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Name: George
Status: student
Grade: 9-12
Location: IL
Country: N/A
Date: 10/2/2005


Question:
Recently I took a tour of a hydroelectric plant (a dam) on a river in Montana. The question I had was where did the electrons come from in the wires to provide electrical power to the grid. More specifically, why did they not have to occasionally replace the wires in the generators because they would eventually run out of electrons? They indicated that they hardly ever replaced the wires (usually if the insulation were to break down - which was very seldom). I was later told that the electrons really did not flow in the conductors. They stayed in the outer orbits of their respective atoms and only "bumped" the electrons in the neighboring atoms which caused the perception of "current flow". This is why the electrons in the generator never depleted their electrons.

This answer seems to contradict everything I have to date learned about electron "flow" in a conductor, but seems to answer the question of a net loss of electrons in a conductor (exhausting its electrons). What is the correct concept to understanding electron flow in a conductor in the example above and will electrons eventually exhaust?


Replies:
George,

The electrons do flow through the wires, but they never leave the wires. This is why you need something called a complete circuit. The power plant pushes electrons through the wire. Electrons do not like to be near electrons because they have the same electric charge. The electrons that were moved forward in the wires then cause more electrons to move. This continues around the circuit. Also, electrons from behind tend to be attracted by the metal atoms that are now missing an electron. They fall in behind to replace the first electrons.

After a very short time, the electrons throughout the circuit fall into a regular pattern. Energy goes from the power plant to the moving electrons. Energy goes from the moving electrons to the devices, such as the light bulbs and televisions of the area. More energy is given to the electrons. More energy is transferred to the lights and TVs. All the loose electrons move at the same time, almost like water flowing around in a circular tube. The power plant tries to speed up the electrons. Electric devices try to slow them down. They quickly reach a balance. You never need to replace electrons because they always come back.

This is why you need two prongs on a plug for it to work. Electrons go in one and out the other. Passing through the device is what allows energy to be transferred.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College


I think you are right, George, it does contradict. Electrons do move down a wire when current flows. They just do in a very hair-brained, randomly zig-zagging fashion. The zig-zagging due to thermal motions is so much faster than the net downstream drift that if you watched one electron you would not be able to see the slow progress in all the frenetic bumping around. But it is the same with water molecules in stream, or gas molecules in a breeze, too. If you can see the molecules collectively, as a fluid, then you can see the drift.

Unfortunately we have no way to tag an electron or drop a leaf in its stream. So we cannot see the electric fluid move; we can only measure the effects of the motion: the magnetic field around the wire, and the energy transferred downstream or the charged-up voltage on a capacitor.

The electrons do not "come from" anywhere. They were always there, part of the metal. The metal is always neutral, with just as many electrons as metal-ions. Having a 1% mismatch in a solid costs more energy than vaporizing the metal completely. Electrons can leave at one end of some metal _only_ if they can come in just as fast at the other end. And vice-versa too: electrons can only enter if and as others leave. This is pretty much why electricity only flows in closed loop circuits. A loop is the only way all parts can be happy with the motion. There are no wire-ends getting robbed or crowded.

Out of almost 10^24 atoms and electrons per cubic cm, it only takes an imbalance of ~10^4 electrons/cm3 to charge the wire up another volt. (Look up capacitance of a wire, charge/voltage in a capacitor, and electrons in a coulomb.) So in 120 volts AC, the wire stays neutral to something like 1 part in 10^18. That is an extremely small mismatch. A million volts might be 1 part in 10^14 imbalanced. Still a really small mismatch.

Another thing that might prompt a power-station engineer to tell this story is A.C.: Alternating Current. Since there are roughly 10^24 electrons/cm3 in the metal, and only about 10^19 electrons in an amp-second, It takes about 1 amp x 1 day to move all the electrons through 1 cm3 of metal. In AC current this flow, though very real and quite stiff, turns itself around backwards 60 times per second (or should I call it 120?). You could have 100,000 amps in a 1cm2 wire, and at 60Hz the full oscillation distance of the "electron fluid" in the metal would be less than 1mm. In this case it is very much like oscillating water pressure in a pipe: the pressure and some work would go downstream fast, but the water never would. It might be interesting to try to visualize a machine that uses energy from alternating water pressure to do some intense work. So he is right: AC is just a game of very fast, really hard-shoving musical chairs.

But it is also true that at every moment power flows only when electrons are drifting left or right.

I would not say they stay in their respective atoms, though. That implies they never get to move even 10^-8 cm. That is hardly moving at all, and it is just not true. They are moving past many atoms very frequently, even when there is no net current but only random thermal motion.

Another caveat: In a metal, stationary atoms share a sea of mobile electrons. That is what defines a metal. The mobile electrons in a metal have diffused locations: each electron can be considered spread-out over thousands of atoms at a given time, and the sum of partial presences of many electrons is what keeps a given atom neutral. A spread-out electron generally cannot get stuck to any one atom. In an insulator it is different: each electron is concentrated in, and stuck to, one place in one molecule most of the time.

Sheesh. Sorry if that is too many perspectives at once.

Jim Swenson


wow, I see several points I can help you with there...

1) the electrons DO flow. Their actual velocity depends on the amount of current, but it's typically about 2 cm/second. However, they are also displacing the electrons from the atoms they are moving to, which causes a wave like effect. This moves at or near the speed of light.

2) A generator or alternator is relying on magnetism to push electrons around. It is not so much pumping the electrons out of the wire though, as simply forcing them all to move along it. So the electrons being pushed out one side of a piece of wire are coming back in the other side.

In the case of an alternator, the current is repeatedly changing direction. So there the electrons move briefly one direction, then right back where they came from. Since they 'bump' each other all the way down any length of wire, electrons very far away can be manipulated even though the specific electrons in the power plant never go that far.

Ryan Belscamper


The electrons do indeed flow. When the circuit is closed, an electric field is set up in the entire wire from the generating plant to the user and back again on another wire -- a closed circuit is required! A wire is a conductor precisely because some of the electrons are free to move when pushed by an electric field.

The closed circuit is required so the electrons can flow in a closed circuit; no electrons are lost!

An analogy may be helpful. Consider a water pump driving water through a closed loop of piping. The pump never runs out of water (any more than an electrical generator runs out of electrons) since the water pumped out by the pump pushes on the water already in the pipe all the way around the closed loop and so drives water back into the pump.

Best, Dick Plano, Professor of Physics emeritus, Rutgers University



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