Talk:Electric current

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Conventional current explanation is incorrect[edit]

"The direction of conventional current is arbitrarily defined as the same direction as positive charges flow."

This is widely believed, but it is wrong, and it is a misunderstanding of the definition of current.

The only arbitrary convention is that charge (unit: Coulomb) is defined in such a way that electrons have a negative amount of it.

Current is defined as the rate of flow of charge. Movement of electrons in a particular direction is a flow of negative charge in that direction, and the rate of flow of that charge in that direction must therefore be negative. The direction of positive current must therefore be in the direction opposite to electron flow.

There's no trick, no additional arbitrary current direction convention. It's just simple algebra. Gwideman (talk) 17:28, 17 February 2016 (UTC)

You have that the wrong way round old fruit. Conventional current flow is much older than the discovery of the electron. The electron was assigned a negative charge to be compatible with conventional current flow, not the other way around. SpinningSpark 18:36, 17 February 2016 (UTC)
What did I have the wrong way around? I said nothing about the order in which these phenomena were discovered, and of course charge and current were recognized before electrons. My point was that the charge of electrons and the direction of positive current are compatible, not alternative competing conventions. Once a particular polarity of charge was established (which *was* arbitrary), the current direction and electron polarity is determined in the way which matches. Gwideman (talk) 08:28, 16 July 2016 (UTC)
I think conventional current has two meanings that overlap. Current, in a wire, is effectively charge density (coulombs / meter) times velocity (meters / second). If you draw the arrow in the direction the charges are moving, then value of current is negative (negative charge times positive velocity). If you draw it in the other direction, then its value is positive.
The other convention regards choosing a reference direction arrow in a circuit. When a circuit is being solved, reference directions for the currents are chosen arbitrarily. The solution may produce positive or negative values. It is conventional, as a matter of convenience, to try to choose those reference directions such that the currents have a positive value. And, in fact, after the solution is determined, the reference directions be inverted so that the currents have positive value.
In a battery powered circuit, conventional current flows out of the positive terminal. It is also conventional to choose a reference direction arrow for the current coming out of the battery that points away from the positive terminal. However, some people will choose the reference direction arrow such that it points into the positive terminal. In that case the value of current is negative.Constant314 (talk) 16:35, 16 July 2016 (UTC)


I’ve had the same thought. That the electron has a negative charge is, today, accepted without argument. If you have a current of electrons moving from right to left in quantity of 1 coulomb/sec and draw the arrow from right to left, then you have to label it -1 amp with the minus sign required because the sign of the electrons is negative. If you want to label it +1 amp, then you have to draw the arrow the other way. It might be worthwhile to note that if the direction of conventional current were reversed then the sign of the electron would have to be reversed.Constant314 (talk) 14:27, 18 February 2016 (UTC)

Semi-protected edit request on 17 June 2016[edit]

Please change in the section Current measurement , the device for current measurement as Transformers into Current transformer which is the more specific device that is used to measure the electric current.

Sathish.polimi (talk) 15:28, 17 June 2016 (UTC)

Disambiguation is done. — Andy W. (talk ·ctb) 00:08, 18 June 2016 (UTC)
  • The article contains:

"Current measurement ...

... Transformers (however DC cannot be measured)"

DC Current CAN be measured, usually by the force on a coil located in a magnetic field.

Transformers are used in AC circuits, not DC. 70.27.152.243 (talk) 03:36, 20 July 2016 (UTC)

It is just saying that transformers can be used for AC current measurement but not DC current measurement. I supposed it could be improved to "Transformers (for AC current only)". Constant314 (talk) 04:12, 20 July 2016 (UTC)
There are current probes which can measure the DC current through a wire, though the magnetic field around that wire, through a ferrite core and Hall effect device. Gah4 (talk) 10:23, 10 January 2017 (UTC)

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Holes[edit]

There is the statement In other materials, notably the semiconductors, the charge carriers can be positive or negative, depending on the dopant used. which seems to suggest that holes only exist in semiconductors. Many metals have hole bands, and in fact Aluminium, a metal widely used for power transmission, has a large fraction of its conduction in hole bands. Gah4 (talk) 10:21, 10 January 2017 (UTC)

I see that the clause about dopant suggests only dopable materials. Perhaps the sentence can be rewritten "In other materials, notably the semiconductors (depending on the dopant used), the charge carriers can be positive or negative." Or maybe it needs to be two sentences with the second sentence adding the information that the charge carrier type in semiconductors depends on the dopant. Never the less, it is interesting that aluminium has considerable hole conduction. If you have a reliable source, I think it would be a good addition to the article.Constant314 (talk) 19:03, 10 January 2017 (UTC)
The dopant is a distraction here. Intrinsic (ie undoped) semiconductor materials also have conduction band electrons and valence band holes both taking part in conduction. What the dopant does is create more of them, usually biased towards one or the other to create p-type or n-type materials. But another point is that holes are not real particles, they are an absence of an electron. The current is still being carried by electrons in reality. The virtual holes are just an analytical convenience. A better example of positive charge carriers is the cations in an electrolyte. SpinningSpark 19:29, 10 January 2017 (UTC)
Holes come from the quantum mechanics of band theory. Both the Hot-point_probe and positive hall coefficient in some materials show the need for holes in explaining conduction in solids. The quantum mechanics rules on indistinguishability don't allow one to track individual electrons through a conductor. Gah4 (talk) 23:42, 17 January 2017 (UTC)
The band structure of aluminium, commonly used in electric power transmission, is described by Ashcroft and Mermin.[1]

References

  1. ^ Ashcroft and Mermin (1976). Solid State Physics (1st ed.). Holt, Reinhart, and Winston. pp. 299–302. ISBN 0030839939. 

Drift speed[edit]

The section on drift speed suggests that the speed of sound is faster than the speed of gas molecules. From Maxwell–Boltzmann distribution, the mean speed of nitrogen molecules is 422 m/s, a little faster than the speed of sound in air or nitrogen gas. Gah4 (talk) 10:48, 10 January 2017 (UTC)