Stupid question of the day....

Every atom in the conductor contributes an electron to the moving herd. If you alter the cross section or shape of the conductor, the total number of electrons taking part in the flow across any cross section changes in proportion to the cross sectional area (with cross section being defined as perpendicular to the local E field that motivates the flow).

Since the current (number of electrons passing through a cross section) has to be uniform, all around a current carrying loop, the average velocity of the electrons must vary inversely to the cross sectional area. If more of them are carrying a given current, they go slower. If fewer have to carry that current, they mist move faster.

I think these rules cover all your cases.

It is not a stupid question--it is just irrelevant. Current flows in various ways, and in almost all cases, the details of the flow is unimportant. The "wires" can be made from metals, semimetals, hot glass, semiconductors, ionic solutions, etc. Each has a different kind of conduction mechanism.

I have taken the probably impossible task upon myself to discourage thinking of conduction as a flow of electrons.

Bill

neither ... research "skin effect"

electron flow (or hole flow is you prefer to think that way) is determined by total circuit resistance. (and applied EMF as per ohms law) decreasing total resistance by increasing contact point surface area will result in increased current flow if all other factors remain the same.

You missed that I guess?

"TimPerry" schreef in bericht news: snipped-for-privacy@adelphia.com...

Most of the times this just aplies to AC (high frequency) circuits

Before you attack this post, saying electrons can only travel at the speed of light, that's incorrect. The electrons themselves can travel any speed, but the voltage wave produced does travel at 300,000 kms per second.

For DC or low-frequency AC, charge flow will be uniform across the cross-section of a round wire conductor (or, actually, any shaped conductor with unchanging cross-section.) If you butt two clean-cut wires against each other, they're now effectively a single wire, so current distribution is still uniform.

The cube situation is more complex. A wire pokes a nearly uniform circle of current into the cubes, and the other wire (by symmetry) sucks it up uniformly across its cross-section, but the current spreads out as it passes through the large cube, most diffuse halfway through and necking down near the entry/exit circles at the wires. The exact current distribution within the cube is complex, usually computed using finite-element simulation. It might be possible to use calculus to compute this distribution, but I wouldn't want to try.

At higher frequency AC, current in a wire tends to avoid the center and crowd near the surface, "skin effect."

John

Before you attack this post for saying that electrons can travel at any speed, keep in mind that Joe probably understands that this includes any speed up to, but not including, the speed of light. ;-)

Thanks for helping out, Joe.

Hmmm...

Copper does have a weak Hall effect. And the current through a round wire does make a circular/transverse magnetic field. So, at very high DC currents, is the current density a bit non-uniform?

John

researching skin effect will give the poster a better understanding of electron distribution in a conductor than short answers on the internet.

ignore all sites that relate to car or home hi-fi audio.

Op [GMT+1=CET], hakte Jamie op ons in met:

Damn perhaps Maxwell can help us out ;)

i am glad some one is on the ball here! :))

wrong:

Thanks to everyone for the great input!

electrons cannot exceed the speed of light in a vacuum. no physical object can.

That said the drift velocity of electrons in electric wires is rarely more than walking speed, the signals are transmitted by the interaction of the electrons electric fields - ie each electron pushes on its neighbours...

signals usually seem to propogate through coaxial conductors at 2/3 the speed of light. iirc they travel no faster in any other type of conductor.

Even in fibreoptic cables the signals (photons) go slower than 300000 km/s the ratio difference is the definition of the refractive index of the optic material.