Potential vs. Voltage: Explained

[quasi426]

Can someone explain to me what the difference between a potential and a voltage are. My understanding as of now is that a potential occurs when two elements of different charge density are compared but not connected to each other. Therefore the potential indicates the possible voltage that may occur if the two elements were to be connected to complete a circuit. I'm sure it is flawed, thanks for the help.

 

[Your.Master]

To my knowledge, voltage and potential are the same thing (but voltage is specifically about electric potential, at least in usual parlance if not always).

 

[misogynisticfeminist]

voltage is either the electromotive force or the potential difference. Electrical potential energy is simply something used to explain cases like...a positive charge when attracted to a negative charge gains kinetic energy (as it accelerates towards the negative charge), where did that energy come from?

Like different points on the Earth, say, your house and the nearby mountain. Your house (assuming it is at sea level) has a lower gravitational potential than the tip of the nearby mountain (a unit mass has less grav. potential energy in your house than on the mountaintop). A ball can only drop from your mountain to your house because of the difference in height.

More specifically, the ball drops due to the difference in gravitational potential.

Similarly, charges in circuits flow (like the ball dropping) due to potential Difference and not potential itself.

 

[Metallicbeing]

Electrically speaking, potential and voltage are the same. The only difference is the way each "term" is used to discribe a given scenerio. When you talk about voltage, you're generally talking about doing work with it. When you talk about potential, you're generally talking about relating one reference to another.
One example using potential would be talking about two systems that are seperately grounded. Grounded, by definition, means this is the zero point of reference in your system. Sometimes, if you take a voltage reading between two seperately grounded systems, you'll find that you have some "potential" between them. This is the correct way to use the term "potential".

 

[lightgrav]

NO! MetallicBeing listens to sloppy speakers ...
after making sloppy ground connections, they SHOULD
use a VOLTMETER to measure POTENTIAL DIFFERENCE.

Electrically speaking, Electric Potential and Voltage
are measured in the same units ... but not the same.
Temperature and Temperature difference are both
measured in Kelvin, but I would say they are different.

If two subtracted quantities are not different than
either quantity, why do they it a DIFFERENCE?

 

[Metallicbeing]

NO! MetallicBeing listens to sloppy speakers ...
after making sloppy ground connections, they SHOULD
use a VOLTMETER to measure POTENTIAL DIFFERENCE.

Electrically speaking, Electric Potential and Voltage
are measured in the same units ... but not the same.
Temperature and Temperature difference are both
measured in Kelvin, but I would say they are different.

If two subtracted quantities are not different than
either quantity, why do they it a DIFFERENCE?

Lightgrav:

Chill out man, it's not the end of the universe. You're right. "Potential DIFFERENCE (relative)" isn't the same as "Potential (absolute)"; just like "Temperature DIFFERENCE (relative)" isn't like "Temperature (absolute)".

I was using "potential" in the "absolute" context in my sentence (was I not?) even though the measurement itself, implies a difference between the two "sloppy" grounds. I was merely saying that there was a "potential" between them.

You're playing semantics. The poor guy just wanted to know how he should view diferrences between "Potential" and "Voltage". What he said in his opening statement is essentially right. He just needs to be able to view each term in its correct context.

 

[Speleosapien]

To Metallicbeing and to Quasi426,

Metallicbeing, I agree with you 100%. And the following contains additional information that others might find useful. In the first few paragraphs, I have used your words and I have applied a few grammatical changes to present our ideas in third person, to correct a couple misspellings, and to expand on your ideas.

Quasi, see my responses to you at the end.

Electrically speaking, potential and voltage are the same. The only difference is the way each "term" is used to describe a given scenario.

When one speaks about voltage, one is generally referring to doing work with the Value (ie., modifying the circuit, presenting a load, measuring the voltage for meaningful calculations to be considered, or to check the results of calculations or modifications made to the circuit, etc.).

However, when one speaks about potential, one is generally speaking about relating one reference to another.

An example for using the word "potential" would be to speak about two systems that are separately grounded.

"Grounded", by definition, means that the system has a zero point of reference.

