# How electrons inside copper wire behave when source is ac or dc?

When a simple copper wire is connected to AC or DC, what actually happens to the electrons inside the wire?

I.e. What kind of path do they follow when they reach the end of the wire? If anyone can provide a video demonstration link it would be great.

Atoms contain several layers or shells of electrons. The hydrogen atom has one electron on the first layer, the helium atom has two on the first layer, the next atom (lithium) has two on the first layer then one on the second layer, etc. Each layer can typically only hold a specific number of electrons.

The best conductors have one atom in their outermost layer, and they are more than happy to give it up. Consider the atom of copper. It has the following electron count in each layer: 2, 8, 18, 1. It will give up that one electron under a weakly charged field, and it will then be positively charged and "pull" an electron from a neighbor copper atom. If you look at silver and gold they are arranged in a similar manner: 2, 8, 18, 18, 1 for silver, and 2, 8, 18, 32, 18, 1 for gold.

You can strip any atom of an electron, but the best "conductors" require only a weak field to do so.

So if I pull an electron off the end of a copper wire, using a weak electrical field, then that atom might pull an electron off its neighbor, and eventually one copper atom somewhere in the wire will lose their electron, but be unable to get someone else's because they're too far away, or interacting with some other field. If I push an electron in the end of the wire, then the copper atom that gets it will have too many, exhibit a negative charge, and essentially push its extra electron onto some other copper atom until it finds an atom that can't get rid of it, or an atom that's missing one already.

You can push and pull electrons onto and off of insulators as well - you do so when you build up static charges, for instance, using cloth and plastic.

But conductors redistribute the charge internally, so if you charge one end of a wire with extra electrons, you can consider the other end of the wire similarly charged.

A battery, often using a chemical reaction, sets up a positive charge on one end, and a negative charge on the other. If you connect a conductor between the two ends, you will force electrons through the conductor as they travel from the negatively charged side (too many electrons) to the positively charged side (too few electrons).

The electrons move one direction only for DC, and they move in one direction then the other for AC. Due to the changing magnetic field (ie, the wire becomes an inductor) high frequency AC signals typically travel near the surface of the wire. You can look up "skin effect" to understand this better. The electrons travel between the atoms of the conductor.

Every time you push 6.28x10^18 electrons through the wire, you've moved one amp of current. That's 6.28 billion billion electrons. However, there are about 4.38x10^22 copper atoms in one meter of 20gauge wire, so if you push a full amp through it, assuming even distribution, you won't get any of the electrons out that you pushed in - you'd have pushed out electrons that were already in the wire. Electrons move slowly, individually, but the charge distributes quickly - as soon as you push in one electron, you find that it's easier to pull one off the other end almost at the speed of light at the other end. It's not the same electron, but the effect and charge is the same.

A good conductor distributes the charge very, very quickly, and doesn't convert much of the movement to heat. If you push the same current through the same size gold wire and the same size copper wire, the gold wire will heat up more, because it's harder for those gold atoms to give up and accept electrons.

I.e. What kind of path do they follow when they reach the end of the wire?

They don't. If there's an open circuit, there's no current.

Current is really just electron flux: 1A current in one direction = 6.24 x 1018 electrons flowing in the other direction. (Thank Benjamin Franklin for that: he's the one that decided the sign convention for current, based on the movement of what he thought was positive charge.)

Current in a conductor is caused, in a sense, by electric fields. In a conductor, the current density J = σE where J is in amps / m2, σ is the conductivity of the material, and E is the electric field.

If you have a wire connected in a circuit with components (e.g. resistors, etc.) to a source of voltage, that voltage imposes electric fields along the circuit, causing current to flow. When the electrons reach the end of the wire that is connected to another component, they move into that component, and continue in a loop around the circuit.

The easiest analogy here is probably the flow of water. Current is analogous to water flow, voltage is analogous to pressure, batteries are analogous to pumps, wires are analogous to hoses or pipes. (Unlike the water analogy, if you cut a circuit, the current will stop, because the conductivity in air of electrons is very low, whereas if you cut a hose, water will spill out.)

