Negative Electrical Voltage and Charge

[Mr. Scott]

In the skepticism forum thread "Is Death A Myth?" we got on a tangent about whether or not an electrical charge or voltage can really be "negative" in an absolute way as well as a relative way.

Here are some starter questions to bootstrap the science forum on what it's about.

Can there be a negative absolute voltage?

Can an absence of electrons create a positive charge, or is the presence of excess protons responsible for a positive charge?

What does it really mean that electrons or accelerated to, say, a million electron volts? Can this be either positive and negative voltage?

Why do we refer to electrical charges as positive or negative and refer to magnetic poles as north or south? Are these terms more archaic than descriptive?

Thanks!
S

[Stimpson J. Cat]

Mr. Scott,

In the skepticism forum thread "Is Death A Myth?" we got on a tangent about whether or not an electrical charge or voltage can really be "negative" in an absolute way as well as a relative way.

Here are some starter questions to bootstrap the science forum on what it's about.

Can there be a negative absolute voltage?
There is no such thing as an absolute voltage. Voltage is just a measure of the difference in potential energy between two points.

Can an absence of electrons create a positive charge, or is the presence of excess protons responsible for a positive charge?
Ignoring the existence of other charged particles, no and yes. If you have no protons or electrons, you have no charge, positive or negative. Add more protons than electrons, and you get a positive charge. Add more electrons than protons, and you get a negative charge.

What does it really mean that electrons or accelerated to, say, a million electron volts? Can this be either positive and negative voltage?
An electron volt is defined to be the energy which an electron will gain (or lose) when accelerated accross a 1 volt potential. The force on the electron will always be in the direction from high potential to low potential. So if the electron starts at rest at a potential of 1 million V, then when it reaches a potential of 0V, it will have gained 1 million eV of kinetic energy. Likewise if it started at 0V, but with 1 million eV of kinetic energy moving towards the potential of 1 million V, then when it reaches that point, it will have stopped, and will then begin to accelerate back in the other direction.

Why do we refer to electrical charges as positive or negative and refer to magnetic poles as north or south? Are these terms more archaic than descriptive?
Both. Positive and negative is natural for electric charge because of the way the two types of charges cancel out. Mathematically, we have to describe one charge as being positive and the other as being negative. The choice of which should be positive or negative is, of course, completely arbitrary.

In the case of North vs South, this just follows from the fact that the Earth is a big magnet. So in that sense, you could say that it is more archaic than descriptive. Also keep in mind that there is no "magnetic charge". The most fundamental quantity in magentism is the magnetic dipole, which must be represented as a vector, rather than as a scaler quantity which is either positive or negative. So the North-South choice is also descriptive.


Dr. Stupid

[Soapy Sam]

Mr. Scott,

(snip) " The most fundamental quantity in magentism..."

Dr. Stupid

Ah! Magentism!
http://www.magenta-club.co.uk/

(Sorry Stimpy. Too good to miss.)

[Stimpson J. Cat]

proper speling si fro nurds :p

[Mr. Scott]

Mr. Scott,


Both. Positive and negative is natural for electric charge because of the way the two types of charges cancel out. Mathematically, we have to describe one charge as being positive and the other as being negative. The choice of which should be positive or negative is, of course, completely arbitrary.

In the case of North vs South, this just follows from the fact that the Earth is a big magnet. So in that sense, you could say that it is more archaic than descriptive. Also keep in mind that there is no "magnetic charge". The most fundamental quantity in magentism is the magnetic dipole, which must be represented as a vector, rather than as a scaler quantity which is either positive or negative. So the North-South choice is also descriptive.

Dr. Stupid

Thanks for the illumination.

Don't magnetic vectors add, subtract, and cancel out like electrical voltages? If we called magnetic poles negative and positive instead of north and south, would the mathematical descriptions, simulations, and predictions of magnetics break down or hold firm?

[ponderingturtle]

Mr. Scott,

. Also keep in mind that there is no "magnetic charge". The most fundamental quantity in magentism is the magnetic dipole, which must be represented as a vector, rather than as a scaler quantity which is either positive or negative. So the North-South choice is also descriptive.


Dr. Stupid

Well it would be nice if there where magnetic monopols, as it makes proving quantization of charge and other things easy. They fit so nicely into theory, now if only they could be observed.

[phildonnia]

Can there be a negative absolute voltage?
There's no such thing as "absolute voltage" at all.


Can an absence of electrons create a positive charge, or is the presence of excess protons responsible for a positive charge?

