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Name: Andy
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Question:
I have a question in regards to electrical current flow. I understand that electrical current is the flow of electrons from a negative source to a positive source. (With the net current flow going from positive to negative.)

But what happens to the electron after it reaches its destination? For example: do the electron that came from the power plant travel back?



Replies:
Dear Andy,

Yes electrons flow from the negative terminal of a battery or generator to the positive terminal because there is an electric field in the wire which exerts a force on them in that direction. Incidentally, since the charge on an electron is quite large and there are so many of them in a conductor, the average speed of an electron (called the drift speed) is quite small. For example, in a 12 gauge copper wire carrying 10 amperes (typical for residential wiring), the drift speed is about 0.0002 m/s. This is certainly tiny, especially when compared to the speed of light (186,000 miles/sec). Electrons typically move at speeds which are a reasonable fraction of the speed of light.

Once the electron gets to the positive terminal, the chemical energy of the battery or the electrical energy of the generator moves the electron AGAINST the force exerted on it by the electric field to the negative terminal where the electric field again pushes it through the wire to the positive terminal.

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


Yes, they travel back, and go all the way around repeatedly. But the time for one electron to make this journey could be longer than you think. What moves fast, when you flip a switch, is the increase of pressure, and the start of the flow of energy.

It is a circuit, a loop, in which the electron goes round and round. Do not forget that whatever voltage source you have is part of the closed circuit. Only difference is, this part pushes the flow as it passes through.

Imagine a closed water-pipe system with a pump somewhere in the line, and a big nearly-clogged filter somewhere else. The pump is the power source, and the filter is the load. Flow is analogous to current, and pressure is analogous to voltage. The power is injected when the pump pushes the water "forwards" to go around the loop again. And the energy gets transferred, it is turned into heat, by the flow-resistance of the filter.

What pushes electrons along might be
- the changing magnetic field in a transformer or generator.
- chemical reactions in a battery or fuel cell. (The current flows through the battery as ions in solution, not electrons in wire.
This makes it hard to plot a full cycle for any individual electron.
You might say it cannot go around twice in a battery, at least until the battery is taken out of service and recharged.)
- getting bumped to a higher energy level by a photon, while in the solid crystal in a solar cell.

Calculating the loop transit time: Suppose you have a toy circuit. Say a 1.5v battery making 1 Amp, through medium-thick wires of 1mm2 cross-section area, 10 cm long going to the light-bulb, and equally 10cm coming back. With the battery and bulb, suppose whole loop-distance can be considered 25cm.

Since by definition an Ampere is a Coulomb/Second, the number of electrons passing each point in the circuit is
= (1.0 coulomb/second) / (1.6 x 10^(-19) Coulomb/electron)
= 6 x 10^(18) electrons/second
To find the sped in cm/sec, all that progress is split among the electrons in one cm of wire. There's about one electron per copper atom, and an atom in a solid is roughly an Angstrom, [10^(-8) cm] wide, giving 10^24 atoms/cm3.

Actually it is always a bit less.
For copper:
Cu: (63gm/mole) / (6x10^23 atoms/mole) = 10^(-22)gm/atom ;
(9gm/cm3) / 10^(-22)gm/atom = 9 x 10^22 atoms/cm3 ;
Assuming they are stacked cubically (not quite true),
cube_root(9 x 10^22 atoms/cm3) = 4.5 x 10^7 atoms/cm;
1 atom = 1/4.5e7 = 2.2e-8 cm = 2.2 Angstrom.
Anyway, I have found I should use ~10^23 Copper-atoms/cm3,
and the same number of electrons too.

1 cm of wire:
1 cm x (1mm2) = 1cm x (0.01cm2) = 0.01 cm3
10^23 electrons/cm3 * 0.01 cm3 / cm-wire
= 10^21 electrons / cm-wire

Finally the speed:
(6 x 10^18 electrons/sec) / (10^21 electrons/cm)
= 6 x 10^(-3) cm/sec
= 0.006 cm/sec

If your loop is 25cm long,
LoopTime
= 25 cm / 0.006cm/sec
= 4000 seconds
= 1 hour + xx minutes

And that is just a little circuit.
for a power plant miles away, it could take weeks.

Jim Swenson


Hi Andy

Actually, what flows in a circuit is charge. The electrons are there to provide the charge, but the electrons themselves do not do much moving. In a 12 volt DC circuit, electrons move at a rate of about 0.3 meters per hour. The reason a bulb lights as soon as the switch is thrown is because the charge in the wires is already there. Think of a wire as a plastic tube just large enough to pass a ping pong ball. Fill the tube completely with ping pong balls (that is the charge). Now, push another ball in the tube. Immediately a ball will pop out the other end. You just caused a flow of charge! For a direct current circuit, such as that with a light bulb and battery, the charge will flow from the battery through a wire, through the bulb, back to the battery through another wire, through the battery and then back again through a wire to the bulb. The battery acts as a pump that moves the charge. In an alternating current circuit like the ones that run household appliances, the charge shuttles back and forth (that is why it is called alternating current) without any total displacement. Because charge still flows (first one way then the other), useful work can still be done. Hope this helps.

Bob Froehlich


If power plants produced DC voltages, the electrons would have to make a complete circuit -- through the power plant, through ground, to your local ground, which most likely is a water pipe.

But power plants produce AC voltages, so the electrons just move back and forth along the wire. In fact they move less than a millimeter during each power cycle, and end up roughly where they started.

-- Tim Mooney


All electrons are alike. Current flow consists of electrons traveling along a wire of atoms, but any one electron does not travel all that far before it recombines, and goes back to being just a part of that particular atom's cloud. But another one picks up, and keeps the current moving.

Steve Ross



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