# How does an AC circuit get anything done? [duplicate]

if alternating current switches directions back and forth periodically it seems like to me that current would go forward a bit and then almost immediately back track the exact same distance, if this is the case then how does AC ever get to where we want it to? Could I be misconceiving AC in some way?

• Possible duplicate of Alternating current – Aaron Stevens Nov 21 at 4:45
• Movement of charge is movement of charge. It usually doesn't matter in which direction – Aaron Stevens Nov 21 at 4:47
• Consider that pistons move only a small distance back and forth in an automobile engine and do so very quickly. How can such a system move the vehicle so far? – JimmyJames Nov 21 at 15:29

The wording of your question suggests that you might have a fundamental misconception about how electricity works (quite a common one, unfortunately, because introductory courses in electronics often get it wrong). In an electric circuit, electric currents do not directly transfer energy. Rather, energy is transferred through the electromagnetic fields in the space surrounding the wires. The only purpose of the wires and other components is to guide the fields to the right places.

This is quite counterintuitive, since electromagnetic fields are invisible and we are used to thinking of electricity as something that flows inside wires. However, the conclusion is easy to demonstrate. The directional energy flux in an electromagnetic field (the Poynting vector) is equal to $$\mathbf{S} = \mathbf{E} \times \mathbf{H}$$ where E and H are the electric field and the magnetic auxiliary field (in simple cases, $$\mathbf{H} = \mathbf{B}/\mu$$).

In the simple circuit drawn below, the electric field points downward (since the voltage is positive in the upper wire). Meanwhile, the magnetic field circles around the current-carrying wires; thus, it points into the screen inside the circuit. Using the right-hand rule for cross products, we can see that the Poynting vector points to the right. Thus, power flows from the battery to the load. One can show that this energy flux is sufficient to explain all of the power delivered to the load.

What if we replace the battery with an AC voltage source? Now when the voltage becomes negative, both the electric field and the current (and therefore the magnetic field) reverse their direction. These changes cancel each other out and the Poynting vector still points to the right. Therefore, power continues to flow from the voltage source to the load – even though every electron in the circuit stays in nearly the same place.

• Err... What about electrons, though? Isn't that a transfer of energy? – Andrew Nov 21 at 17:37
• But there is an electromagnetic field inside the wires... why don't you say anything about that energy? – thermomagnetic condensed boson Nov 21 at 19:08
• @Andrew the electrons move slightly counterclockwise around the circuit when the voltage is positive, and slightly clockwise when the voltage is negative. No electrons are transferred from the source to the load – if that happened, charge would build up on the load side of the circuit, and the voltage on that side would quickly become large enough to prevent more current from flowing. – Thorondor Nov 21 at 22:27
• @thermomagneticcondensedboson Because there is virtually no electric field inside the wires, since they are made of a conductive material. – Thorondor Nov 21 at 22:28
• @thermomagneticcondensedboson In Ohm's law $J = \sigma E$, $\sigma$ is the conductivity, which is very high (for simple problems like this one, effectively infinite) in a metal wire. Therefore, to allow a finite current density, the electric field must vanish. – Thorondor Nov 22 at 6:46

Imagine you have a crosscut saw with two handles, one on each end and a person holding each handle. The saw rests on a log to be cut. One person pulls their end of the saw and the teeth cut into the log. Then the other person pulls their end and the blade comes back, cutting further into the log.

The point is that the saw can cut the log even though it only goes back and forth, retracing its path over and over. In the same sense, AC power can perform useful work on the "pull" part of the AC cycle as well as the "push" part.

• This is an odd analogy, the saw does work because it consistently moves in the direction that it's cutting (generally downward) if it only moved back and forth, it would achieve nothing. – DBS Nov 21 at 15:26
• @DBS That's not really how saws work. They shave off the top surface of the wood as they go across it. One a slight amount of downward force is required. – JimmyJames Nov 21 at 15:32
• @DBS The saw moves downward by gravity because the wood that it cuts through is no longer there. – Tashus Nov 21 at 15:32
• In this analogy, Direct-Current is more like a bandsaw, which is a belt with teeth that spins continuously in one direction. While AC is more like a saw with double-sided teeth, both directions apply cutting effects and an oscillating motion gets the best out of it. – Ruadhan2300 Nov 21 at 16:07
• @DBS The problem with analogies is that they can be taken too literally; read it this way - given that there exists a mechanism that makes the saw interact with the wood at the contact point (the aforementioned slight downward force, but we'll "black box" that), the back & forth periodic motion is a viable option to do useful work, with the two people being analogous to a battery. (P.S. Imagine extending the ends of the saw to form a circuit with the power source (maybe a single person or an engine) being far away.) – Filip Milovanović Nov 21 at 16:09

If alternating current switches directions back and forth periodically it seems like to me that current would go forward a bit and then almost immediately back track the exact same distance, if this is the case then how does AC ever get to where we want it to?

See this microscopic view of current, and understand that the electrons do not travel fast within the conductor, but have a drift velocity. This drift velocity , according to the classical electrodynamics equations, is what generates the current measured in the lab. In a DC circuits the electrons move slowly towards the potential , in an AC current they move back and forth, but the macroscopically generated current has a direction from high potential to low potential.

The reason that AC has prevailed in the transmission of electric power is practical considerations. When DC is transmitted one has to generate AC at home to get motors running, for example.

• The Drude's model is known not to be an accurate description of reality. The speed of the electrons that conduct is about the order of the Fermi speed, i.e. about 1% of light speed in metals. – thermomagnetic condensed boson Nov 21 at 19:28
• @thermomagneticcondensedboson Of course a model that does not include quantum mechanics cannot be the "real" one, as main stream physics is based on quantum mechanics being the underlying frame of classical. But it is useful, as the planetary model of the atom was useful (and still is) when physicists first found out that it described the data in the correct direction. – anna v Nov 22 at 5:15

and then almost immediately back track the exact same distance

Current travels, very approximately, one foot per nanosecond. Mains 50Hz cycles have a period of 20,000,000 nanoseconds. It's very far from "almost immediately".

• This doesn't really address the fundamental misunderstanding that is confusing the asker. The distance traveled by the current each half period is not really material to the way the work is performed, and it does indeed travel the same distance in the opposite direction during the second half of each period, regardless of the distance traveled. – Tashus Nov 21 at 15:39
• The asker thinks that 1/60th of a second is "almost immediate", but for an electric field moving at (nearly) the speed of light, that's enough time to travel across a continent, or zoom through the circuits in your device many thousands of times. The confusion is (in part) due to not understanding the vast difference between the speed of humans vs the speed of electricity. – Foo Bar Nov 21 at 16:02

Alternating current transmits powered by varying electric fields which makes the electron oscillate this oscillating energy transmitted through the wire in the form of electric fields and the energy is transmitted where it needs to be transmitted.

For example in a electric heater the alternating current oscillates electron in the filament which produces heat. But in dc heater there is a continuous flow of electron which generates heat due to resistance.

• But DC current can cause heating, transfer power, etc. too. This answer isn't helpful, as it basically just says AC works because it works. – Aaron Stevens Nov 21 at 5:01
• The edit didn't really help alleviate @AaronStevens concerns, since it's the resistance that results in heat in an AC heater also. If anything all you've done is made it less clear. – Trotski94 Nov 21 at 12:59