Help me understand what voltage is.

[Leros]

I've been having a really hard time understand what exactly voltage is. I've read wikipedia, I've read my textbook. I have no problem working with voltages. Equations are equations. I just can't wrap my head around the concept of voltage.

In my mind:

Current is the amount of electrons flowing. In the hydraulic analogy, this is the gallons/minute. I understand that just fine.

Voltage is the difference of electric potentials between two points. I can't visualize what that actually means. Any help?

 

[SRoode]

Think of current as water flow,
think of voltage as water pressure.

Voltage is potential energy. If you experience a voltage drop, think of it as a pressure drop. The resistance can be compared to a pipe restriction. The water flow in a single loop of piping will remain constant (just as it would in an electrical circuit) restricted only by the total resistance in the pipe. Pressure (or voltage) would steadily decrease in the line as resistance eats it away.

 

[ZeroNine8]

Imagine you have 2 buckets of water, one on your roof and one on the ground. If they are connected by a pipe, then current would be the amount of water flow through the pipe and voltage could be thought of as the difference in height of the two buckets. (potential energy converted to pressure, as the previous poster said). Both have an impact on how quickly the water is transferred (power).

 

[Born2bwire]

Imagine you have a chicken at one end of your fence run. Now after you remove the chicken's head, it will run around somewhat randomly but it may eventually tend to move from one side of the run to the other. The tendency for the headless chicken to move to the other end of the run is voltage.

 

[BrownTown]

Hmm, well that overcomplicated chicken example would more closely represent the electric field, the voltage is the integral of the electric field over some length.

 

[Born2bwire]

Originally posted by: BrownTown
Hmm, well that overcomplicated chicken example would more closely represent the electric field, the voltage is the integral of the electric field over some length.

Actually, I just wanted a headless chicken. Maybe if we add up the amount of blood that the chicken expends over its movements towards the opposite end of the coop we could say that's voltage...

 

[Born2bwire]

To give you a serious answer to your question, the main problem is that voltage is not a tangible property, but it is a very useful number of merit. Specifically, E = -\nabla V, that is, the electric field is the gradient (vector derivative so to speak) of voltage. Thus, voltage is the electric potential. There is an analog for the magnetic field, B = \nabla \times A, or, the magnetic flux density, B, is equal to the cross product of A, which is called the vector potential.

So if we include the time dependent coupling of E and H,
E = -Gradient(V)- d/dt (A) <-- these are really partial derivatives with respect to time
B = Cross(A)

The potentials A and V are not really physical, but they allow us to solve a lot problems much more easily than if we strictly worked with E and B. For example, voltage allows us to work with the vector E as a scalar. In addition, voltage is the integral of the electric field over distance. One of the nice results of this is that the voltage is only dependent upon the field at the end points of the curve of integration. So if we take the potential difference, V(a)-V(b), then it does not matter how we draw the curve of integration. And in real life, you always take the potential difference, V(a)-V(b).

As an aside, I have done some simple work with the problem of a dipole antenna above a layered medium, ie: radio antenna on top of the Earth. The way to solve this problem is to reduce the E and H fields of the antenna into the vector potential A. Then you can solve the problem using simple Fresnel reflections of plane waves. So this is another case where reducing the E and H fields into potentials allows us to solve complex problems easily.

 

[Navid]

Originally posted by: Born2bwire
Imagine you have a chicken at one end of your fence run. Now after you remove the chicken's head, it will run around somewhat randomly but it may eventually tend to move from one side of the run to the other. The tendency for the headless chicken to move to the other end of the run is voltage.

I now have a headache and a stomach ache laughing! All because of your post!
You should be banned! :laugh:

 

[randay]

lol. isnt voltage the giant robot that got formed from 5 smaller robots?

And I'll form the head!