Yes. And there are different types of zero references. Earth-grounding is used in most cases to provide a safe shunt-to-earth ground for currents that have "strayed" out of their normal operating scenarios. Supposedly, if the massive chassis of an electromotor or turbine-powered generator becomes energized because of an internal fault, the massive grounding straps will shunt the current to Earth so that humans or other machines in the area do not become conductors to ground.
The "Earth-ground" conductor, to which the Neutral conductor is connected, has also the possibility of conducting part of the current returning from the load.
But there are also "Ground-planes" that have nothing to do with Earth-ground. These Ground-planes provide local power supplies and components of a particular system or PCB (printed circuit board) its own point of reference. Many times, these Ground-planes will have significant differences in potential compared to Earth-ground or even compared to other system's ground planes. It is best to leave such Ground-planes floating and its associated system totally isolated.

Sometimes, if one takes a voltage reading between two separately grounded systems, one will find that there is some "potential" between them. This is the correct way to use the term "potential".

Yes. Now, if one wishes to somehow make some kind of inter-connection between the two systems, simply determining that a potential exists is NOT the end of the story. If the potential exists between the two Zero-point references, care must be taken to consider the effects of connecting the two together, especially if the two systems are powered by different PHASES.
Other considerations:
Is the potential difference the result of some cross-talk from the phase line? Or from some other "leakage" from other in-system component. Does the potential contain some AC characteristic like 50/60 Hz, or does it contain a characteristic from RF radiation, such as a microwave antenna or radio station in the area? Generally, such AC problems can and should be corrected using decoupling capacitors of appropriate values and sufficient numbers to reduce the AC component to within reasonable or workable limits. Now, I know what you are thinking. We were first talking about DC, not AC potentials.

DC potentials can also exist between two separate systems for some very similar reasons. In-system components can influence the local Zero-reference. The reason you can measure a potential (or voltage difference) between the two systems' Zero-reference is that there may be a great distance(electrically) between the two "ground planes", or the Neutral conductor may be insufficient in size(diameter) to eliminate the difference. Even the copper- Neutral conductor and Earth-ground conductor present "some" resistance to current flow. (Remember what happens to your power saw when you power it using a 50 meter power cord? It runs slower because the cord itself is "dropping" some of the voltage.)

Additionally, if a connection is to be made between TWO SEPARATELY GROUNDED BUILDINGS, consider that there could be some difference in the way the buildings are themselves grounded. Could there also be a difference in the materials used to build the foundations that could be creating some kind of dis-similar materials reaction with the Earth itself. Could there be different types of ground-rods used? (Copper-rod?, copper-plated?, copper pipes?, iron pipes?, steel pipes?, aluminum?, Insulating materials used somewhere in the pipe or conductor train to earth-ground?, improper binding, hence a resistance?, dissimilar metals used in the grounding circuit, hence a voltage potential created?.) Might one foundation grounding be saturated with water, and the other high and dry, virtually insulated and "floating"? It's all connected somehow, and altogether, there could be numerous combinations of little µVoltages running throughout. True, Building and Electrical Codes exist to standardize this, but... we are speaking about Reality.).

Yes, one could simply connect the two "ground-planes" together with a conductor large enough or short enough to present virtually no resistance. But take care. What effect will there be to the power supplies or to the delicate semiconductor circuits of a system if the Zero reference is suddenly raised or lowered by an arbitrary 0.5 Volt or 2.3 Volts compared to its input phase or compared to its ability to relate to the signals received from yet a third system. This potential must be considered, measured and the direction of current flow determined. By eliminating the resistance previously existing in the Neutral conductor, you are now introducing to each system the full effect of the voltage difference, basically adding a battery or two between them. Current will flow.

But, better that this current flows in the Neutral conductor than through your data lines. (Data lines should be isolated anyway by use of isolation transformers, or optoelectronics.)