It depends. In ordinary atomic matter, there is no net charge since positive charges balance negative charges. If you remove negative charges, then it becomes positively charged. You could say this is because of the "absence of electrons", or because of the "excess of protons".
An absence of electrons in general does not create a positive charge. For example, a perfect vacuum would not be positively charged.




What does it really mean that electrons or accelerated to, say, a million electron volts? Can this be either positive and negative voltage?

Electron-volts are a unit of energy. Very simply, Energy = Charge * Voltage. An electron-volt is the charge on an electron, times one volt. An electron with one-million electron volts is moving as fast as it would if it moved through a voltage difference of one million volts.

[Stimpson J. Cat]

Scott,

Don't magnetic vectors add, subtract, and cancel out like electrical voltages?
Well, not exactly like electrical voltages. Voltages are scalars, and magnetic fields are vectors. But yet, they do add, subtract, and cancel out.

If we called magnetic poles negative and positive instead of north and south, would the mathematical descriptions, simulations, and predictions of magnetics break down or hold firm?
There's no "if" about it. "North" and "South" don't even appear in the mathematical descriptions. In the math it is just vectors, each component of which can be positive or negative.

When we say that end A of a magnet is magnetic north, all that really means is that if we line up the magnet parallel with the magnetic field of the Earth, then end A will be pointing north.


Dr. Stupid

[Jimbo07]

Scott,

Well, not exactly like electrical voltages. Voltages are scalars, and magnetic fields are vectors. But yet, they do add, subtract, and cancel out.



Actually, this is merely due to looking at the quantities involved. Here, I think, the potential has been confused with the field.

Voltage is an electric potential.

The electric field is expressed with vector quantities. The field is related to the potential as the Electric Field is the negative gradient of the Electric Potential. That is, V decreases as you move along the electric field.

The electric field has sources and sinks (at + and - charges), so the divergence (stuff moving away) of the electric field has a relation to amount of charge at a source. Magnetic fields do not have sources and sinks (no monopoles (at least not experimentally yet)), so the divergence of a magnetic field is 0.

The Electric Field E and the Magnetic Field B (or H, depending which constants you are using) are expressed as vectors and you can do vector manipulations with them (ex. E X B gives the Poynting vector).

...

Bleah.

...

There are charged particles besides protons and electrons (antimatter is just a pain to produce ;) ). For most normal matter, postive charge is said to belong to protons and negative charge is said to belong to electrons. In a semiconductor material, you can remove electrons from a region and create 'holes.' These holes are due to ions missing electrons (having a surplus of protons).

[Mr. Scott]

In a semiconductor material, you can remove electrons from a region and create 'holes.' These holes are due to ions missing electrons (having a surplus of protons).

But such a hole doesn't do anything -- it's an absence of something that does something and is a conceptual/semantic convenience. Attributes and actions of holes in semiconductors are a result not of the absence of the electrons that may fill them, but of the protons lacking companions that are left behind. When we say a semiconductor hole has an action it's like suggesting that a vacuums pull at things, isn't it?

[Jimbo07]

But such a hole doesn't do anything -- it's an absence of something that does something and is a conceptual/semantic convenience.

Uhh... here someone who knows more than me would have to get into the 'philosophy' of the mechanics of material properties. :boggled:

Is a potential well actually "nothing"? :confused: That's what happens in a hole-electron recombination. The free electron comes along and says, "Gee, I don't have enough energy, I'm going to fall into this well" (except for a statistical bunch that will get transmitted/reflected at all boundaries, etc.). If you do Hall Effect experiments on pieces of semiconductor, you can see a directionality in voltage (and thus, charge buildup) that for all intents and purposes looks like the holes are moving!

however, getting away from semiconductors...

I'm really not sure what you're getting at. I think we would have to move from the realm of classical electromagnetism to statistical mechanics. A plasma (ions stripped of electrons) could have a collection of charged particles such that averaged over the whole aggregate, from a far enough distance, the plasma looks neutral. However, if you could get down to a small enough region (very small), there could be a definite directionality to the local E. There can also be non-neutral plasmas.

[Mr. Scott]

I'm really not sure what you're getting at.

What I'm getting at is challenging the idea that there is a negative and a positive voltage or charge -- that classical electrical semantics have misnomers running though it that interfere with clear comprehension of the principles involved.

