2
\$\begingroup\$

I assume when I hook up a 12V battery to a wire, all the free electrons in the wire move. If I hook up 24V, they move faster so more charge passes a area cross section for i=C/s. But in AC, their movement is constrained. They move at 60Hz. So what happens if I increase the voltage? More electrons have to pass a cross section, so just some electrons move, and then more of them do if I increase the voltage? Maybe I am confusing myself :)

\$\endgroup\$
  • 3
    \$\begingroup\$ The wire will vaporize long before you can reach any limit on the amount of charge you can transfer per second. \$\endgroup\$ – Ignacio Vazquez-Abrams Mar 25 '14 at 22:30
  • \$\begingroup\$ Unless you want to take heat out of the equation. Have a look at this physics SE question on superconducting wires. physics.stackexchange.com/questions/1060/… \$\endgroup\$ – krb686 Mar 25 '14 at 23:50
6
\$\begingroup\$

But in AC, their movement is constrained. They move at 60Hz.

This shows a misconception. The AC frequency (60 Hz) tells you how often the current switches direction, not how fast the individual carriers are moving.

If I hook up 24V, they move faster so more charge passes a area cross section for i=C/s.

This is true to a point. But you have to remember that electrons are moving around randomly in all directions, and a reasonable current in an ordinary wire just "biases" the direction of this motion slightly in one direction or the other. If you have a higher current, the bias is even a bit more in the direction of that current, but not every electron is moving with the current and the effect on the speed of each electron is fairly small.

I assume when I hook up a 12V battery to a wire, all the free electrons in the wire move.

This is not really true. All of the electrons were already moving. Applying a voltage just made the average of that motion slightly to one direction or the other.

Is there a limit on the charge that can flow in a wire?

Practically, the maximum current in a wire is limited by the resistive self-heating of the wire. If too much current is carried, the wire will get hot enough to melt itself or cause a fire hazard. Typically we choose a wire large enough to limit its temperature rise to 20 or 30 degrees C under expected loads. On this basis, you can find tables online giving the ampacity of round wires depending on diameter or pcb traces depending on width and copper thickness.

\$\endgroup\$
  • \$\begingroup\$ I think he's really just saying as a general rule, the electrons don't move (aren't long-term biased) in any particular direction with AC, so I don't think he has a misconception there, assuming he meant to say "they cycle at 60Hz". Also, I think this answer has some good clarity on some points, but have you really answered his original question? \$\endgroup\$ – krb686 Mar 26 '14 at 2:00
  • \$\begingroup\$ @krb686, good point. Edited. \$\endgroup\$ – The Photon Mar 26 '14 at 2:55

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.