To Quasi426, who had begun this string. I understand your frustration over this question. This discussion has occurred more than once at our BerufsSchule (Trade School). You had written:

"Can someone explain to me what the difference between a potential and a voltage are. My understanding as of now is that a potential occurs when two elements of different charge density are compared but not connected to each other. Therefore the potential indicates the possible voltage that may occur if the two elements were to be connected to complete a circuit. I'm sure it is flawed, thanks for the help."

I agree with your original understanding in general except for a one word. Where you had written, "Therefore the potential indicates the possible "voltage" that may occur if the two elements were to be connected...". I respond, that you probably meant to say that, "potential indicates the possibility that "current" would occur if the two were connected. If the voltage difference (the potential) exists, then, absolutely yes, ...if the two are connected, current will definitely flow. Voltage does not flow; current flows. Voltage exists, or lies on ..., or is applied to... Current flows through...

Also, "Charge Density" is yet another subject for discussion we need not get into for the purpose of the discussion of "potential vs. voltage". Suffice it to say, if you have a difference in potentials(which is also referred to as "potential"), then it can be measured in units of Voltage, and there is the certainty for current flow if the two are connected with any conductance having any resistance less than Infinity.

 

[sophiecentaur]

It may be worth mentioning that Electric Potential (or gravitational) is, strictly, defined as the energy needed to take a unit charge (or unit mass, in the other case) from infinity to the point you are dealing with.
The reason for taking infinity as your reference point is that you have to choose somewhere and if you choose zero distance, it introduces problems when you do
1/r1 - 1/r2.
You can have an object at any potential but yo can't use the energy unless the charge / mass goes to another potential.

 

[Dale]

Can someone explain to me what the difference between a potential and a voltage are.

I think that the descriptions here are reasonably correct, but a little overcomplicated. Here is how I think of potential and voltage:

If you have any conservative force then the force can be described as the gradient of some scalar-valued function. This scalar-valued function is called the "potential". This defines the potential for arbitrary conservative forces. The electrostatic force is a conservative force and therefore it has a potential which is called "voltage".

All of the above discussion about differences is due to the fact that if you have two scalar valued functions that are the same except for a constant offset between the two then their gradients will be the same. So the potential is only defined up to a constant and only potential differences have any physical significance.

 

[sophiecentaur]

Voltage is Potential Difference, surely?
You have to connect both ends of your meter and you can't be sure that the potential of 'Earth' is actually zero. Did you count all the spare electrons on the Earth before you did the measurement of Volts?

 

[tiny-tim]

Sound-bite …

electric potential is potential energy per charge​

… and since potential energy is just another name for work done (by a conservative field), that also makes it electric field "dot" displacement per charge …

which is voltage! ​

 

[Naty1]

voltage or volts is one unit of measure of electrostatic potential...

Wikiepdia has a decent discussion with some basic math relationships...

http://en.wikipedia.org/wiki/Electrical_potential

for most uses voltage is a measure of potential...and both measure differences rather than absolutes...in a fields and waves class for example you might be asked to determine the electrostic potential (formula) for a given charge distribution...thats not the terminology typically used in a circuit with batteries, resistors,etc...where voltage and current (amps) is the conventional terminology...

 

[sophiecentaur]

Both have the same units (JC^-1, of course) but, if they weren't distinct quantities, why have a term 'potential difference'?
The only realistic reference for electric potential must be 'infinity', surely.

 

[Dale]

You have to connect both ends of your meter and you can't be sure that the potential of 'Earth' is actually zero.

The only realistic reference for electric potential must be 'infinity', surely.

You can arbitrarily define any point you want to be your zero reference.

 

[arithmetix]

I think that there is no difference whatever between "potental" and "voltage". You can call it different names in different contexts if you want but that's up to you, if a voltage can be measured between two points there is potential between then, whether current is flowing or not.
You can call it e.m.f. too, and other namesbut we're still talking about voltage = electrical pressure = emf = potential.
(The voltage which exists in the term voltage is not voltage at all, the true voltage bites you when you mis-handle it.)

 

[mikelepore]

Even "potential" without the word "difference" is really a potential difference, because then the reference is taken either at infinity or a ground. The potential at a point is the work per unit charge that is done to move a positive test charge from the reference point to the given point. The {potential difference between point A and B} is the difference {the potential difference between point A and the reference point} minus {the potential difference between point B and the reference point}.

