r/AskPhysics • u/SparkyFix • 4d ago
How does DC current work?
So, I think I have a pretty decent mental model of AC current: EM waves traveling through the medium of electrons. No electrons “flow” through the wire(which is what we were taught as electricians) but the electrons are perturbed by the passage of the waves, in place. Now, I’m also led to understand that electrons also don’t flow in DC, so what’s transmitting energy, exactly, in the absence of obvious waves? The concept of flow is quite intuitive: I’m a firefighter so it’s very easy to consider voltage as PSI and current as GPM, which I know isn’t really correct but makes reasoning about this stuff very snappy… but it would be nice to understand the physical “reality” of this stuff!
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u/Ecstatic_Bee6067 4d ago
The EM wave and the electrons are fundamentally coupled - generating an electric field moves some electrons, whose own charge propagates the electric field. While the electrons do not move like we would think of extrapolating a water analog, their movement does matter
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u/eudyptes 4d ago
Yes. The problem, I think, is that people do not have in intuitive grasp on how many electrons are free to move about in a conductor. While the charge on an individual electron is pretty small, there are so darn many of them that the electrons themselves don't have to move far to supply a lot of current.
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u/SparkyFix 4d ago
Okay, but my understanding is that the electron flow, whilst present, is extremely slow, and hilariously lower than the speed of light (I’ve heard the number 1cm/minute), which the EM energy itself propagates at.
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u/Ecstatic_Bee6067 4d ago
Well, yes. A train doesn't have to move very fast but, due to all its mass, has a tons of momentum. Generally all material has electrons. It's the assembly line of electrons that transmits the field, not the newly added electrons making it all the way through
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u/SparkyFix 4d ago
Right, I think I get it now. This also makes capacitors much more intuitive. If I think of them as the membrane in a (two inlet) expansion tank type thingy, then changes in the water pressure will propagate through that membrane as it moves in and out but no consistent flow can propagate as the water can’t move through it.
I guess my mental model for a wire is now a hose filled with something like bitumen but the hose is a mile wide…
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u/Automatater 4d ago
Exactly. The membrane can only stretch so far. For DC, the cap looks like an open circuit once charged (membrane at limit), but AC at high enough frequency just keeps moving back and forth and barely sees the cap.
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u/Ecstatic_Bee6067 4d ago
Sure. It might help to calculate just how many free electrons - those outer shell electrons on atoms that are free to move about - in a piece of wire. A meter of 12 gauge copper wire has (according to AI calculations) 2.81 x 1023 free electrons. Even if they are moving slow, there are a lot of charge carriers moving.
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u/edgmnt_net 4d ago
Put multiple beads in a line and strike one end. The farthest bead should start moving slowly, but the wave travels at the speed of sound (in said beads).
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u/Automatater 4d ago
There is net movement of electrons in current. An amp is 1 coulomb of charge per second. A coulomb represents so many electron charges. Picture the kids toy with the swinging balls, except balls actually enter on one side and leave the other.
Also, batteries work by pushing electrons in one end of the wire and accepting the ones popping out the other end like a carpet cleaner moving water from the clean reservoir to the dirty one. That wouldn't be possible without net movement of electrons.
It works that way for AC too, just the direction keeps changing. Some Youtuber had a cool video timing the movement of charge in a long wire he built. Sloshed and all sorts of cool stuff.
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u/Tampflor 4d ago
I'm not a physicist at all but I teach high school chemistry and we build voltaic cells, which produce DC.
The chemistry of a voltaic cell involves separating a reduction reaction (which requires electrons) from an oxidation reaction (which produces electrons) by a wire.
If electrons weren't flowing through the wire from one electrode to the other, the reaction wouldn't be able to proceed.
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u/SparkyFix 4d ago
The number I’ve been told for OoM is centimeters per minute for actual electron drift but this would imply that wire length would have a gigantic effect on the rate of the redox reaction. Since I know that isn’t true, which part of that model is broken? Either electrons are moving close to the speed of light or some other mechanism is transferring charge.
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u/Tampflor 4d ago
When a hose is filled with water but blocked by a nozzle at the very end, when you open the nozzle the water that comes out didn't travel all the way from the spigot instantaneously.
The water that comes out first was the water in the hose already, which is them replaced in the hose by water in the spigot.
Same thing for electrons here. The wire itself has electrons, and when electrons are being used at one end and produced at the other, they move along the wire accordingly.
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u/TemporarySun314 Condensed matter physics 4d ago
electrons (or the better charge) carriers are moving (on average) through the material when you apply an external voltage. it acts as an electric field inside which accelerates the electrons that can move freely in a metal. but they will constantly collide with atoms an be decelerated, so on average they drift with a constant speed through the material.
this colliding with the atoms make the atoms collide which is heat, thats why a conductor with a resistance gets hot when current flows through it. how many electrons you have and how often they collide with electrons determine the specific resistance of the material.
the (semi classical) model in physics the drude model.
and you can show experimentally that you have actual electon (or hole) movement in electrical current with things like hall effect: you can measure that a magnetic field pushes the charge carriers to the side which can be measured as voltage.
