Electron flow

Status
Not open for further replies.
S

sparks1

Senior Member
Location
Massachusetts
Can someone explain this,
For example, If 10 amps of Alternating Current flows in one direction through a wire passed a given point (A ) in the positive half cycle of the sine wave, then it must flows in the opposite direction passed point (A) in the negative half cycle of the sine wave.
From this statement it would appear that the relative motion of the electrons moving through the wire at point (A) cancel each other, but we know the current must be flowing because the lights are still on and there must be some type of motion involved. Do the electron drift more in one direction?
I hope this doen't stir up a hornets nest!
 
Re: Electron flow

That's a valid question that everyone who really thinks about AC usually asks at some point.
it would appear that the relative motion of the electrons moving through the wire at point (A) cancel each other, but we know the current must be flowing because the lights are still on and there must be some type of motion involved.
Click to expand...
That's correct. The electron flow reverses direction 120 times per second (twice per cycle) so they don't actually "travel" any distance.

But as long as they are moving, they transfer the energy that they aquired from the source generator (or transformer winding) to the load device, such as a heater or motor.

The sketch below represents power flow in a DC circuit, however, to illustrate what happens in an AC circuit, think of it as a video.
Now, press re-wind and then fast-forward 60 times a second. :D :cool:

Ed

power.gif


[ February 22, 2005, 01:57 PM: Message edited by: Ed MacLaren ]
 
Re: Electron flow

Even in a DC circuit, linear motion is exceedingly small. To move an electron 1 inch, the electron would have to cover a distance 12,700,000,000,000 times its own size.
 
Re: Electron flow

sparks,

The individual electrons do not travel very far because there are so many of them; they merely push against each other so that there is motion throughout the wire. I will try to find a reference on this.

As far as delivering energy, think about rubbing your hands together to warm them. Your hands don't go anywhere, but the heat is generated anyway.
 
Re: Electron flow

I am going to step up to the podium, and once again give my lecture on ?Let?s never mention ?drift velocity? again.? Here goes:
Let?s never mention ?drift velocity? again. It represents truth. It is also irrelevant. :eek: What a single electron does, as it drifts back and forth along a wire, means nothing to the professions of electrical engineering and electrical construction. All that matters is the overall impact of the motion of electrons through the wire.
Click to expand...
End of lecture. Now I'll give my answer to the original question.

Consider a person watching electrons, while standing your ?Point A.? Suppose the person can see the entire circuit path, in the same way a person can sit in a box and watch an entire car race. In a car race, each car will travel once around the track, and then do it again, and then do it again, and the person watching will see each round trip transit of the track.

The person standing near a wire at Point A, watching current flow, will see each electron go around the circuit, and then go around the circuit again, and then go around the circuit again. If the circuit length is 100 feet, then the same electron will go around the entire round trip journey over 80,000 times (in the same direction! :eek: ) in the first positive half cycle alone. During the next sequential negative half cycle, the same electron will go around the entire circuit in the opposite direction over 80,000 times. Then it will start traveling in the original direction, and will make the round trip over 80,000 times during the next positive half cycle.

During each and every trip around the circuit, the electron will be picking up energy from the source, will give up some of that energy in heating the circuit conductors, and will give up the rest of that energy to the load.

I believe that that is how you should visualize the flow of current.

Now back to the podium.
Please do not feel impelled to report on the lies I just told. I am well aware that no single electron will make the entire trip around the circuit. I know that a single electron will merely jump to the next atom, and that it will be a different electron that continues the journey from there. I say again: True, but irrelevant.
Click to expand...
 
Re: Electron flow

This is how I think of it,
If I stand at point (A) and throw a single electron forward in one direction to point(c) 20' away heres what happens.
Let us call point (A) 0 degrees. When it passes the 10' mark at point (B) it's at 90 degrees. It continues its journey until it reaches point (C) at 20'. Now its cemf causes it to reverse direction at 180 degrees and move back towards 0 degrees. When it arrives back to point (A) at zero degrees the electron has moved a total distance of 40' and has travelled 360 degrees.
My point is, the electron has in fact moved or travelled some distance. This is indeed an Interesting phenomenon for anyone who think about AC. It would appear that the electron hasn't moved from point (A) at all!
Thanks for your input!
 