• From the last statement it's clear that you've never seen an electron fountain :) Mar 9, 2012 at 9:32
• well, OK, corona discharge + ion jets + such things require high electric fields to overcome the breakdown strength of air. :-) Mar 9, 2012 at 13:17
• Ahah just kidding :P Mar 10, 2012 at 9:54
• "he's the one that decided the sign convention for current, based on the movement of what he thought was positive charge" ... not that there's anything wrong with that: physics.stackexchange.com/a/17131/176 May 16, 2013 at 19:38

A metal conductor is a sea of free electrons held in a potential well by the positive charge of the atom kernels that make up the metal. Here's how it works: Some electrons are tightly bound to the nucleus of the atoms, and some are free to wander. The tightly bound ones don't move, but the free ones can go wherever they want ... sorta. Heat (brownian motion) makes all of these particles jostle around and they go faster as the temperature rises. Since some electrons are free to move, the jostling tends to bounce them further from the rest of the atoms. A cloud of electrons starts to form beyond the surface of the wire, and it gets bigger as we heat things up. As the electron cloud moves further out, the atoms that are stuck in place (in a crystal lattice actually) develop a positive electric charge that tends to pull the electrons back. So there is a balance between jostling due to heat that tends to make the electron cloud expand (something like the molecules in a gas making it want to expand when heated) and the electric field that develops because negative electrons are spending some of their time further away from the wire than the positive atoms left behind. The net effect is that all the electrons have to stay near the wire, but they move further out as temperature rises. There are a bunch of things that happen because of this 'sea of electrons'.

First, it is a sea and we can make an analogy to the ocean. Along the east coast of US there is something called the Gulf Stream. It is a current in the sea. It moves a few miles per hour and carries lots of water northward. In the ocean there are also waves. If there were a earthquake in the Atlantic, a resulting tsunami would move across the ocean at 600 miles per hour. So we have experience that in a sea, waves can move very fast while current moves much slower. In a wire it is much the same. When you apply a positive potential to the end of a wire, the electrons in the cloud around the wire are drawn to it. Actually, your positive charge now competes the positive charge of the atoms, and some of the electrons will shift in your direction. Some may even physically move into your positive charge you applied, but mostly the electron cloud at the end of the wire will shift toward you. Once they shift, the ones a bit further in will see the shift because now there are fewer negative electrons on the side toward you. So they will shift. This process propagates down the wire, each batch of electrons shifting because of the change in field due to others shifting. When the 'wave' gets to the other end of the wire, the cloud there will shift toward the opposite end, exposing more of the positive charge of the atoms, so that you will see a positive potential at the end. But it doesn't happen immediately. The field in cable has to change and that takes time. Now here's the really interesting part: electric fields move at the speed of light outside of the wire, but they move VERY SLOWLY within the wire. I don't have exact numbers, but outside the wire fields are speeding along at 3x10^8 meters/sec. Inside the wire it isn't even one meter per second. If you apply DC, it takes a very long time for a single electron to actually travel down the wire to the other end. But, if you apply a positive pulse to the wire, you will see a positive pulse at the other end at about the speed of light (if you put an insulator around the wire it actually goes a bit slower, but thats a detail for the moment). How can this be? If fields travel very slowly inside the wire, how does the pulse get to the other end so fast? It does so because of the field AROUND the wire. A wire, for AC signals especially, acts like an inside out waveguide to some extent. Fields can't get going inside the wire, so they stay near the surface, and only jostle the electrons near the surface. For DC, the fields can finally penetrate the whole wire and get everything moving, but for AC the field is reversing at regular intervals, so just as it gets going into the wire a bit, it reverses and has to start over. The net effect is that currents in wires travel in a narrow region near the surface: this is called the 'Skin Effect'. I don't think it was discovered by Dr. Skin (but I could be wrong), I think it just refers to the current sticking to the surface, or 'skin', of the wire. If you wonder how much this matters: a very very very lot. Tons. Great gobs. I have professionally built cable equalizers for video signals. The skin effect has allowed me to earn a good salary for a few years. Take a 24 gauge wire (say, Cat 5) and apply a signal that has frequencies from very low (say 30 Hz) to reasonably high (say 5MHz). The low frequencies can penetrate much further into the copper, and so they actually see a much bigger cable. The high frequencies only see a thin tube. What's the difference? Resistance! Signals flow much more easily in a thick wire than a thin tube. So the high frequencies get smaller and smaller as you go down the cable. For a video signal, it means that your picture gets blurrier and blurrier and eventually the color will disappear. After traveling through a mile of Cat 5 cable the 5MHz parts of a video signal will be about a million times smaller than the low frequencies.
One other thing that this 'sea of electrons' explains: cathode rays. In the good old days electrical signals were amplified by vacuum tubes. The vacuum tube itself had a filament (which had current forced through it so it glowed orange hot) and a grid (kinda like a metal screen) next to the filament. Further away was something call a plate (which was just a metal plate with a terminal connection). When the filament was hot, the sea of electrons expanded and a lot of electrons would wander pretty far from their home wire. If you applied a positive charge to the grid, it could pull some of the those electrons clean off of the filament, and if, at the same time, you applied a positive charge to the plate, they would scurry off across the vacuum inside the tube and land on the plate causing a current. So the grid could control the current through the tube, and that was the first electronic amplifier. It was invented from the first light bulbs. In fact, Edison almost invented it, but never finished the experiment, so the nod goes to a gentleman named DeForrest. (I think ... maybe I should check Wikipedia). If that plate was a screen coated with phosphors, it became a CRT (cathode ray tube) and that became the first television. So there's a lot that can be explained by this view of a wire/conductor as a sea of electrons held loosely in place by their parent atoms. I'm not sure if that's what you were looking for, but it always helped me once I learned it. Best of luck. Dave