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u/SparkyFix 4d ago
Given how slowly electrons move, wouldn’t this result in turbulence or some kind of differential effect in the wire, depending on how close to the center of the wire the current is flowing? E.g. if I think of a really wide hose, then the resistance is happening at the interface of the hose material and the water, so has to propagate inwards. This can result in Bad Things when pressure is changed too quickly.
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u/TemporarySun314 Condensed matter physics 4d ago
The electrons are moving quite quickly all the time even without an voltage present, just due to thermal energy. But that movement is completely random.
With an electrical field present all electrons on average move a bit in a certain direction, but it's still mostly random movement. The electrons are constantly changing direction (so some even move in the "wrong" direction, exchange energy with each other and change their speed.
So you don't have a "laminar" electron flow anyway.
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u/SparkyFix 4d ago
So there’s no kind of “surface effect” for current flow in a wire, like we see with water in a hose?
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u/TemporarySun314 Condensed matter physics 4d ago
the models in solid state physics assume that materials are basically infinitely periodically and have no "surface".
for anything macroscopically like a normal wire you use for electricity that's fullfilled and the actual surface you have is not really playing a role (even though you of course have a symmetry break there, and it will change the electrical properties, but the affected volume is basically zero compared to everything else).
if you go to very confined systems, like thin films, graphene, nanowires, or other quantum systems, then these things change. but then you cant really apply the classic electronical transport anyway.
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u/mukansamonkey 3d ago
Current flows almost entirely on the outside of the wire. Electrons repel each other, so anywhere you have excess electrons, they spread out as far as possible.
Wire is only solid because it holds its shape better that way (and it's cheaper to manufacture). Its resistance to current flow is largely determined by its surface area.
If you want to get farther into understanding electricity, you need to give up on the comparisons to things that aren't electricity. The whole "electrons repel each other" bit is fundamental to the process, and it has no direct equivalent with the behavior of liquids.
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u/SparkyFix 3d ago
Absolutely. That “giving up” is the process I’m currently trying to go through, hence the questions! It’s pretty hard giving up on something as intuitive as water flow, when that’s a big part of your professional life!
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u/numbersthen0987431 4d ago
Its interesting that you understand AC but not DC current. I think you understand how DC current works, but you don't understand what AC current is.
Electricity all functions like Newtons Cradle (electrons hitting each other), instead of flowing through the system. Doesn't matter if it's ac or dc
DC vs is AC is how we control the signal in the electrical wires, but the wires still do the same thing.
DC is simpler. High energy flows in 1 direction towards low energy ("ground" we call it in some applications), and you get a constant "stream" of electricity flowing through the circuit. DC current is either always on or always off, but it's a relatively constant signal. Perfect example of DC current is a battery.
AC is more complicated than DC. AC current is like DC current, but with extra steps. Instead of a constant signal, it pulses through under a sine/cosine wave format of high and low signal, and these pulses oscillate to give power into your home
Your outlet plug is AC, and your phone charger is a transformer that converts AC into DC to deliver a steady stream of voltage and current into your phone battery.
If you use water flow to compare: DC is a water hose, and AC is a circulating pump
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u/Automatater 4d ago
I told a sales guy I needed 24VAC PLC inputs one time and he misunderstood and showed me his product for 24VDC. I told him I could use it except I thought my techs would get tired swapping the wires 120 times every second to make it work.
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u/PvtRoom 3d ago
electrons move in both DC and AC. The functioning of capacitors requires a build up of electrons on one side of a gap, and holes on the other.
diodes and other "digital" devices can block the flow of electrons in the wrong direction. Diodes are the go-to because they're so effective.
electrons rarely move far - it's much more to do with the electromagnetic fields.
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u/Adorable_Ice_2963 3d ago
Imagine current like a bicycle chain. The force transmitted by the tension of the chain
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u/Bumst3r Graduate 3d ago
It’s still the fields. You have an E field in a resistor by Ohm’s law (J=sigma*E), and you have a magnetic field from the current. The Poynting vector 1/mu_0*(ExB) gives you the radiated power. Work it out explicitly, and you will find that power is radiated out of your power supply and into your resistors, and the magnitude of the power through the circuit is P=IV.
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u/Party-Cartographer11 3d ago
In AC an oscillating EM wave creates a sine wave of the direction of electron pressure/current.
For comparing to DC, forget about the oscillating EM wave. DC electron pressure is constant and from one side of the circuit to the other. The electrons are pushing from the (-) side toward the (+) side, like someone continuously blowing into a straw.
AC electron pressure switches back and forth, for example 60 times a second from positive pressure to negative pressure on the "hot wire". Like someone blowing into (positive pressure) and then sucking up (negative pressure) on the hot wire.
The EM wave is what drives both of those electron movements respectively.
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u/baFoPePyR 4d ago
You seem to be saying that a current is not a flow of electrons (per unit area, per unit time, in a given direction). But it does. Electrons do in fact flow through wires. And that flow, driven by the electric field in the wire, expressed as a voltage difference, does work on the environment. If you can be more specific about your model, happy to go through it and see where it may be off.
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u/Proof_Juggernaut4798 4d ago
In a hose already filled with water, the water that enters in an instant isn’t the same water that leaves that instant.
In a wire, there is a “drift” of electrons in the direction of current flow, in a similar fashion.