Re: Electron flow

For example, If 10 amps of Alternating Current flows in one direction through a wire passed a given point (A ) in the positive half cycle of the sine wave, then it must flows in the opposite direction passed point (A) in the negative half cycle of the sine wave.
Click to expand...
This is correct.

From this statement it would appear that the relative motion of the electrons moving through the wire at point (A) cancel each other
Click to expand...
This is not true because the peak negetive and peak positive voltages don't exist simultaneously. There is only one voltage at any instant.

Bryan has what I think is really good analogy for this.

Consider an up side down bicycle. somebody using the peddles to move the back tire back and forth (both directions). If the tire changes speed from zero to maximum and back to zero and then does the same thing in the opposite direction the tire speed could accurately describe a sine wave. Now imagine putting your hand on the tire as it spins by. The heat caused by the friction is a result of energy being transfered from the guy turning the peddles in the same way a generator sends enegry to a light bulb. (He probably can't go as fast as AC of course).
 
Re: Electron flow

Originally posted by sparks1: This is how I think of it: If I stand at point (A) and throw a single electron forward in one direction to point(c) 20' away here?s what happens.

Let us call point (A) 0 degrees. When it passes the 10' mark at point (B) it's at 90 degrees. It continues its journey until it reaches point (C) at 20'. Now its cemf causes it to reverse direction at 180 degrees and move back towards 0 degrees.
Click to expand...
Sorry, but this is far too far from the truth to be a useful analogy. :( Between the 0 degree point on a voltage curve and the 90 degree point on the same curve, the electron will have traveled 4 million feet, not just the 20 feet you describe. In that amount of time, for a 100 foot circuit path (50 feet each way), the electron will have gone the entire round trip (source to load to source and start over aging) over 40,000 times. :eek:

{ASIDE: If anyone is tempted to talk about electron drift, I beg you to let it go. See my earlier "lectures from the podium."}
 
Re: Electron flow

Originally posted by sparks1: Now its cemf causes it to reverse direction at 180 degrees and move back towards 0 degrees.
Click to expand...
An electron does not possess the property of counter-emf. Some types of loads do, but not the electron that brings energy to the load. The thing that causes the electron to change direction is an action by the source, not a reaction by the electron. The source creates an electric field throughout the wire, and it is that field that does the pushing. It is not one electron that pushes the next electron along, in the way that one water molecule pushes the next molecule along the hose. The same field pushes all electrons throughout the entire circuit at the same moment, and in the same direction. When the source causes the direction of the field to reverse, then the motion of the elections will change directions.
 
Re: Electron flow

Originally posted by bphgravity:
Even in a DC circuit, linear motion is exceedingly small. To move an electron 1 inch, the electron would have to cover a distance 12,700,000,000,000 times its own size.
Click to expand...
This is the kind of "movement" I'm trying to understand. It's is important to talk about this topic at some point in our careers. We should all have at least some real concepts of how electrons and linear motion relate.
 
Re: Electron flow

[/QUOTE]This is not true because the peak negetive and peak positive voltages don't exist simultaneously. There is only one voltage at any instant.

This is true

But what about the total value of linear motion .The direction of linear motion appears to cancel each other in both directions of the sine wave. We know that there is some type of movement involved because the lights are still on!
 
Re: Electron flow

This is how I was explained electron flow.

Imagine you have a large pipe. The pipe is completely stuffed full of ping-pong balls. It can't take another. If you try to put a ping-pong ball in the front of the pipe, the only way to get it in, is to shove out a ball at the end of the pipe. So, the pipe is a piece of wire and the balls would be electrons. So, actually the electrons don't move hardly at all, but the end result is total movement. This is also how I have been explained that electricity is very fast. If the ping pong balls are jammed so tight together, any movement of one, would in fact move them all. Push a little on the first one in line, and the one at the very end would move at the almost exact same time. I imagine the analogy is probably chalked full of errors, but this is how our instructor was able to help get our mind wrapped around it.
 
