# How can AC current power anything?

I understand the difference between AC and DC. What I don't understand is how does AC power anything when it's reusing the same electrons over and over as they are moving back and forth?

A visual picture is this link at 0:35.

Wouldn't it require new electrons? Eventually?

• Note that DC current doesn't "use [up] electrons" either. You could view DC current as the same electrons going around and around in a circle, like race cars - there are no "new" electrons added to the circuit. When race cars go over a section of a race track, they heat it up. Enough cars going by can heat it up a lot. It doesn't matter if the cars are going around and around in circle or going back and forth, the track still gets hot. So it is with an incandescent light bulb or electric heater and the electrons. – Todd Wilcox Oct 12 '15 at 18:14
• And speaking of cars, you might as well ask how the engine can power your car, when the same pistons move back and forth over the same few inches without going anywhere, and the crankshaft just goes in circles. – hobbs Oct 14 '15 at 4:57
• But you're OK with the idea that the wind can power things, even though it's just the same air molecules blowing back and forth? – David Richerby Oct 14 '15 at 10:15
• I often visualize electricity as water. For AC it would be water in a pipe moving back and forth--think of all the uses it could be put to. If a paddle blocked the flow with a "Stick" coming outside the pipe you'd have a stick moving back and forth that could be used to power any kind of mechanism--even though the actual water stays the same and always remains in pretty much the same region. Water in a pipe, although an imperfect analogy, can be used to visualize most electronic interactions surprisingly well. – Bill K Oct 14 '15 at 17:48
• AC power anything No, AC is used for transmitting energy over distances and powering electrical devices while DC is used for electronics devices. AC must be converted to DC in order to power any logic or electronics circuit including the ones inside electrical devices to control those – phuclv Oct 14 '15 at 18:38

@The Photon's answer is quite extensive, the only thing missing is, how electrical energy is now actually transferred. In a simple case where you just have some kind of ohmic load, it is exactly the same as for DC, just with switching polarities.

If you want a picture, imagine a saw: It is pulled through the same block of wood, back and forth. The same sawteeth enable it to remove layer by layer, as there is a force (and power) applied while moving into both directions.

For the electrons, it is quite similar. An alternating voltage keeps pushing them through some load. As they pass through the load, they are moving from a high-voltage node before the load to a low-voltage node after the load, giving off the energy difference between the first and second state.

Then the AC polarity is inverted and again, they are on a high-voltage node, passing through the load, to a low-voltage node. Again, their previous state had more energy, thus the energy is transferred into the load.

• The saw analogy is excellent, I will have to remember that! – Stig Hemmer Oct 13 '15 at 8:08
• Though eventually the saw teeth wear out and you do need a new saw. – OrangeDog Oct 13 '15 at 11:00
• Well, that's where the analogy ends. The energy is in fact not just used to generate heat and overcome the binding energy in the wood, but also the binding energy in the sawteeth. Even if it just leads to some reorganization in those teeth. You could extend that example to sufficiently suitable materials, but they would end at the normal quantum mechanical wear and tear. You just can't reach the level of a single electron with any analogy in our macroscopic world. – J A Oct 13 '15 at 15:17
• In the DC case it's like a chainsaw where the same teeth keep going in the same direction in a loop – user2813274 Oct 14 '15 at 2:47
• The saw analogy might also help explain the true RMS value of AC. Imagine the "DC saw" - either a chainsaw or circular blade doing a certain amount of cutting per unit time. Now the AC saw is rocking back and forward in a sinusoidal motion. To make up for the times it is traveling slowly (as it changes direction) the AC peak speed must be higher than the DC saw by a factor of SQRT(2) - about 1.41 - to achieve the same rate of cutting. – Transistor Oct 14 '15 at 10:05

The energy used in an electric circuit isn't "contained" in the electrons and electrons aren't used up when energy is consumed in a circuit.

The energy in circuits can come in several forms:

electric fields: Produced when positive and negative charge carriers are separated from each other.

magnetic fields: Produced when charge carriers are in motion.

kinetic energy: Not normally considered part of the electric circuit energy, but it comes into play as an intermediate step as energy in the circuit is transformed from electric to magnetic forms. Or, for example, when an electric field accelerates a charge carrier which then gives up its kinetic energy to produce thermal vibrations in a resistive material to produce heat.

electromagnetic radiation: Produced when an oscillating electric or magnetic field creates a self-sustaining oscillation in the electromagnetic field.