• You succinctly explained the Skin effect. However,I have a much more sillier and more fundamental doubt.It is related with the generation of alternating electric field. Please explain the alternating field and the electron's wiggling behavior as cause and effect. Like,if AC is cause,the positive cycle causes electrons to accelerate more,&negative cycle impedes them,so this forms clusters of electrons at some points and voids at other. Thus,effective field due to positive ions and electrons makes the alternating contour. But if AC is cause, then how do electrons help in propagation, or do they? Apr 17, 2013 at 6:00

When an electric current flows, electrons move from the negative towards the positive pole at a very small speed, something in the order of $0.02 mm/sec$ in a standard wire towards a light bulb. Electrons move in the opposite direction of what we call the current. When they reach the end of the wire, they actually transfer into the material of the terminal, bulb, or whatever. The ease of electron mobility is what we call conductance.

In AC circuits, the electrons actually wiggle about a little bit, depending on the frequency of the AC, following the polarity of the current.

• thanx for your reply but question remain unanswered i want to know that how electrons flow inside wire when source is ac.like wise as we know that electrons flow through surface of conductor Mar 8, 2012 at 12:21
• I dont understand your question, really. Can you rephrase the original question please? Mar 8, 2012 at 13:05
• I think he wants to know if in the AC electrons, or their movement, happen in the surface of the wire, or exactly in the middle, since in the AC the electrons "come and go" Mar 9, 2012 at 17:11

When DC is applied, free electrons in copper begin to leave the negative terminal of the battery and go into the positive terminal of the battery. They move very slow [ A reference can be found in B L Theraja, Electrical Engieering]. They don't just reach and finish there journey. The current is because of their movement in a particular direction, not because they reached their destination and they must die now.

Electrons do not die. They would simply continue its path from copper wire into the battery (which is also a low resistance conductor).

AD current is likewise. The electrons simply move back and forth. As another answer say they move very slowly so it must really be a small wiggle. But not that they are very elastic. That means if one electron moves at the start of the condutor, it move another electron at the end of the conductor. Sot he motion is totally unform. The and there is always electrons moving from copper to batter and battery to copper. In both AC and DC.

Just to expand the analogy with water; suppose to have a long pipe, with some water source at one end and a valve in the other: the pipe is full of water and when you open the valve, it starts spilling "instantaneously". You don't say that the water travelled at infinite speed through the pipe, just that it was inside it and waiting for a way to go somewhere.

The same happens with electricity: the electrons are in the wire, and when you apply the voltage, they start moving. You see almost instantly the effect because there were some "waiting" at the end of the wire, pushed by the ones near to them and so going to the source. So, even if electrons are slow, the signals propagate much faster (2/3 c is a common reference) because of this chain reaction.