 

[f95toli]

Originally posted by: Born2bwire
The potentials A and V are not really physical, but they allow us to solve a lot problems much more easily than if we strictly worked with E and B. For example, voltage allows us to work with the vector E as a scalar. In addition, voltage is the integral of the electric field over distance. One of the nice results of this is that the voltage is only dependent upon the field at the end points of the curve of integration. So if we take the potential difference, V(a)-V(b), then it does not matter how we draw the curve of integration. And in real life, you always take the potential difference, V(a)-V(b).

I think you are confusing a few issues here. You are right that the vector potential is not "physical" (it is not an observable) and therefore gauge dependent.
However, voltage IS an observable since it is related to potential energy.
A particle with the charge 1 Coloumb held at a electric potential 1V will have potential energy of 1J. A more "physical" picture is that voltage determines the distribution of energies of the quasiparticles with respect to the Fermi surface (in a semi-classical picture applying a voltage essentially "tilts" the Fermi-level). Also, using second quantizaion you can sometimes define a voltage operator in terms of creation and annihilation operators of the electric field.

Morever, voltage also happens to one of the corners of the metrological triangle (resistance. current, voltage) and therefore forms the basis of the SI-system. If we ever manage to close the triangle (at the moment we can not measure current directly with good enough precision, we should be able to do that in a few years) we will be able to define most SI units using voltage.
We can measure voltage with almost ridiculus precision using the Josephson AC effect which relates frequency and voltage (the conversion factor is 2*e/h=483 GHz/mV). Since we can measure frequency with high precision we can therefore also measure voltage with the same precision.

 

[CTho9305]

However, voltage IS an observable since it is related to potential energy.
A particle with the charge 1 Coloumb held at a electric potential 1V will have potential energy of 1J. A more "physical" picture is that voltage determines the distribution of energies of the quasiparticles with respect to the Fermi surface (in a semi-classical picture applying a voltage essentially "tilts" the Fermi-level). Also, using second quantizaion you can sometimes define a voltage operator in terms of creation and annihilation operators of the electric field.

Voltage is not absolute though - voltage only has meaning when it's relative to something.

 

[Born2bwire]

Originally posted by: f95toli

Originally posted by: Born2bwire
The potentials A and V are not really physical, but they allow us to solve a lot problems much more easily than if we strictly worked with E and B. For example, voltage allows us to work with the vector E as a scalar. In addition, voltage is the integral of the electric field over distance. One of the nice results of this is that the voltage is only dependent upon the field at the end points of the curve of integration. So if we take the potential difference, V(a)-V(b), then it does not matter how we draw the curve of integration. And in real life, you always take the potential difference, V(a)-V(b).

I think you are confusing a few issues here. You are right that the vector potential is not "physical" (it is not an observable) and therefore gauge dependent.
However, voltage IS an observable since it is related to potential energy.
A particle with the charge 1 Coloumb held at a electric potential 1V will have potential energy of 1J. A more "physical" picture is that voltage determines the distribution of energies of the quasiparticles with respect to the Fermi surface (in a semi-classical picture applying a voltage essentially "tilts" the Fermi-level). Also, using second quantizaion you can sometimes define a voltage operator in terms of creation and annihilation operators of the electric field.

Morever, voltage also happens to one of the corners of the metrological triangle (resistance. current, voltage) and therefore forms the basis of the SI-system. If we ever manage to close the triangle (at the moment we can not measure current directly with good enough precision, we should be able to do that in a few years) we will be able to define most SI units using voltage.
We can measure voltage with almost ridiculus precision using the Josephson AC effect which relates frequency and voltage (the conversion factor is 2*e/h=483 GHz/mV). Since we can measure frequency with high precision we can therefore also measure voltage with the same precision.

I'm not saying that voltage is not an observable. The physical relation would be that voltage is akin to the amount of work that is required to move a unit electric charge. But the term "electric potential" is more indicative of the mathematical relationship, that V is the scalar potential of the electric field. By the same token, A is the magnetic vector potential of the magnetic flux density. The other reason why I say that it isn't really "physical" is because voltage is generally an indirect property. Current is easy for most people to visualize because it's the movement of charges. Power is easy because it is observed in the form of energy like heat or light. Resistance brings about the idea that one material impedes the flow of electrons more than others. But voltage, voltage is generally an indirect property. Like how a voltmeter measures voltage by looking at the current flowing through a series resistance. Or a galvanometer that uses Ampere's Law to cause a deflection in the coil coupled with a resistor.