Re: Electron flow

Originally posted by sparks1: The direction of linear motion appears to cancel each other in both directions of the sine wave. We know that there is some type of movement involved because the lights are still on!
Click to expand...
I think you are beginning to learn, but there are still a few basic concepts you need to grasp.

At the risk of broaching the topic that I prefer to avoid, I will concede that a given electron will move down the line slowly. It will jump from one atom to the next. A different electron will jump from the second atom to a third. The first electron might stay put for a long time, before it makes a jump to an atom further down the line. But by that time, the direction of the electric field may have reversed, so that the first electron might jump backwards towards its original home.

So yes, in terms of the migration of individual electrons, their motion one way and then the other way might very well result in very little actual movement.

Can we now PLEASE stop bringing up that ?electron drift? topic?

This is what I want you to understand: If you stand at one point along a wire, and watch the electrons go by, you will not be able to distinguish any single electron from any other. You will not be able to discern whether a single electron blowing by you came from only one atom away, or whether it has been making the complete round trip over 80,000 times during the present half-cycle. It does not matter which electron is moving past a given point. If you get enough electrons going by (specifically, 6,240,000,000,000,000,000 per second), then you get one amp.
 
Re: Electron flow

Originally posted by yanert: This is how I was explained electron flow.
Click to expand...
The ping pong analogy has its merits, and like all analogies has its shortfalls. It works very well for explaining the flow of water through a hose. As I mentioned earlier, no single water molecule will move until it is pushed by its neighbor, which had been pushed by the one further upstream, with the pump doing the initial pushing.

But think of this: If the center of an atom were the size of a basketball, then the relative size of an electron would be about like a BB pellet, and that pellet would be a dozen or so miles away from the basketball. On the atomic scale of things, there is a lot of empty space in there.

Once again: Electrons are not pushed by other electrons. Rather, the voltage source creates an electric field for the entire length of the wire. Every electron is pushed by that field. Every electron feels the same push, in the same direction, at the same point in time. With a switch open, there is no field, no push, no current. Close the switch, and the field instantly appears at every point in the entire circuit path, causing the push, and causing the current to flow.
 
Re: Electron flow

Yanert's "ping-pong ball in the pipe" is a good analogy. I used a similar one.

I found that many of my students were confused by statements they had heard referring to "electricity moving at the speed of light".

What moves at (or near) the speed of light is what I call the "impulse".

Imagine a circular train track full of train cars. In other words, the cars occupy the complete track, so that the caboose is just in front of the engine. (By the way, there is no slack in the couplings.)

At the same instant that the engine starts to move, every car in the train starts.

Ed

[ February 22, 2005, 09:47 PM: Message edited by: Ed MacLaren ]
 
Re: Electron flow

Electrons are not pushed by other electrons. Rather, the voltage source creates an electric field for the entire length of the wire
Click to expand...
That almost sounds like you are saying "electrons don't repel each other".

A voltage source consists of a surplus of electrons at one point, the negative terminal, and a deficiency of electrons at the positive terminal.

Ed
Electron1.gif
 
Re: Electron flow

Ed,

I too think it is a good analogy, and I think that electrons do push each other since we know that like charges repel. Furthermore, the charges of electrons and protons are intimately involved in the electrostatic field. You cannot have an electric field without charge. So, it is debatable that the field rather than the electrons, causes the movement. We could say it is the energy of the electrons that creates the field.

I would add though that we must not confuse the "holes" mentioned in your diagram with "hole" current in the valence band of semiconductors.

Another analogy is a sound wave. For example, we can see lightning miles away, but it may be several seconds before the hear the thunder. The sound travels quickly, but the individual air molecules hardly move. Same, same.

So, electric signals travel at phenomenal speeds, but the individual carriers (electrons) move quite slowly.

[ February 22, 2005, 10:24 PM: Message edited by: rattus ]
 
Re: Electron flow

thanks for the links. I had been told that the speed of a flowing electron was 5 or 6 feet or so per second, and one of the letters attached to attached site (BHgrav's) points out why that is true.

thanks, paul
 
Status
Not open for further replies.
Top