 

[Naty1]

The only realistic reference for electric potential must be 'infinity', surely.

no, but that's how we define potential energy, like gravitational potential. I think it's convention lot's more than a "right" or "wrong"...

 

[Born2bwire]

no, but that's how we define potential energy, like gravitational potential. I think it's convention lot's more than a "right" or "wrong"...

Another point here being that you can have systems where the potential is divergent, making the potential at infinity infinite.

 

[Phrak]

Electrical potential is measured in volts.

'Voltage' is the common term for electrical potential; two syllables are shorter than seven.

As far as we know, a unique zero of electrical potential does not exists; the laws of physics are unchanged when all electrical potentials in a system are replace with a value offset by a constant. Electrical potential is said to be globally gauge invariance.

 

[sophiecentaur]

I don't what the disagreement is about. My definition of potential merely comes from textbooks and I can see why that is the definition which is used.
Infinity is the only realistic reference for good practical reasons.

Of course we all use terms in a sloppy way but when asked for a definition I would try to provide the most bomb-proof one I can think of. In pretty well every situation I can thinmk of 'voltage' is synonymous with 'potential difference' because it is a complete description of the situation. A birdie on a power line is not concerned with his absolute electric potential. He is just happy that thaPD between his feet is a few pV.

 

[Dale]

Of course we all use terms in a sloppy way but when asked for a definition I would try to provide the most bomb-proof one I can think of.

The problem is three-fold. First, your definition of potential is not sufficiently general since all conservative force fields have a potential, not just the electrostatic force field. Second, it is overly complicated requiring you in all cases to calculate the field out at infinity just to determine some constant which has no physical significance whatsoever. Third, even in electrostatics it fails with some very elementary charge distributions which cannot be zeroed at infinity, such as a line charge.

The correct "bomb proof" definition of "potential" is the one I gave in post #9 above. Mathematically:
$$\mathbf{F}=-\nabla U$$
where $\mathbf{F}$ is any conservative force field and $U$ is the associated scalar potential.

 

[sophiecentaur]

Help me with this one. Is the line charge of finite length?

 

[sophiecentaur]

Dalespam
here's another thing. It's all very well defining potential In your way but - spot the differential! To find out the actual potential you have to Integrate!
So apart from having produced an answer, you still haven't told us what should be the limits for the inevitable integral. I don't think your definition is good enough.

I have a feeling that OP was aimed at more of a nuts and bolts answer and I'm trying to produce one. Slinging 'Del's in just manages to confuse folks like me. But I think I have revealed a problem. Have I?

 

[mikelepore]

My definition of potential merely comes from textbooks and I can see why that is the definition which is used.
Infinity is the only realistic reference for good practical reasons.

It depends on the application. In the wiring of a house, the reference potential is that of the cold water pipes. All exposed metal in the house, from the screws on a light switch to the handle on a refrigerator door, are connected to be at the same potential as the cold water pipes.

 

[Dale]

Is the line charge of finite length?

No.

It's all very well defining potential In your way but ... I don't think your definition is good enough.

It is not my definition of potential. It is the physics definition of potential.

But I think I have revealed a problem. Have I?

Not particularly, but let's step through it just for the sake of clarity.

spot the differential! To find out the actual potential you have to Integrate!

Yes, as with most physics equations the definition of potential is a differential equation, and integration can indeed be used to solve any linear first order differential equation (but not more complicated ones). As you probably know, whether you use integration or some other technique to solve a differential equation you always wind up with an arbitrary constant of integration. Your insistence that the potential be defined such that it is zero at infinity amounts to requiring a specific value for the constant of integration when, in fact, any arbitrary constant will satisfy the equation. There is no physical reason to make this restriction and, as mentioned above, there are some good reasons not to make it.

So apart from having produced an answer, you still haven't told us what should be the limits for the inevitable integral.

There are no limits. When integration is used to solve a first order linear differential equation it is always an http://en.wikipedia.org/wiki/Indefinite_integral" (aka antiderivative).