As an analogy, consider a swinging pendulum. Energy is constantly being transferred between potential energy and kinetic energy in a swinging mass. But the mass of the pendulum isn't used up and it never has to be replaced (at least, not as a result of the pendulum's operation).

Edit: We could also of course talk about photodiodes and piezoelectric transducers and motors and gamma ray scintillators and other devices that allow a circuit to transform energy to various other forms. I'm ignoring those special cases here and just talking about the energy that's involved when doing circuit analysis.

• +1 I like this answer a lot. I very much like "they aren't used up when energy is consumed". However at the quantum level, might it be slightly contentious to say 'Electrons don't "contain" energy'? AFAIK, electrons raised to higher energy states must represent, have or 'encode' more energy somehow. Also, AFAICT, their ability to move is reduced by removing energy from a system. I am not a nuclear Physicist, so I apologise if have misunderstood the mechanisms. The statement 'they aren't used up when energy is consumed' seems clear and unequivocal. – gbulmer Oct 12 '15 at 16:27
• @gbulmer, you are right. I'll try to reword it. – The Photon Oct 12 '15 at 17:05
• I suspect it isn't even as noticeable as a blemish; I just thought you could polish it to perfection :-) – gbulmer Oct 12 '15 at 17:07
• The pendulum example pretty much hit the nail on the head. So the potential energy that an electron can produce hypothetically never decays? – Luke Oct 12 '15 at 19:31
• The electron can have kinetic energy, it can be contributing to a current that produces a magnetic field (with associated energy), it can have electrical potential energy because it is in the vicinity of a positive charge, etc. All of those can be transformed to other forms of energy, but the electron itself is not used up in the process. – The Photon Oct 12 '15 at 20:17

I sense you have a misunderstanding of how DC energy is transferred from source to load which is hindering your ability to understand how AC energy is transferred.

The picture many people have in their heads is that the power source somehow gives energy to electrons. The electrons then flow down a wire carrying this energy and then somehow release the energy when the electrons flow through the load. I would bet that your mental picture of electricity is something like this. And if that is close to how you view electricity, then the question of how an AC energy source transfers energy is perplexing. Afterall, electrons aren't flowing back and forth 50 or 60 times a second from the lightbulb in your kitchen all the way the way back to the generator at the power plant. We know electrons move much, much slower than that (they move on the order of a meter an hour, depending on a number of factors like current, size of the conductor, etc.). And given that there are transformers in between your kitchen light and the generator, it makes even less sense, since they are 2 different electrical circuits that have different electrons in them. The wires aren't even connected.

But this is not how it works. Energy isn't carried from source to load via electrons. Energy doesn't even flow down the wires. Instead, electrical energy travels from the electrical source to the electrical load via an electromagnetic (EM) field in the space surrounding the source, wires, and load.

Look at the picture below of a DC circuit consisting of a battery, some wire and a resistor. The green arrows represent the magnetic field that arises due to current flow. The red arrows represent the electric field due to the voltage source. The blue arrows represent the energy flux density, or the Poynting vector, which is the cross product of the electric and magnetic fields. The Poynting vector can be thought of as the rate of energy transfer per area.

Notice the flow of energy is from the battery to the resistor. Also notice that the energy flows into the resistor not from the wire but through the space surrounding the wires.

If you replace the DC source with an AC source, you should be able to convince your self - by looking at the electric and magnetic fields - that the Poynting vector still points from source to load even though the current is switching directions. Because the Poynting vector is a cross product of the two fields, its direction stays the same even as the fields are changing.

There have been some questions in the comments about the scientific validity of what I've said above. How electromagnetic energy travels in circuits has been known for some time ... since at least the late 1800's. The Poynting vector, named after John Henry Poynting who explained this theory in a paper in 1884, entitled On the Transfer of Energy in the Electromagnetic Field. The paper is pretty readable and explains the theory pretty well. He explains:

Formerly a current was regarded as something travelling along a conductor, attention being chiefly directed to the conductor, and the energy which appeared at any part of the circuit, if considered at all, was supposed to be conveyed thither through the conductor by the current. But the existence of induced currents and of electromagnetic actions at a distance from a primary circuit from which they draw their energy has led us, under the guidance of Faraday and Maxwell, to look upon the medium surrounding the conductor as playing a very important part in the development of the phenomena. If we believe in the continuity of the motion of energy, that is, if we believe that when it disappears at one point and reappears at another it must have passed through the intervening space, we are forced to conclude that the surrounding medium contains at least a part of the energy, and that it is capable of transferring it from point to point.

