbitrary distribution of charges in some volume of space (and that the distribution outside of that volume is such as to have negligible effect within the volume). Now imagine placing a test charge at some point (call it A) within the space and that you can, magically if necessary, exert whatever force is needed to hold it stationary. Now imagine moving it from point A to some other point B. Just like moving a physical object around in a gravitational field, you will have to do some work (force times distance) along the path between A and B. That work (on a per unit charge basis) is, by definition, the potential difference between A and B. Voltage has units of joules-per-coulomb, meaning that it requires 1J of energy (work) to move 1C of charge from some starting place to a new place that is 1V higher in potential.

 

A simple way to look at this is that electrons repel each other because they have the same polarity of charge. This repulsive force is the only place that "pressure" comes from. A positive charge is merely a relative lack of electrons, or a place that has a lesser concentration of electrons as compared to the first place. Pressure is measured as voltage and a voltage is always a voltage as compared to someplace else.

If you look at the top of the Chat page, there is an article called, "Ohm's Law for noobies". It really breaks this down to basics.

 

In a plasma, which is generally described as a high temperature ionized gas, it is very easy for electric charges to move around as compared to regular air or a vacuum.

Electric gas-discharge tubes (neon, mercury vapor / fluorescent, high pressure sodium, metal halide, xenon, etc) are high resistance when cold but low resistance when hot.

Usually you need a high voltage pulse to get power flowing through the cold gas, but as the gas heats up the resistance goes down, and the voltage needed to operate it also goes down. Power limiting is needed or the gas will get hot enough to melt the glass container and escape.

High up in the Earth's ionosphere, there are high temperature ionized gases, where tens of thousands of amps can flow around easily and effortlessly, at nearly zero volts.

 

I guess another way to ask my question is that why different cells with different components or structures produce different voltages? What is essential for the difference in the voltage output? Is it because one cell can produce more electrons at a period time than the other?

If you look at the top of the Chat page, there is an article called, "Ohm's Law for noobies". It really breaks this down to basics.

I have read the page, and it's helpful. However, there's one place I don't quite understand. Say an non-rechargeable battery doesn't store electricity, rather it produce electricity by chemical reaction. If the battery is rated at 12V 2.4amp-hour and the radio only need 1.2 amp, inside the battery which condition is more true:
1.) electrons being produced at the rate of the load required
2.) electrons being produced at a constant rate unaffected by the load require, and electrons will be stored somewhere in the battery before being send to the positive side

One more question, why batteries in series produce the voltage equal to the sum of those batteries?

Sry for throwing so many questions to your guys, but I am really curious about these.
Thanks.

 

I'm not that good at chemistry but, I know that the voltage produced is caused by which chemicals are used. Batteries do not finish their chemical reaction a few minutes after they are built and have all the electrons sitting there, waiting for an escape route. I must assume that the voltage developed acts as a suppressor to the chemical reaction. When you allow some current to flow, the chemical reaction resumes at the rate allowed by the voltage drop caused by the load.

Now, suppose you went to the store and bought (2) five gallon containers for compressed air. You take them home and use a compressor to add air until each of them has 5 psi. Then connect them in series. You will measure 10 psi because one volume of air is pushing on the other volume of air. Same thing for batteries.

If you put the 2 cans of air in parallel, you will still have 5 psi but twice as much air. Same with batteries.

If you really want to know why different chemicals produce different voltages, you should ask on a chemistry forum.

 

I have read the page, and it's helpful. However, there's one place I don't quite understand. Say an non-rechargeable battery doesn't store electricity, rather it produce electricity by chemical reaction. If the battery is rated at 12V 2.4amp-hour and the radio only need 1.2 amp, inside the battery which condition is more true:
1.) electrons being produced at the rate of the load required
2.) electrons being produced at a constant rate unaffected by the load require, and electrons will be stored somewhere in the battery before being send to the positive side

No electrons are being produced or stored. The chemical reaction in the battery separates charge by the physical movement of +/- ions (atoms). The charge separation creates a electric field between the cell electrodes that slowly moves (not produces or creates) the free electrons in the conductors of the current to the load and back.

