# Stupid question of the day....

Discussion in 'Electronic Basics' started by AllTel - Jim Hubbard, Jul 30, 2005.

1. ### AllTel - Jim HubbardGuest

I am curious about what would happen to an electrical current in 2
situations.....

Assume that you have 2 wires that, when joined, complete a closed electrical
DC circuit with electrons flowing thusly.....

------------ ============
eeeeeeeeee eeeeeeeeeeeeeee
------------ ============

If you flattened out the end of each wire where they connect , would the
resulting electron paths be more like figure A or Figure B?

Figure A

--- ===
--- ===
--- ===
--- ===
eeeeeeeee eeeeeeeeeeeeeeee
--- ===
--- ===
--- ===
--- ===

Figure B

--- e e ===
--- eee eeeeee ===
--- eeeee eeeeeeeeee ===
--- eeeeeee eeeeeeeeeeee ===
eeeeeeeeeee eeeeeeeeeeeeeeeee
--- eeeeeee eeeeeeeeeeeee ===
--- eeeee eeeeeeeee ===
--- eee eeeee ===
--- e e ===

(Please note that the vast # of "e"lectrons shown in Figure B is simply to
show the path's of electrons. )

The second portion of my question is....If the flattened portions were
increases in mass (if each wire were connected to a metal cube and the cubes
were brought together to complete the circuit) how would it effect electron
flow where the cubes touch?

2. ### John PopelishGuest

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.

3. ### Repeating RifleGuest

<snip>

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

4. ### TimPerryGuest

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.

5. ### Andrew GabrielGuest

You missed that I guess?

6. ### AlexanderGuest

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

7. ### JoeSixPackGuest

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.

8. ### John LarkinGuest

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

9. ### John PopelishGuest

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.

11. ### John LarkinGuest

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

12. ### TimPerryGuest

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.

13. ### AlexanderGuest

Op [GMT+1=CET], hakte Jamie op ons in met:
Damn perhaps Maxwell can help us out

14. ### John FieldsGuest

---
That's not true. The electrons diffusing through the flattened
portion of the wire would result in a charge flow profile more like
Figure B, given the understanding that none of the electrons would
follow a straight-line path through any portion of the wire.
Further, the assumption is made that the cross-sectional area of the
wire remains constant at the connection.
---

[/QUOTE]

15. ### JamieGuest

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

18. ### Alltel - Jim HubbardGuest

Thanks to everyone for the great input!

19. ### Jasen BettsGuest

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.

20. ### John FieldsGuest

---
Wrong.

Since an electron has a rest mass, m0, and since:

m0
mr = -------------------- ,
sqrt (1 - (v²/c²))

its relativistic mass, mr, will tend toward infinity as its
velocity, v, approaches that of light, c.