Voltage

[Ifat Glassman]

What exactly do we measure when measuring voltage (or potential) of something?

Theoretically, electrical potential is the sum of work that had to be done to bring a positively charged particle from infinite distance from the system to it's current location, where it has this specific potential that we measure with our voltmeter.

Well theory is fine and well, but what do we actually measure with a voltmeter?

Can there be voltage if there is no charge accumulation (in comparison to some other point in space)? And if not, how is it possible that a wire has X voltage on it, even though there is no charge accumulation at any given location in the wire?

Second question: voltage spreads through a wire in the speed of light. What is it exactly, that spreads through the wire?

[Felix]

What exactly do we measure when measuring voltage (or potential) of something?

Theoretically, electrical potential is the sum of work that had to be done to bring a positively charged particle from infinite distance from the system to it's current location, where it has this specific potential that we measure with our voltmeter.

Well theory is fine and well, but what do we actually measure with a voltmeter?

Okay. First of all, you can only measure voltage or potential difference. There is no absolute zero-potential. This is chosen on a case by case basis just for simplification. It's just a mathematical model. You can't measure potential.

Also, the entire idea of using a potential-difference-model to describe voltage only applies to Electrostatics (when there are no moving charge carriers).

To explain how exactly a voltmeter works, I'd have to dig up the script in my basement.

I can do that if necessary, but hopefully someone else will tell.

What you basically do is measure the accumulated effects, charges have on one another, usually forces charges apply to one another, if I remember correctly. From these effects you can judge back to which voltage must be there to cause such an effect. You can't really measure voltage in the strict sense. All you can do is measure effects.

Can there be voltage if there is no charge accumulation (in comparison to some other point in space)? And if not, how is it possible that a wire has X voltage on it, even though there is no charge accumulation at any given location in the wire?

Again, we're in Electrostatics, the electrical field results from the position of charges in space and so does the potential field and therefore also the potential field difference or voltage. So any voltage between two points has to be the result of the position of charges in space.

Since you can pick the point where the potential is zero, you can say that a charged wire has zero potential and an uncharged wire would therefore have a (non-zero) potential. The voltage between the two, however, would remain the same no matter where you put the zero potential.

Second question: voltage spreads through a wire in the speed of light. What is it exactly, that spreads through the wire?

Electromagnetic waves. Electric and magnetic fields change inside the wire over time according to Maxwell's laws.

Please note that this question relates to Electrodynamics and no longer to Electrostatics.

I think that this is what causes most of the confusion.

In electrostatics, there is no change of fields over time, therefore nothing spreads. Electrostatics is an idealized model to explain simple electrical effects where charges don't move. One assumes that any time-dependent effects have already taken place and that you now have no change anymore.

I hope this helped a little.

[y_feldblum]

http://www.physicsforums.com/ not providing answers?

[Lagroht]

Disclaimer: I am not a physicist.

electrons and protons attract each other, by convention 1 electron has a charge of -1 and a proton has a charge of +1.

(wikipedia: elementary charge)

This attraction is a force, which can be denoted in Newton (force required to give 1 kg an acceleration of 1 m/s^2).

1 joule = 1 N * M, in other words the amount of energy to accelerate an object with a force of 1 Newton, over a distance

of 1 meter. (of course if the actual object you apply the force is heavier than 1 kg you will not get the 1 m/s^2

acceleration, but if the object is in fact 1 kg, you will)

and if you keep on supplying energy to fuel your force application you can express this in watts (1 w = 1 joule /s)

now note that the charge of 1 coulomb equals the charge of 6.25 * 10^18 electrons and that 1 volt equals 1 joule / 1 coulomb.

since 1 electron has a given charge, so do 6.25 * 10^18 electrons, but what can be different is the amount of force applied to them;

this is determined by the existence of an equal or unequal amount of protons on the other size of the conduit; This is what determines

the amount of potential energy of a given charge point. (in other words the number of joules per coulomb).

"One Coulomb © is the amount of charge such that a force of 9.0×10^9 N occurs between two point like objects with charges of 1 C separated by a distance of 1 m."

(from wikipedia)

So if you look at 1 joule / 1 coulomb = 1 volt, what are we talking about if we talk about 14 volts potential difference between points A and B?

I guess we talk about 14*6.25*10^18 joules of potential energy spread out over 6.25 * 10^18 electrons caused by a surplus of electrons at 1 of the points

relative to the number of protons at the other point. In reality the point probably will not contain exactly 6.25 * 10^18 electrons, but are we only talking

about a fraction (of joules per coulomb) which yields the same voltage.

Also note that this indicates that given a fixed number of electrons in a given point the voltage associated with that point is strictly relative to any other point.

Note how this makes sense, the number of electrons at a given point can not change due to a comparison with a random given point;

only the attractive force of the point you compare with changes, it also explains how a high voltage can be harmless; if the charge (number of electrons) of a given point is low, no matter how large to force applied to them; if they start moving they will always produce a low current.

In other words: the number of joules per coulomb of a given point is different given the charge of whichever other point you compare that point with.

I guess the wikipedia definition is talking about the situation where 1 point has 6.25 * 10^18 electrons and the other has 6.25 * 10^18 protons. Apparently this

generates 9.0×10^9 N of force. Quite massive, so in normal electronics we are talking about much lower potential differences.