 

Wikipedia: electrochemical cell
http://en.wikipedia.org/wiki/Electrochemical_cell


The charge on the plates in a battery is always held constant if nothing else is happening. By connecting a load, you are disturbing that balance, so the chemicals react until the internal charge is restored.

The general equivalent is going out to a lake and scooping water. You have disturbed the level of the lake, and gravity acts to refill the "hole" you created until levels are balanced again.

In this case the chemicals react until one of the chemicals is depleted and the charge can't be replenished. The chemicals cannot "anticipate" loads, though usually you have lots of surface area available for reaction, to handle whatever loads may occur.

The reaction slows down due to loss of reactants and/or loss of surface area, so the battery can't maintain the output voltage, but will "top itself off" if allowed to rest and the remaining chemicals have time to react.

 

nsaspook makes an important point. Matter (electrons) is/are not being created or destroyed in a battery. It is the availability of electrons that is being produced. As he said, the chemicals only seperate the existing electrons to make a higher concentration in one place and a lesser concentration in another place.

 

It seems much clear to me now. Thank you guys for the help. I really appreciate it.

 

Now, suppose you went to the store and bought (2) five gallon containers for compressed air. You take them home and use a compressor to add air until each of them has 5 psi. Then connect them in series. You will measure 10 psi because one volume of air is pushing on the other volume of air. Same thing for batteries.

Huh?

How would you hook them up "in series"?

No matter how you connected them, you would end up with a ten gallon receiver charged to 5psi. You still have the same total number of molecules of air in the same total volume at the same temperature.

 

You're right. Must have had my head up my...
Anyway, it seems to have convinced the OP, even if it was a bad example.

 

You're right. Must have had my head up my...

I know the feeling well!

Anyway, it seems to have convinced the OP, even if it was a bad example.

And that actually concerns me a tad, as well!

 

Have you ever come up with a physical analogy to adding batteries in series?
I could use that kind of help. So could several visitors to our site.

 

The one I usually resort to, and which seems to work even though it is pretty kludgy, is to think of a battery as a crane that can lift a weight through a certain height, say 10ft. Now imagine that our "electrons" are bowling balls that our "circuit" is a chute that the bowling balls roll down on their way to "ground". If I have one "battery" (my crane) that is capable of picking up a bowling ball from a tank that is around its feet, lifting it 10ft and releasing it into a tank at that height and returning for another ball in one second. That means that my battery can deliver a "current" of 1 ball/sec at a "voltage" of 10ft. Now, imagine I have 10 of these batteries and they are all at ground level. Combined, they can deliver 10 ball/sec, but they can still only deliver them from a height of 10ft. Now imagine that I stack the cranes on top of each other so that each crane delivers its balls to the tank at the foot of the crane on top of it. Now the circuit can start from a height of 100ft, but I can only deliver 1 ball/sec.

 

I just pasted that into "notepad". I will probably have the opportunity to use it.

 

I just think of the batteries as they physically are. Picture 1.5V AA batteries standing on end.

stack one on top of the other and now they're 3V "high"; if an electron fell off the top (picture a small bearing or BB or marble) it would "drop" 3V to the "ground". Stack another and it's 4.5V "high"; an electron would drop 4.5V to ground .

take those same 3 batteries and stack them all on ground level, side by side (parallel), and it's only 1.5V "high", but 3X as many electrons are able to drop to ground.

 

Now, suppose you went to the store and bought (2) five gallon containers for compressed air. You take them home and use a compressor to add air until each of them has 5 psi. Then connect them in series. You will measure 10 psi because one volume of air is pushing on the other volume of air. Same thing for batteries.

This pneumatic analogy is somewhat misleading, containers filled with air have no inherent polarity, so connecting them in series would not increase the pressure.