You can go into more detail as you have in describing voltage, but I do not think that the OP is going to make any more sense of what it is if he's already having trouble with the basic classical model.

 

[Pulsar]

Ever seen a dam, with all the water behind it? Then a little stream far far below?

The difference in height between the water behind the dam and the little stream at the bottom is voltage.

 

[f95toli]

Originally posted by: Born2bwire
But voltage, voltage is generally an indirect property. Like how a voltmeter measures voltage by looking at the current flowing through a series resistance. Or a galvanometer that uses Ampere's Law to cause a deflection in the coil coupled with a resistor.

I understand what you are saying, but the point is that voltage is NOT an "indirect property". The Volt belongs to a very exclusive club of units in that it can be measured directly (using the Josephson relation) WITHOUT refering to (or even measuring) either current or resistance; all we need is time.
Hence the Volt is a fundamental unit.

But I agree that this discussion is probably of no ue at all to the the OP

 

[Biftheunderstudy]

You also have to remember that alot of physical observables have no meaning unless they are relative to something, take velocity for instance. Everyone will agree that its an observable, but it needs a reference frame to even mean anything. Also, the vector potential may not be physical in classical mechanics but it does make a come back in qunatum mechanics, something about the deflection of electrons around a solenoid. The only field active there is the vector potential so its actually doing the deflecting.

 

[bobsmith1492]

A good analogy that takes reference points into account:

Think of some marbles lying on a hilly terrain. The balls will tend to roll downward toward the lowest point they can get to, following the steepest and easiest-to-roll-on path they encounter.

Now, the marbles are electrons, the slope of the hill is the electric field, and the elevation at any particular point is the voltage. The difference in elevation is the difference in voltage (potential).

I think that was the best analogy I've heard. Does it help?

 

[f95toli]

Originally posted by: Biftheunderstudy
Also, the vector potential may not be physical in classical mechanics but it does make a come back in qunatum mechanics, something about the deflection of electrons around a solenoid. The only field active there is the vector potential so its actually doing the deflecting.

This is known as the Aharonov-Bohm effect. However, note that the effect itself only depends on the enclosed flux which can be expressed in terms of B. On the other hand the effect appears despite the fact that the B-field is zero in the region where the electron can travel, i.e. in classical physics nothing would happen since there is no Lorentz forze on the particle.

 

[Smilin]

The Voltage = water pressure, Amperage = water flow analogy is always a good one.

Pressure as a whole is great for voltage/current analogies to some degree.


Here's the thing. If you get a bunch of atoms together then strip off all their electrons you got this big gob of atom nuclei. You now have way too many protons and if one stray electron gets anywhere near them it's gonna get yanked into orbit around one of them real fast!

Then say you have a copper wire. All it's electrons are intact and spinning happily around their protons....all of them nice and lined up in a row to make the wire. Take the end of this wire and jam it into the aformentioned gob of stripped atoms. All the electrons in the wire are gonna flow into the gob of protons like dominos falling. Then it stops.

If you have the other end of the wire plugged into some other gob of atoms with intact electrons they'll flow continuously until either all the protons on the far end now have an electron, or you run low on atoms with their electrons intact and things kinda get into a 'pressure' equilibrium.

Voltage is going to be how many atoms in the gob are missing their electrons. If all of them are then you have high voltage. Plug some other gob of matter into this and electrons are going to really, really be pulled into the mass of protons. If only a few are missing electrons then the voltage (pressure) for electrons to flow in will be lower.

Now amperage is how many electrons are heading over at the same time. When you jammed that wire in earlier the electrons flowed in from one atom to the next in a dominos fashion. Now imagine you have a really thick wire. Then things can flow like some 1000 wide domino cascade. Lots of electrons are flowing by at the same time. Does this change the voltage? Well it means the gob of protons are going to get their balanced share of electrons much faster.