 

[sophiecentaur]

'PD' has the same number of syllables as 'Voltage' and fewer letters, Phrak! ;-)

DaleSpam
How does the fact that you can't define the potential of a line charge of infinite extent referred to infinity have any bearing on practicality? The same could apply to many other charge distributions, I'm sure. I haven't come across many practical situations where an infinite distribution is involved.
I don't have a problem with the Differential expression which involves potential but it doesn't actually give you a way of working it out unless limits are specified.
This is dragging out too long. My only point is that the term 'Voltage' is a very practical one and refers to a measurable quantity (with a voltmeter, at least in principle). A voltmeter has two terminals and measures a difference in potential. It doesn't care where it is being used. The very word 'Potential' has the implication of the availability of energy / work - which must involve the idea of adding up the work over some distance - and the path needs to be defined.

If two scientists on different planets were told to go away and find a suitable reference against which to define Electrical Potential, do you think they would be likely to agree on the same planet to use? Referring it to Infinity would be the only mutually agreeable way, I think.

 

[Born2bwire]

Practicality does not come into it at all. If you sent your scientists off to find a reference they would have to agree that there is no reference because infinity is not a possibility for all problems. In addition, because a scalar potential is gauge invariant, there is no correct reference point. I can always apply a valid gauge transformation and change the reference without affecting the physics or results. I could define the voltage at infinity for problems that prove to be finite to be -3\pi without loss of generality.

Another point though, the path does not need to be defined. That is one of the main points about DaleSpam's definition. By noting that a potential results in a conservative force (explicitly implied by the gradient of the scalar potential), the path taken through the potential is irrelevant to the net work done between two points. Only the endpoints matter, not the path.

 

[sophiecentaur]

OK about use of the word "path". I appreciate that; bad choice of mine.

But, as you say "Only the endpoints matter,".
There have to be two points and that gives you the difference in potential. That was my only original message - that Potential is a lot less useful than Potential Difference and it is PD that 'means' Voltage.

There is only 'other end' point which everyone in the Universe could come up with and couldn't disagree about in a definition for 'absolute Potential'. It would just have to be infinity (I can't agree that practicality doesn't come, necessarily, into Science).

Apart from the distributions of infinite extent (not a practical situation, I think) what arrangements cannot have their potential calculated wrt infinity? If an integral can be solved then why not for infinite limits (bearing in mind that there must always, surely, be some inverse distance law operating)?

Btw, at what stage in Physics education / advancement does absolute Potential stop being defined, referenced to infinity? I clearly didn't get that far but I am always prepared to learn:-)

 

[Dale]

How does the fact that you can't define the potential of a line charge of infinite extent referred to infinity have any bearing on practicality?

Practically it allows simplifications of problems. Let's say that you have a charged ring of some weird geometry with some straight segments and some curved segments, some in plane and some out, and let's say that you are only interested in the field in some small region near the middle of one of the straight segments. Using the standard approach you can just approximate the potential near the middle of the line segment as the potential around a line charge. Using your approach you cannot make that same approximation since the potential at infinity is infinite, so you would have to evaluate the entire complicated geometry of the whole ring in order to get a finite potential. In the region of interest you will wind up with the same field as everyone else, but only after a lot more calculations.

The same could apply to many other charge distributions, I'm sure. I haven't come across many practical situations where an infinite distribution is involved.

Then you haven't actually used it much in practical situations.

I don't have a problem with the Differential expression which involves potential but it doesn't actually give you a way of working it out unless limits are specified.

This is getting repetitive. I already addressed this above. There are no limits, it is an indefinite integral. I even provided a link so that you could learn more about indefinite integrals.

A voltmeter has two terminals and measures a difference in potential. It doesn't care where it is being used.

Precisely. You are arguing my point now. There is no need for the reference to be connected to "infinity".

If two scientists on different planets were told to go away and find a suitable reference against which to define Electrical Potential, do you think they would be likely to agree on the same planet to use? Referring it to Infinity would be the only mutually agreeable way, I think.

