Voltage

[Kevin Weddle]

We are accustomed to maintaining differences of potential in electronics. But isn't the voltage level, of let's say a rock of copper, an absolute. I believe that in theory, you could just use one probe to measure voltage. Never seen a voltage meter with one probe, but there could be one, since voltage is the amount of energy possesed by a certain number of electrons. There does not have to be a voltage difference.

 

[jstevenperry]

Voltage is not the number of electrons in a block of conductive material (nor the energy contained within them). It is a *potential* (as you mentioned) for those electrons to flow, which, by definition, means it MUST BE measured relative to some other point. Unless compared to some other, different, potential, the number of electrons in a block of conductive material cannot be said to have a *voltage* as they are all electrically common. Your voltmeter with one lead will never measure a meaningful voltage.

Your voltmeter with one probe makes for interesting philosophy (like the Zen koan of "one hand clapping") but is of no practical use.

Maybe there's a EE on this forum that can clear it up for you. My degree's in physics, and so I'm probably coming at this from a more theoretical position...

HTH

 

[Kevin Weddle]

Voltage is not the number of electrons in a block of conductive material (nor the energy contained within them). It is a *potential* (as you mentioned) for those electrons to flow, which, by definition, means it MUST BE measured relative to some other point.

A difference of potential makes electrons flow. Voltage is the amount of energy in joules divided by the number of electrons in coulombs. You can't measure the voltage difference between the AC outlet and the earth, yet we know the voltage difference is enough to shock someone. The two probe meter becomes ineffective when trying to measure the potential difference that definitely exists. We might have to go elsewhere besides electronics to determine this voltage. If so, do we connect this device between the two points, or could it be possible to determine one point first, then the other point, and simply subtract to find the difference.

 

[jstevenperry]

You pose an interesting question. Is there a potential difference even if I don't measure it (by connecting a Voltmeter to two points in the circuit and measuring the difference between the two points)? I guess the answer, from a quantum mechanics standpoint would be, well, maybe.

But, unless we take a measurement (and thus interfere - albeit slightly due to the Voltmeter's incredibly high input impedence - with the system) we don't know what the potential difference would be.

Still, since voltgage is always measured as a difference in potential between two points in space, what is the physical meaning of voltage of a single point? I think we're in the realm of some serious theoretical stuff (good choice, btw, of a place to post this on the forum).

I think I'm in over my head...

 

[shekhar_dandya]

"Potential" of a point in space,as I was taught in school, is energy spent in bringing a charge from infinity to that point within the vicinity of test charge,which is exerting the force on our charge.
So energy spent is stored as

 

[jstevenperry]

Nicely stated. My point exactly. It is a "difference." Which means its nature is inherently linear (and thus requires two points in space).

 

[prateeksikka]

hi kevin!
i agree with your point that we can measure voltage at a point with one probe only.but there is one condition ....
to give us this right,we are forced to assume earth at zero potential wrt anything else on it.
take for example a +5 V can be measured w.r.t 0V with 1 probe only.there would be no probe at all.But u need a second probe to see if the other lead is at -5V to see if the p.d is 10V and not 5V !!!!!
i hope u are more clear....
;D

 

[Ldanielrosa1]

What is the difference between the left hind wing of a duck?

The higher it flies the much!

 

[gogo2520]

I guess I don't unstand the question. One point would be the same no matter what because you would have nothing to compare it to. Unless you had a second Point to compare it to there is no differance.
    And hopefully that ducks right wing is the same or the duck wouldn't be able to fly stright.
                        have fun
                              gogo