He goes on to say:

Starting with Maxwell's theory, we are naturally led to consider the problem: How does the energy about an electric current pass from point to point — that is, by what paths and according to what law does it travel from the part of the circuit where it is first recognisable as electric and magnetic to the parts where it is changed into heat or other forms?

The aim of this paper is to prove that there is a general law for the transfer of energy, according to which it moves at any point perpendicularly to the plane containing the lines of electric force and magnetic force, and that the amount crossing unit of area per second of this plane is equal to the product of the intensities of the two forces, multiplied by the sine of the angle between them, divided by $4\pi$, while the direction of flow of energy is that in which a right-handed screw would move if turned round from the positive direction of the electromotive to the positive direction of the magnetic intensity.

He then goes on to show how energy enters and heats up a wire:

It seems then that none of the energy of a current travels along the wire, but that it comes in from the non-conducting medium surrounding the wire, that as soon as it enters it begins to be transformed into heat, the amount crossing successive layers of the wire decreasing till by the time the centre is reached, where there is no magnetic force, and therefore no energy passing, it has all been transformed into heat. A conduction-current then may be said to consist of this inward flow of energy with its accompanying magnetic and electromotive forces, and the transformation of the energy into heat within the conductor.

Richard Feynman also talks about this in his lectures on physics. After an explanation of this phenomenon, Feynman derives how a charging capacitor gets its energy, then says:

But it tells us a peculiar thing: that when we are charging a capacitor, the energy is not coming down the wires; it is coming in through the edges of the gap.

Feynman then, like Poynting, explains how energy enters a wire:

As another example, we ask what happens in a piece of resistance wire when it is carrying a current. Since the wire has resistance, there is an electric field along it, driving the current. Because there is a potential drop along the wire, there is also an electric field just outside the wire, parallel to the surface. There is, in addition, a magnetic field which goes around the wire because of the current. The E and B are at right angles; therefore there is a Poynting vector directed radially inward, as shown in the figure. There is a flow of energy into the wire all around. It is, of course, equal to the energy being lost in the wire in the form of heat. So our “crazy” theory says that the electrons are getting their energy to generate heat because of the energy flowing into the wire from the field outside. Intuition would seem to tell us that the electrons get their energy from being pushed along the wire, so the energy should be flowing down (or up) along the wire. But the theory says that the electrons are really being pushed by an electric field, which has come from some charges very far away, and that the electrons get their energy for generating heat from these fields. The energy somehow flows from the distant charges into a wide area of space and then inward to the wire.

• Why does the electrical field (red) in the resistor point in the same direction as in the battery? – Clawish Oct 14 '15 at 0:17
• @Eric - re: "the energy flows into the resistor not from the wire but through the space surrounding the wires." Is that statement based on a scientifically founded principle? If yes, where is the science to support it? I've never seen that explanation before today. – zeffur Oct 14 '15 at 19:57
• @zeffur, yes, of course. "We have shown that the Poynting vector is not confined to the interior of the circuit, but flows through all space from the battery to the resistor. Part of the electromagnetic energy takes the shortest route, which is typically shorter than the distance along the wires. A small part of the energy follows very long paths from the battery to the wire. Maxwell’s equations suggest that in an ordinary device such as a flashlight some of the energy makes a very long space odyssey from the battery to the bulb, exploring every cubic nanometer of space in the process." – Eric Oct 14 '15 at 21:44
• @zeffur, that was taken from this paper: arxiv.org/pdf/1207.2173.pdf See also this: cq-cq.eu/Galili_Goihbarg.pdf or just google "poynting vector circuit" and you'll find lots of info. – Eric Oct 14 '15 at 21:44
• @zeffur: I think I first learned about the Poynting vector in my 3rd or 4th year at college getting my EE degree. Apparently everyone seems to think that fact is only relevant to antenna design. You may find "In a simple circuit, where does the energy flow?" relevant. – davidcary Oct 14 '15 at 22:53

What you need to know is P=IV I is the electrons going back and forth. During the time when the electrons are moving back, V is always negative, so the sign of P = (-)*(-) is positive. So positive work (for example heating the tungsten filament of a light bulb) gets done during both forward and backward flow of current.

Ignore the electrons. Learning about electricity through electrons will mislead you most of the time. For one thing, they're going in the wrong direction. Secondly, they're travelling at the wrong speed. Drift velocity is much slower than the speed of an electrical signal.