All this stuff happens about as close to instantly as we can really comprehend (not quite speed of light but someone here could probably give you a number). The trick to making electricity usefull is somebody has to take that gob of protons (stripped atoms) and keep stripping the electrons off so that new ones coming in don't cause things to become balanced and stop.

Batteries use a chemical process to do this. Generators use electromagnetism. In some form or another they keep pulling electrons out of some gob of matter so that a voltage or pressure exists for new electrons to go in. Remember though - voltage is "potential"...nothing happens unless you connect a wire or other chunk of electron-rich matter to your gob of protons.

You also need the place where electrons are coming from to get a steady supply of new ones or again, the whole process will quickly stop. Forming a complete electrical circuit is one of the keys here. One side of a battery will be short on electrons so voltage or pressure exists for things to flow there. Connecting a mass of atoms will allow the actual flow to start. If you connect the other end of the mass of atoms back to the opposite end of the battery (where the arriving electrons are being chemically shoved to), a continuous supply of electrons will be available for the flow to happen. You now have a circuit with a continuous flow of electrons around in a circle and a battery/generator acting as sort of a 'pump' to keep the gob of atoms on one end of it short on electrons and the gob of atoms on the other end packed full with electrons.

The other way to keep the flow happening without a circuit is to use the earth. The earth is pretty much an unlimited supply of atoms with their electrons intact. The earth in this manner can really act as a wire itself. Plug one end of a battery into the earth. Plug the other end into a lightbulb. Plug the other wire coming out of the lightbulb back into the earth....
One end of the battery has excessive electrons the other end is short of electrons. It will pull electrons in from the earth on one side, and out to the earth on the other. In the meantime the electrons are flowing by the lightbulb so it lights up for you.

You could also just take the wire coming out of the wire and stick it back into the other end of the battery to do the complete-circuit thing as well.





ok, I've babbled. Not even sure if that was helpful. hope so

 

[marulee]

Fair enough!

 

[silverpig]

Originally posted by: Born2bwire
Imagine you have a chicken at one end of your fence run. Now after you remove the chicken's head, it will run around somewhat randomly but it may eventually tend to move from one side of the run to the other. The tendency for the headless chicken to move to the other end of the run is voltage.

That's more like diffusion.

 

[blahblah99]

Originally posted by: Leros
I've been having a really hard time understand what exactly voltage is. I've read wikipedia, I've read my textbook. I have no problem working with voltages. Equations are equations. I just can't wrap my head around the concept of voltage.

In my mind:

Current is the amount of electrons flowing. In the hydraulic analogy, this is the gallons/minute. I understand that just fine.

Voltage is the difference of electric potentials between two points. I can't visualize what that actually means. Any help?

Imagine two water towers connected by a pipe at the base of the towers. THe pipe is closed off... now fill up one tower. The difference in the levels of water in the towers is the "voltage", and the amount of water flowing through the pipe is the "current". If you open the pipe and allow water to flow to the other tower, the two towers will eventually be equal and there will be no more water flow, hence, no more "voltage" since the two towers are now at equal water level.

 

[zinfamous]

Originally posted by: silverpig

Originally posted by: Born2bwire
Imagine you have a chicken at one end of your fence run. Now after you remove the chicken's head, it will run around somewhat randomly but it may eventually tend to move from one side of the run to the other. The tendency for the headless chicken to move to the other end of the run is voltage.

That's more like diffusion.

diffusion isn't random. it's the flow of particles from areas of higher concentration to areas of lower concentration. if using the chicken as a particle, then the opposite side of the fence would have a lower concentrattion of chickens (0).

But this isn't relaly diffusion b/c the chicken isn't moving to the less concentrated area b/c it prefers too, it just has no head and no idea where it is going.

Also, I'm not so sure diffusion is qualified with only one particle; as it could never reach an equilibrium point. I.E., A single particle will always inhabit the most concentrated space.