Sigh.
I’ve been working in Electronics for almost 50 years. I’ve designed electronic circuits that have been produced in the tens of thousands. But, I finally realized that I have a fundamental gap in my knowledge - what is “Voltage”?
Current is easy - the passage of charge-carriers past a point.
But, voltage is much more obscure. I know all the analogies (it’s like “pressure”), but I’m still at a loss to understand where it is “stored.” Is it in the electric field?
If I have a single electron, what property of it defines its voltage? Is it a physical change (like mass or velocity or size)?
There are a ton of references on the Web which try to define what voltage is, but they all seem to end up simply using an equation that describe it’s effects, but not what it is. For example. people mention taking a charge and moving it in an electric field (thus giving it energy), and raising its voltage. But, after that is done, if I were to inspect that electron, what makes my little high-voltage electron different from any other electron?
Voltage isn’t “stored” anywhere; it is a measure of the difference in electrical potential. Asking where it is stored is kind of like asking where the intervening floors are ‘stored’ in the height difference between the ground floor and the fifth story of a building.
I cannot answer your question. But I’ll offer an analogy that might let us think a bit about the “what is is” question.
Where is gravitational potential energy stored? It’s not in that bowling ball at the top of the ladder. It’s in the height between the ball and the ground below. What is gravitational potential energy? It’s position. As measured against a vector field of potentials.
Perhaps, arguably, it’s the tension or stretching of that vector potential field caused by the existence of the object itself in that position. If the bowling ball magically poofed into or out of existence, the rest of the field would respond by changing a la the rubber sheet analogy. The “effort” (I’m a bit reluctant to say “energy”, which has a very specific meaning in physics) to make those changes was what is stored in the surrounding field by the ball poofing into existence and it will be liberated were the ball to poof out of existence.
Real objects don’t poof. They move. But the consequences of motion are the same as poofing. Just smeared out in time.
It might help to consider that voltage isn’t a “fundamental” unit, like mass or charge. It isn’t “part” of an electron or anything like that.
It’s entirely dependent on the path you are measuring along. I mean, that’s fundamentally what it is: the line integral along a path within an electric field. It’s “how hard is it to move a charge from point A to point B along path L”.
I’m an EE, and I still don’t have a good “feeling” of what voltage “is.” I sort of think of it as height above the ground, and how it relates to the gravitational potential energy of an object with mass.
But I don’t worry too much about it. I think I have a pretty good “intuitive feel” of voltage when it comes to circuits, and that’s all I care about.
Where is the water pressure that supplies your home stored? It’s essentially the same answer.
You can see the water itself, you can feel the pressure, but you don’t really talk about ‘storing’ pressure. The pressure is a consequence of wherever your local water tower is (how the gravitational potential comes about). The water itself can be stored up there and used to generate a pressure drop (relative to some local ground) but it is not the pressure itself.
Likewise, you can feel the result of voltage but you don’t think about how it is ‘stored’. Charge can be stored and can be used to generate a voltage drop (relative to some local ground again, natch!) but the charge is not the voltage itself.
A good analogy would be perhaps gasoline (or any flammable). The chemical propertis of a such a substance mean that given the trigger, the atoms will re-asociate so they bond with free oxygen (which is ripped from its weak O2 state also). The voltage potential is stored in the chemical properties. Essentially, the electrons are weakly bonded in a battery and like the spark, providing a path where they will be able to escape - a conductor to another location which is not expelling electrons so forcefully but will absorb them - they will escape. Voltage is a measure of how much the electrons have the “urge” to escape.
A capacitor is even more obvious in this regard. Excess electrons have built up meandering between the conductor plates, attracted to the other side of the capacitor where there is no repelling charge, but unable to get through the electrolyte separator.
Or currents are induced in a moving magnetic field that pulls such loose electrons in a certain direction, where they either can escape through a conductor or cannot.