Electricity transmission in a metal looks a lot more like a "Newton's Cradle": an electron goes in one end, force is transmitted through repulsion of electric fields, and an electron goes out of the other end.

(Situations where you do need to care about electrons: semiconductor junctions, cathode ray tubes, gas discharge devices, thermionic valves.)

• Electrons don't go in the wrong direction. We just arbitrarily assign them negative sign. Think of it this way: if you had electrons that went in the other direction, you will break physics, and probably your circuit in the process. – PyRulez Oct 13 '15 at 21:53

I just wanted to explicitly state that electricity is merely energy which is used to move electrons. Electrons are never made, or lost, or charged, or consumed. All of the work done with electricity is done with the movement of electrons.

To use the cliched analogy of water mechanics, imagine a channel of water with a turbine in it. If the water is not flowing, the turbine doesn't turn and no work is being done. If the water is flowing continuously (as in direct current) the turbine will also spin continuously and work is being done. Likewise, if the water flowed back and forth (alternating current), the turbine would also spin back and forth, and work is being done. At no point is the status, quality, or quantity of water ever changed, other than with respect to the flow.

An alternating turbine is just as useful as a continuously spinning turbine, but must be applied differently. Also, as with electricity, if the correct mechanisms are applied, the rotation from an axle attached to a continuously rotating turbine can be converted into an oscillating axle, and vice-versa.

You don't worry about the electrons for circuits in general; in super tiny devices like on an IC, possibly.

Depends on how deep into theory you want to go, but in general you think of electrons flowing like water in a hose, once the water is put into motion, that's what does the work, what force put the water into motion?

The transformer is just 2 coils of wire close to each other, it only works because of AC, the copper wires react with the CHANGE in current, if it was DC it would sit there and no power gets thru. When the current changes? That's when the power is transferred inside the transformer from one coil to the other.

so if you put DC in a coil of wire it becomes a magnet. If you move that magnet around and another coil is nearby? it will pick up current. Its definitely not free energy though. A car's alternator works like this, the center part becomes a magnet (the part that spins) and coils are wound and set close to that spinning armature and pick up current, usually 3 coils. One (dangerous) way to test if an alternator is working, is turn on the engines key to Run, dont start it, and put a magnetic screwdriver at the center of the alternator pulley, if the alternator is on? the screwdriver will be strongly pulled into that pulley. If not? its usually because the brushes are worn or the alternator is no good.

I think the explanations for how the alternator works will help to visualize AC

The applied force (Voltage) in a circuit causes an electric field that causes electrons (charged atomic particles) to move in a specific direction (very fast, but a very short distance). Those electrons affect other nearby electrons by bumping them (electrons magnetically repel each other, so the applied force is transferred through the conductor atoms extremely fast). Those other electrons slightly resist that bump & heat a bit, but most of the energy is cascaded through a circuit as an energy wave that eventually makes its way to a device to do some work (e.g. light a bulb, cause a very resistive material to heat up, or windings in a motor to cause a magnetic force to spin a motor rotor, etc). The electrons that surround the atoms in a conductor only act as a medium for energy to flow through them--much like water in a pond that reacts to a dropped pebble. You don't need more water for the energy wave to flow through the pond--but once the energy is dissipated (or the electric current stops), the show is over--that's the nature of electrical energy transfer.

• You're treating electrons like little billiard balls that bump into each other and transfer energy mechanically. That's not how it works. – Eric Oct 14 '15 at 21:48
• @Eric - the bump that I described is electromagnetic--not mechanical. – zeffur Oct 15 '15 at 3:36
• Either way, that's not how it works. – Eric Oct 15 '15 at 4:20
• What exactly do you disagree with? An electric field will in fact change the path of a charged particle (-electron) and increase its energy state which will cause higher kinetic energy...which will eventually lead to more interaction with other electrons/atoms. – zeffur Oct 15 '15 at 8:37
• I disagree with your entire answer. Energy isn't transferred down a wire by electrons bumping each other (whether you want to call it electromagnetic bumping or not). The heat doesn't come from electrons resisting that bump. Rather, wires and resistors heat up because they absorb energy from the outside of the wire. Poynting showed pretty clearly that the amount of energy absorbed by the wire from the outside was equal to the amount of heat that was dissipated by that wire. – Eric Oct 15 '15 at 9:08

It is the movement of the electrons that transfers energy from one form to another. The electrons do not get used up, they just move and in the process transfer energy from one point to another.