17

Electrons flow from low potential to high potential in DC, i.e. from negative terminal to positive terminal.
But how do they flow in AC, as the polarity changes every 10ms for 50Hz?

Secespitus
  • 103
Vivek
  • 171

4 Answers4

19

Electrons do not "flow" in AC unlike in DC, where they physically move from negative to the positive terminal of the EMF source. Observe that by definition current is rate of flow of charge. In alternating current, the electrons just oscillate about their mean position. Yet they constitute a current[alternating current here], because there is flow of charge through the cross section of the wire.

Tejas P
  • 989
  • 2
    initially I imagined they oscillate in the "vertical" plane (given a horizontal wire) and couldn't see how this creates flow of charge through the cross section. So they must be oscillating "along" the wire, right ? – Ciprian Tomoiagă Apr 07 '17 at 13:40
  • @Tejas I had this same question. Thanks for the answer. – Wrichik Basu Apr 07 '17 at 14:53
  • 5
    @CiprianTomoiaga: yes, indeed. AC is a longitudinal wave. – Javier Apr 07 '17 at 14:56
  • 1
    @Javier "AC is a longitudnal wave". This explains how AC transmits energy and so many other questions that people have. Very nice statement. It connects our intuition about waves with AC. I thought about it just now for the first time. Unfortunately its not 100% correct.. but still very nice. – Kartik Apr 07 '17 at 15:21
12

  • If you plug a battery to a circuit, you have direct current (DC). Charge starts moving and speeding up from positive towards negative terminal. At some point they reach maximum speed and we have a constant steady current.

  • If you now unplugged the battery, turned it around and reinserted it very quickly, the moving charge would slow down, stop shortly and start moving backwards, then speed up backwards and soon reach a steady flow.

If you could turn around the battery very, very fast, you would constantly see charges speeding up, slowing down, moving backwards, slowing down, moving forwards etc. Maybe so fast that the current never has time to becomes steady in any direction. You will then call it alternating current (AC).

Steeven
  • 50,707
  • 2
    Well, a steady current is not necessary for AC, really. Consider an AC square wave: The current reaches a steady state in the positive half, then switches, then steady negative, then switches, and so on. Yes, in the context in this question a simple answer is good, this comment is merely a footnote. – Kroltan Apr 07 '17 at 13:33
  • I do not fully agree with this answer. In a very pure conducting metal crystal sample at low temperature, if you plug a battery you will not necessarily get a DC. You might get Bloch oscillations. The electrons, or better to say quasielectrons, will oscillate back and forth with a frequency proportional to the electric field's magnitude. – untreated_paramediensis_karnik May 27 '18 at 08:00
2

It is worth copying this analysis of what a current is:

current

So the electrons only move with a drift velocity

Although your light turns on very quickly when you flip the switch, and you find it impossible to flip off the light and get in bed before the room goes dark, the actual drift velocity of electrons through copper wires is very slow. It is the change or "signal" which propagates along wires at essentially the speed of light.

italics mine.

Using the calculator given for a copper wire of 1 mm the drift velocity is 0.94X10^-3m/s.

The only difference between AC and DC is the forced change of direction with the AC frequency.

anna v
  • 233,453
  • This answers perpetuates the belief that Drude's model is an accurate description of reality. In reality, there is no such thing as a drift velocity for electrons in a conductor. The electrons are not even localized. They are not like classical point particles having a well definite speed like air molecules would at 1 atm and room temperature. – untreated_paramediensis_karnik May 27 '18 at 07:53
  • @lobotomized_sheep_99 The same is true of the Bohr model, it can be shown that its successes can be derived from the quantum model , connected to average values of the quantum variables. http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/hydr.html The same will be true here, except it is too complicated to derive an accurate qm solution. – anna v May 27 '18 at 10:47
  • I almost agree. I would say it is the most probable value of the distance between the electron and the nucleous that is connected to Bohr's radius, not the average value. They are useful models in that they lead to "correct" numerical values for some things, but not all. But they are not a good description of reality. – untreated_paramediensis_karnik May 27 '18 at 15:03
1

The flow of electrons is overrated: even in DC they move at a rather leisurely pace (we are talking about fractions of inches per second in the mean). What moves at near the speed of light is their mean displacement.

This displacement is a back-and-forth in AC. Imagine sucking a bit of air at a straw in a glass of water and then blowing into it. Most of the water in the straw will remain there, the speed with which you suck and blow can be quite relaxed, yet the level in the glass will fall and rise almost instantly.

user151274
  • 19
  • This answer is wrong. There is no such thing as a drift velocity for the electrons in a conductor. The electrons are not well localized and they do not posses a well definite speed either. The most accurate speed one could possibly assign to the quasielectrons that conduct electricity is Fermi speed, which is orders of magnitude greater than "a fraction of inches per second". So -1 for this obsolete Drude's model based on answer. – untreated_paramediensis_karnik May 27 '18 at 07:56