The point is that there is no need whatsoever for them to agree. They will make all of the same physical predictions regardless of what values they might pick for the constant of integration.

sophiecentaur, I have lost interest in this conversation, it is going in circles. The definition of potential is what it is, and your aesthetic preference for defining the potential at infinity to be 0 is irrelevant. The gauge invariance of the potential has immense practical value in many circumstances, particularly in quantum mechanics. It allows you to pick any value that simplifies your problem. In fact, you can even split your problem up into separate sub problems and use a different gauge to simplify each sub problem. You really should learn the definition as it is instead of trying to change it. There are a lot of good reasons for the standard definition.

 

[sophiecentaur]

OK
Fair enough. I can see what you are saying about making calculations possible.
My definition was, clearly, a bit 'Schoolboy' but it is written down all over the place in textbooks!
It would seem that we are in fact agreeing that potential has only any real use when expressed as a difference and that you have to specify some reference.

btw, can you give me a practical situation? Perhaps you are referring to cases where a practical situation can be approximated to an infinite one - which can then be solved?

 

[Dale]

btw, can you give me a practical situation? Perhaps you are referring to cases where a practical situation can be approximated to an infinite one - which can then be solved?

I gave one above, but here is another one of immense importance in the field of neuroprosthetics and neurophysiology:

Let's say that you want to analyze the flow of ions across a small patch of a neuron's cell membrane. If the patch is small enough (e.g. patch clamp experiments) then the local fields are well approximated by considering the membrane to be a pair of infinite sheets of charge. A sheet of charge has the same problem at infinity as a line of charge, so you couldn't use that approach if you insist on a 0 potential at infinity for a sheet of charge. If you relax that requirement then the problem becomes relatively easy to solve and the results are quite accurate and powerful.

 

[sophiecentaur]

I get it now - thanks.
You avoid having to consider 'end effects'; smart.

 

[dE_logics]

A section of my notes -

Electric potential -
As we have all read since childhood, its the work done (in joules) to bring 2 charges together/apart (from/to infinity) of the same/different polarity, this is just an example, actually its the work done to move a charge through an EF (and cause E.F is a conservative field, the same energy is stored in form of P.E).
The reason for taking infinity is cause if a point is taken, it will no longer be called potential, but something else (the next heading actually).
Its Potential = W/Q by this the unit of potential will be j/c which is called 'voltage'.
So if we encounter such an arrangement such that the work done per columb is one joule, then the potential is too 1 V.
This definition is working as expected, its a known fact that In a constant current source if the resistance in the circuit is made to increase, the potential increases too, that is to pass through the resistance, more work needs to be done.

Potential difference -
Usually the electric potential computed is WRT earth, in fact electric potential is an understatement cause its not defined WRT what it has the potential (its usually WRT earth) and is relative to infinitely (that is work done to bring that charge from infinity to that point).
The term potential difference is a complete statement, cause in this the PD is relative, I mean, if the PD between Earth and some point is 1.5, then the work done to transfer a charge form that point to Earth is 1.5 J.
If its not WRT earth, and the potential difference WRT another point jumps to 20 V, then the work done to transfer a charge form that point to Earth is 20 J, so P.D is relative.
Though P.D is a major criteria determining the work done by a charge (or many charges), it can happen that if with the same P.D, the resistance of the circuit is decreased, more energy is produced at the same potential difference.
The reason for this is that definition of potential difference is work done per charge, so if the no. of charges per unit time (rate of charge flow) increases, though the work done per charge will remain constant, so the total energy delivery will change.
Initially just assume that the momentum (and so velocity) of electrons in a circuit is directly proportional to the P.D, it will be explained later...what actually happens.

Its to be taken into account that in a circuitry having a constant voltage of EMF E, the the energy dissipated in the outer circuit (that is out of the battery), is equal to the EMF of the battery, as we talk about a constant voltage source delivering energy, this definition described is the aim of the constant voltage source that is no matter what happen constant voltage source will deliver the rated energy per charge...of course some can't do that by 100% that is have a bit of inaccuracy.