DC Current: Why does copper wire not physically change while conducting current?

Thread Starter

Dolmetscher007

Joined Mar 21, 2019
17
I can learn all the memorizable theory and equations there are, but I have always struggled to understand the actual physics behind conductivity.

Copper atoms have electrons on their outer-most shell that are free to move from one atom to another with little resistance (free electrons). Once one of these atoms becomes delocalized (i.e. pop's off), that Copper atom is now a positively charged ion called a "cation." These delocalized copper electrons are referred to as a "sea of delocalized electrons."

So... if you have a piece of copper wire that is 6 inches long, for example, and you connect it to a power source of any kind, and begin to run a current through that wire... the sea of delocalized electrons that begins to flow is made up of electrons—electrons are sub-atomic particles—sub-atomic particles have mass—as mass moves it physically changes from one place to another. Why does our 6-inch piece of wire not change at all? As this "sea of electrons" reaches the end of the wire, why do they not collect or bottleneck, or... in some way change the shape of the end of the wire?

I think maybe my mental picture is just false from the very beginning. If you connect the 6-inch piece of wire to the positive terminal of a 9V battery, no current is flowing. Correct? You could take a piece of copper wire, and connect it to JUST the positive terminal of a 600V power supply, and stick the other end in your mouth, and you will be completely fine. However, when you connect the copper wire to the negative terminal, that is when the current will flow, correct? In the case of 9V battery, the wire doesn't physically change, because it is simply acting like a bucket-brigade, taking electrons that are coming from the battery, passing those electrons along it chain of copper atoms that allow these electrons to pass through, and returning them to the other terminal of the battery, so that the chemical reaction in the battery can re-energize the electrons and send them around the loop again. i.e.... a circuit! So the wire ends up with the same number of copper electrons that it had before the current was sent through it.

I guess that answers my question huh? Copper wires do not physically change after a current has flowed through them, because current can only flow when a circuit is closed into a loop.
 

crutschow

Joined Mar 14, 2008
23,826
Copper wires do not physically change after a current has flowed through them, because current can only flow when a circuit is closed into a loop.
Basically, yes.
You can add a charge to a wire, which will displace a few electrons from the wire based upon the wire capacitance to ground, but that likely has no effect on the effective size of the atoms and thus the size of the wire.
Certainly any change would likely be far to small to measure.
 

wayneh

Joined Sep 9, 2010
16,129
Your kitchen faucet doesn't change every time you open or close the valve. The water coming out the end is replaced entirely by water entering at the valve. There is no bottlenecking, there is just flow.

This water analogy fails to capture the essential nature of a circuit, that it's a closed loop. Water flow from a faucet is more one-way. I suppose you could think of water leaving the faucet to be analogous to electrons hitting ground potential.
 

spinnaker

Joined Oct 29, 2009
7,837
It does physically change. You just can't see or feel it, until you turn up the current.
OK then that brings up another question. When you increase the current over the current rating of the wire, it will start to heat. If the wire gets hot enough the wire will eventually melt and break. But what is happening at the atomic level. What changed that caused the wire to break?
 

SamR

Joined Mar 19, 2019
1,528
Electrons have 0 mass and a negative charge. Atomic mass is the number of protons (positively charged) and neutrons (no charge) of an atom with a relative mass value of 1 assigned to them. There are an equal number of charged particles (electrons and protons) in an atom. Electrons are energy matter and treated as particles (particle theory) or waves (wave theory). At 0° Kelvin (absolute zero) electrons have 0 energy. As we add energy (measured as thermal heat) electrons become energized and start to move. As the energy increases the outer orbital (least attracted) electrons break free from their orbits around the positively charged nucleus of the atom and start to move from an area of greater concentration to an area of lesser concentration (both in density and electrical charge). They move in a straight line (unless acted on by magnetic forces) until they strike another object and that collision releases energy. So as we increase the flow of electrons (moving from an area of greater density to lesser density - to +) there are more collisions releasing more energy measured as heat. Generate enough electron flow (current) and the collisions increase and heat increases and that copper wire starts to glow "red hot" and will melt.
 

SamR

Joined Mar 19, 2019
1,528
OK then that brings up another question. When you increase the current over the current rating of the wire, it will start to heat. If the wire gets hot enough the wire will eventually melt and break. But what is happening at the atomic level. What changed that caused the wire to break?
There was enough energy induced to overcome the attraction/bonds between the atoms.
 

dl324

Joined Mar 30, 2015
9,351
Copper wires do not physically change after a current has flowed through them, because current can only flow when a circuit is closed into a loop.
The wire can change.

When I worked on microprocessor projects using copper metalization, we worried about electromigration when the current was primarily in one direction and self heat when it was bi-directional. Over time, wires (or vias which were also copper) could develop opens. When we used aluminum wires, our parts were guaranteed for 7 years. For copper, that dropped to 3 years; but we ran simulations at much higher frequencies than the parts would normally operate at.
 

SamR

Joined Mar 19, 2019
1,528
Basically, yes.
You can add a charge to a wire, which will displace a few electrons from the wire based upon the wire capacitance to ground, but that likely has no effect on the effective size of the atoms and thus the size of the wire.
Certainly any change would likely be far to small to measure.

Actually it does have an effect. As energy is added to the system the atoms "vibrate" faster and move farther apart. As heat (measurement of energy) increases the atoms move apart and the object increases in size. ie metal expands when hot. Which is why there are sag calculations for overhead power lines for example and the resistance also changes w/ temperature. It may be a "small" value, but it's there.
 
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BR-549

Joined Sep 22, 2013
4,938
Heat from resistance causes a phase change in the wire. And if I remember right, wire is current rated by the change in temperature. Maybe that's changed now.
 

SamR

Joined Mar 19, 2019
1,528
Current increases (electron speed/flow) as temperature increases because as the atoms move apart the amount of "empty space" increases causing less resistance (collisions) to electron flow. Electrons (like me) don't move as fast when it's cold.
 
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crutschow

Joined Mar 14, 2008
23,826
Of course if copper heats up, it will expand.
But I don't think the slow movement of electrons below the point of any significant heating, or the addition/subtraction of a few electrons from electrostatic charge has any physical effect on the wire.
 

SamR

Joined Mar 19, 2019
1,528
Electrons can move at nearly the speed of light (depending on resistance) so they are a mite quick. There is always a physical effect, but it can be so small as to be considered almost nothing (which it is at 0° K). Which reminds me of Hans Camenzind's (inventor of the 555 timer) book "Much Ado About Almost Nothing (Man's Encounter with the Electron)". A good read I wholeheartedly recommend.
 

SamR

Joined Mar 19, 2019
1,528
Yes, but their drift velocity in a wire is very slow (typically less than a mm/second).
So If I flip the switch in the house for the lights in the remote garage 30 meters from the switch in the house it takes 3,000 seconds to reach the bulb and another 3,000 seconds to return to ground before the light comes on?
 
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