It is, in some sense, actual pressure, not just an analogy to pressure. Electrons repel each other. When you raise a conductor to a high voltage, you are applying an actual physical force that squeezes those electrons closer together. The amount is tiny because the electromagnetic force is so strong, but it’s not zero. Raise the voltage high enough and the electrons will leap away to escape. At low voltages, the electrons just bounce around the conductor until they are pushed to an area with a more neutral charge density.
I’m really surprised by the statement that voltage is path dependent. I would have assumed the opposite - that the electrical potential between 2 points in a field is path independent. Maybe I’ve misunderstood what you’re saying.
Water doesn’t compress much under even great pressure either. But an awful lot of force can be stored in that tiny amount of shrinkage. IOW, the “spring constant” of water is ginormous. So is the “spring constant” of the electromagnetic force.
Indeed, plus the “spring” is “pre-stressed”. The positive charges (the atomic nuclei) cancel out nearly all of the force, so it’s only the differences that you observe. If you somehow disappeared the positive charges, the remaining electrons in a wire would explode like a bomb.
What will really bake your noodle is that magnetism is due to relativistic effects. Even though the average speed of electrons in a wire is only millimeters per second, the relativistic length contraction at that speed is enough to change the apparent charge density enough to affect the path of other charged particles. Electromagnetic forces are intense, but most of the time they’re almost perfectly cancelled.
No, you are correct. In a static E-field the path taken between two points doesn’t matter. It might in a time-varying field… my physics course days are far behind me, and I honestly can’t remember.
I meant more generally that what voltage “is” has to do with the distance between two points and the strength of the field between those points.
It is the path integral of the path you measure along. Which in a field that’s constant over time will amount to the same total as a straight line startpoint-to-endpoint measurement. The latter measurement being a lot simpler to perform.
Where it gets interesting is when the field is time-varying and you’re interested in the “perceived” voltage along a non-straight path for some not-speed-of-light particle experiencing that voltage along the way. Like the parts inside an AC motor.
It had been long enough since I did this calculation that I lost my intuition about the scale we’re talking about.
Consider a 1 cm^3 cube of copper. It weighs 9 grams, has 0.142 moles of copper, and 2.48e24 electrons.
They’re at an average distance of 7.39e-11 m, and the field energy at that distance is 3.12e-18 J.
But you have to consider all pairs of electrons, which is N2/2, or 3.08e48 pairs.
Multiply through and we get 9.59e30 J. The Tsar Bomba (the largest nuclear device ever built) is only 2.1e17 J. It’s about seven hours of total energy output of the sun.
Note to self: if I built a teleporter, double-check that it teleports both the nuclei and the electrons.
Yowza!! I had a vague notion it was stupefyingly large energy. But not that stupefyingly large.
OTOH … if you did make that noob mistake, nobody on the planet you teleported to would ever be the wiser. Neither would you.
If somebody wanted to invent a Death Star torpedo, a rail gun firing 1cm^3 cubes of copper with magick disappearing protons or electrons would be a good start.
Voltage can be created with chemical reactions or mechanical force.
A generator has wire windings and magnets. The strength of the magnets, number of windings sets up the voltage that can be produced. So in a way the voltage can be thought of as stored in the magnets. But until they are moved relative to the coils, there is not voltage. To move the magnets requires energy. So you could take a step back and also say the voltage is stored there. But more accurate to say it is converted from whatever source is providing that energy.
In chemical batteries, the voltage is stored in chemical reactions that are set up to happen when the battery is connected. Some of the reactions take place before the load is connected, but not a lot. The chemicals have atoms/electrons that will move and produce electric current if they have a place to move to. So the voltage is stored in the potential chemical reactions that are waiting for a place to send those atoms/electrons that they can release/absorb.
Like a generator, chemical batteries are setup in specific ways to produce a certain voltage. 1.5 volts is a common amount for a lead acid battery. So you arrange series of chemical reaction containers in series to produce a higher voltage than 1.5.
I know you already know most if not all of this. But maybe I stated it in a helpful way.
