# AC Current and Forward Movement

Okay - maybe not the most advanced question, but one I have not seen a satisfactory answer to.

DC current is easy to understand. The current and electronics flow unidirectional, and you can understand how by connecting a circuit, a flow goes from the source to the recipient end of the circuit, and by passing through some form of resistance (say a filament on a light bulb), generates heat.

As for AC current, I get it. Once the circuit is on, the electrons pass back & forth. I understand that concept. What I do not understand is how the circuit ever gets created in the first place. Bear with me:

If ultimately, the current is flowing back and forth, how does it ever move forward from the source to the end-point of the circuit in the first place? I have heard of a golf-ball in a pipe analogy to explain how an AC circuit works... but what no one explains is how the current flows when first putting all the golf balls into the pipe in the first place (as an analogy).

Are the electrons already there (i.e. - a component of the material over which current flows)... but even then, if the current oscillates back-and-forth, how does it ever move forward to create a circuit?

The only analogy I thought of that might explain this is that the current itself (the flow of electricity) moves unidirectional, but the electrons oscillate. So an analogy in that case might be that the current is a river, while the electrons are fish swimming back and forth?

Apologies in advance for such a naive question, but this I have never been able to get a sufficient explanation for this.

if the current oscillates back-and-forth, how does it ever move forward to create a circuit?

The current does not create a circuit - a circuit is created by wires and various electric components (e.g., resistors, capacitors and inductors), which together form a loop.

In the simplest case, a circuit is just a loop of wire, filled with free electrons, which will move if an electric field is applied.

So, if you insert a battery in such loop, the free electrons will start moving (simultaneously, all around the loop, including the battery) in one direction. That's a DC.

If you quickly change the polarity of the battery, back and forth, the free electrons will be changing their direction (oscillate) as well. That's an AC.

For the analogy with golf balls and a pipe to work, the pipe has to form a loop as well and someone has to push the balls around the loop either in one direction (DC) or back and forth (AC).

• This makes more sense. In other words, to use an analogy, the garden hose is already full of water (i.e. - the wire is already full of free-electrons ready to push back or forth (A/C) when you flip on a switch. Commented Oct 14, 2018 at 14:34
• @Brad48 That's right, but the analogy with a garden hose is not very good because the hose does not make a loop, which is always the case with electric circuits. A better analogy involving water is a water heating system, where a pump is pushing water though a loop or a closed circuit.
– V.F.
Commented Oct 14, 2018 at 14:41

When an AC circuit is closed, the electric field moves through the wire at practically the speed of light. Since wires are conductors, there is approximately 1 free electron per atom of the conductor (usually copper), so there are always an ENORMOUS number of free electrons available to be accelerated by that electric field. The drift speed of each electron is very slow, even under the influence of the alternating electric field in an AC circuit, so the electrons are "sloshing" back and forth through a very small distance about a fixed point in the wire.

So, how is the circuit complete if there is no net flow of electrons through the wire? I like to use the analogy of a full water hose with a hand-held nozzle on the end of it. When you squeeze the handle on the nozzle, the first drop of water coming out the end of the hose is NOT the drop of water that just entered the hose on the other end. As one drop of water comes out of the nozzle, one drop of water enters the hose on the other end. If you could alternate the pressure on the hose and pump water into the nozzle end, as one drop entered the nozzle, one drop would exit the other end of the hose. This alternating pressure would give you an alternating water flow.

AC current is very much like this. The free electrons in the wire represent something akin to an incompressible fluid in the wire. As one electron enters the wire on one end, an electron leaves on the other end of the wire, and vice verse.

For the circuit to conduct a current we don't need to have a specific electron from one end of the circuit to make it to the other. WHen we have a circuit, electrons are moving across the entire circuit, and those electrons that are closer to the load will reach the load.

• I get that the electrons do not need to travel across the current, but the way A/C has been explained to me is that the current in an A/C Current itself travels back & forth.....my question is if the current travels back and forth, how does it ever move create a circuit in the first place, as how does the current ever reach ultimately the ground? Commented Oct 14, 2018 at 13:21
• @Brad48 I think your basic understanding of current may need some more thought. Imagine that you probe the circuit at a random location and time, the momentaneuous current there is defined as the charge that passes per unit time. The current in this scenario does not care what the current was before or what it will be afterwards, the only thing that differs with AC is the direction of the current. Commented Oct 14, 2018 at 13:53
• I appreciate the help on this.....Is the current always there (in other words, I always viewed turning on a light switch the equivalent of turning on the spigot to a garden hose. The water flows from the source to the end. Is a more apt analogy a hose in which, the hose is filled with water, and the nozzel is closed shut. Not until you open the nozzel does the current flow. In other words, the current is always present in the copper wire, but until you open the circuit, it stays in a fixed state (electrons included). Commented Oct 14, 2018 at 14:13
• @Brad48 One could start with that assumption. Depending on how far your interest/education goes, this assumption will hold for some time. The current "is always there", but the current itself isn't really electrons, but rather electrons in uniform motion. I'd imagina a pipe which is filled with water. The water can only flow out when one end of the pipe is higher then the other (potential energy drop). Now imagine an idiot jerking this around and thus manages to create an ac current ;) Commented Oct 15, 2018 at 4:33