# Does current decrease inside a wire or does it increase it's velocity?

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• ananunes06
ananunes06
I've been really confused on how electricity and circuits work
1. If eletrons "slow down" inside a resistor (or wire, I'm considering it's resistance to be ≠ 0) because of collisions with the lattice, then wouldn't the electrons pile up? The charges accumulating doesn't seem to be good.
2. Some people say that the velocity increases inside a resistor, so that the current remains the same along the circuit, just like "the narrowed section forces the fluid to hurry along". But if the current remains the same, then how could there be a decrease in voltage, once V = R.I?

That’s just not how electricity works. Where are you getting this from?

In this video from electroboom he says that the electrons velocity increases when passing through resistance

ananunes06 said:
.... if the current remains the same, then how could there be a decrease in voltage, once V = R.I?
I am not sure how to answer that. The drop in voltage across a resistor does not imply that current is also dropping across the resistor. They are two different things. But yes, the amount of charge passing any point along a circuit has to be the same everywhere otherwise it would pile up, which clearly it can't.

russ_watters said:
I am not sure how to answer that. The drop in voltage across a resistor does not imply that current is also dropping across the resistor. They are two different things. But yes, the amount of charge passing any point along a circuit has to be the same everywhere otherwise it would pile up, which clearly it can't.
It might not be what actually happens, but now I'm think of the voltage drop more like a hill; it must have a difference in high between two points, what doesn't mean that a few balls rolling down this hill would slow down or anything like that. Is that "right"?

ananunes06 said:
It might not be what actually happens, but now I'm think of the voltage drop more like a hill; it must have a difference in high between two points, what doesn't mean that a few balls rolling down this hill would slow down or anything like that. Is that "right"?
I'm not sure that analogy works very well. While imperfect, the water analogy used in the video is better in my opinion.

If you consider resistors and connecting wires to have the same cross section then, since the current is the same in both you must deduct that the drift velocity has to be the same.
But yes, there is a kind of pile up on one side of the resistor, together with a dilution on the other side. It is this tiny, miniscule difference in charge density at the interface between materials of different resistivity that create the difference in electric field: small, near zero electric field inside the copper wires and strong electric field inside the resistive materials. In accordance with the local form of Ohm's law: $$\vec{j} = \sigma \vec{E}$$

If you consider resistors made of the same material as the wire, then you can increase the resistance by reducing the cross section. This will increase the current density for the same current. In this case it's the surface charge that is responsible for the steering of the electric field lines (and the corresponding charge flow lines) inside the resistor.

PS
I wouldn't put much trust into Electroboom videos. Try FloatHeadPhysics videos instead: he's a guy who gets the fundamentals right. A rarity on YouTube. And IIRC he has a video that answer your very question.

Last edited:
Nik_2213 and Dale
Don't forget that AC may behave very differently, with higher frequencies increasingly restricted to the surface. Hence multi-strand 'Litz Wire', used up to a few Mega-Hertz...
https://en.wikipedia.org/wiki/Litz_wire
This also discusses 'skin effect' depths...

As I understand it, beyond 'Litz Wire', you must increasingly treat any link as a 'Transmission Circuit', respecting impedances, reflections etc etc. In extremis, evert the wire and use wave-guides...

ananunes06 said:
It might not be what actually happens, but now I'm think of the voltage drop more like a hill; it must have a difference in high between two points, what doesn't mean that a few balls rolling down this hill would slow down or anything like that. Is that "right"?
Only incline (hill) is not enough. Friction must be included too. Choose the angle of friction for the angle of slope and you will get the constant velocity.

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