Putting classical semantics aside, I see electrons having a positive electric charge or voltage, and protons having a positive proton charge or voltage. Saying that there is a negative voltage seems to me like saying there are minus five fish in a pond, or that there is a negative absolute air pressure in a hose (the mythical pull of a vacuum). I don't mind talking about holes in semiconductors but worry that its too easy to start forgetting that the hole acts because of the unmated proton near the hole -- not the mythical pull of a missing electron.

That's what I'm getting at, and if I'm wrong, I'd like someone to refute the idea with a slam dunk. If that's already been done, then I guess my physics is not advanced enought to have seen it.

S

[drkitten]

Putting classical semantics aside, I see electrons having a positive electric charge or voltage, and protons having a positive proton charge or voltage. Saying that there is a negative voltage seems to me like saying there are minus five fish in a pond, [...]

That's what I'm getting at, and if I'm wrong, I'd like someone to refute the idea with a slam dunk.

The simplest slam dunk is simply that "charge" and "voltage" are two different things. (They're even measured differently -- charge is measured in coulombs, voltage is measured in volts.)

Voltage, as pointed out upthread, is a measurement of a difference in potential energy. The potential energy can arise either from an excess of electrons or an excess of protons (or other more obscure particles that we don't really need to worry about outside of a physics lab). But since protons and electrons are supposed to balance, having too many protons is the same as having too few electrons.

Think of it this way -- is your coffee too sweet because there's too much sugar in the cup for the coffee, or not enough coffee in the cup for all the sugar?

[Cuddles]

What I'm getting at is challenging the idea that there is a negative and a positive voltage or charge -- that classical electrical semantics have misnomers running though it that interfere with clear comprehension of the principles involved.

Voltage is a slope. It can slope either up or down, so it's value is either negative or positive. No semantics are involved.

Charge come in two flavours. If you put equal magnitudes of the two together they cancel out. In all respects they act mathematically identically to positive and negative numbers, therefore we call them positive and negative. No semantics here either.

I can understand your confusion with semiconductors and holes, but it seems odd to challenge simple scientific principles when it seems you have very little understanding of the actual science involved.

[Jimbo07]

What I'm getting at is challenging the idea that there is a negative and a positive voltage or charge -- that classical electrical semantics have misnomers running though it that interfere with clear comprehension of the principles involved.

drkitten has gotten at the difference between charge and voltage, so both can be addressed - separately.

Cuddles said that protons and electrons have some property which is cancels out in near proximity. We call this property charge, and the cancelling gives neutral. The unit of charge is a coulomb [C].

An unbalanced charge gives rise to an electric field, E. When a second charge is brought in, a force is experienced. From a positive charge to a negative, it looks like this (neglecting field lines in multiple dimensions):

+ ----------------> -

The field has units of force per charge or [N/C]. I used math-speak when I said negative gradient of potential. Cuddles said it better. If you think of the electric field creating a hill, then different heights on that hill have a different potential energy. If you move from one 'height' to another you have changed your potential energy. The change is dependent on a distance up the hill [m] giving [Nm/C] or [J/C] or volts, [V].

Where is ground level for these hills? I'm probably over-extending the analogy, but you can choose some reference which might be 'sea-level.' Now we know it is possible to be at points on the hill above and below sea level. It is a consequence of this 'chosen' reference that allows you to see -ve voltages in electronic circuits (for ex. the power leads of an op amp might be wired to +,- 12 V). The important thing is that there is a 24 V difference between the two leads. One is 24 distances up the hill over the other one.


Putting classical semantics aside, I see electrons having a positive electric charge or voltage, and protons having a positive proton charge or voltage. Saying that there is a negative voltage seems to me like saying there are minus five fish in a pond, or that there is a negative absolute air pressure in a hose (the mythical pull of a vacuum).

So... you can try to redefine electrical systems, but you might run into a wee tad resistance from the engineering and physics communities. There might be some unfortunate nomenclature, but it's no worse than defining fuel energy in terms of BTUs :p .


I don't mind talking about holes in semiconductors but worry that its too easy to start forgetting that the hole acts because of the unmated proton near the hole -- not the mythical pull of a missing electron.

No employed electronics engineer should forget that regions with surpluses of holes are electron deficient. In fact, there is something unclear with the way you've stated it here. Holes are due to missing electrons. A hole can move because electrons get promoted from hole to hole, making the hole seem to move in an opposite direction, but the protons do not move.

But you don't need to work with semiconductors. You could put a metal plate in the middle of a capacitor and call it 0 V. Then to one side it would appear you have a buildup of positive charge and on the other, a negative charge.

[Jimbo07]

Oops! Notice what I did in that last paragraph? :mad: