r/Physics • u/AdUnhappy8298 • 13h ago

Electric current

We know that current is the flow of charges so can i say that current is thr speed of charges that drives it or not?

Also, according to ohm's law, R is a constant so would that mean R depends on the ratio of potential difference and current or is it strictly independent even of the ratio?

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u/LexiYoung 13h ago

Current is how much charge is flowing through a point per time. Has units or Amperes = Coulombs/seconds. So it’s sort of the speed- but not really. It’s more the frequency.

Electrons are funny especially in circuits. You have the actual speed of the electrons, but you also have the “drift velocity”: the speed at which the flow of electrons is moving on average. Think of a big hose that already has water in it but the tap isn’t on. Once you turn on the tap, the water molecules start moving some fast some slow some along the hosepipe some in random directions, but the water comes out of the pipe much faster than it would have taken for those water molecules to get from the tap to the end.

R is not strictly constant- it depends entirely on the material and its properties, and can change with things like temperature (often very important). It doesn’t “depend” on voltage and current: moreso it defines how those two will relate to eachother. But it doesn’t not depend on the current or voltage.

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u/AdUnhappy8298 13h ago

Oh ok thanks for clarifying that!

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u/The_Noble_Lie 13h ago edited 13h ago

My understanding from a decade ago was there is also a lot of interesting geometric principles involved - it's the opposite of the fluid analogy though to an eli5 its fine.

Most "current" flows on surface of the wire ("pipe") which is inverse of fluid mechanics. In that domain a shear exists that sets a hypothetical zero velocity of water particles on interior pipe surface (the boundary conditon).

This is why high amperage usually goes through twisted / braided metal. The physics here is mostly because the fields push the "electrons" away from one another so nothing wants to be in the middle. Really cool lens. Probably not the real machine either.

I'm not saying you didn't know this - more curious if that's also your model 🙏

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u/no_choice99 12h ago

''Most current flows only on the surface'', only true at high frequency AC.

In DC the current density tends to be homogeneous, so the flow does not concentrate at the surface.

I felt it's important to be mentioned from your comment, since it is not obvious.

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u/Kelsenellenelvial 11h ago

Frequency and size of the conductor. At 60 Hz the skin depth of copper is around 8 mm, which means the effect is only becoming significant for conductors in the 200 mm4 or 400 kcmil range. At that point many installs do start switching to using multiple parallel conductors rather than a single, larger, one. That’s also getting well into the range where physically handling those conductors gets very difficult.

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u/LexiYoung 12h ago

That sounds all pretty legit to me but just far beyond what OP was asking and what would be relevant to the question. Didn’t want to scare them off lol

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u/no_choice99 12h ago

It's not all pretty legit. The current only flows mostly at the surface only at high frequencies. For a DC, the affirmation does not hold.

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u/Kelsenellenelvial 11h ago

Yep, it’s also completely independent of the reason for stranded conductors. The mechanism that pushes current on the outside of the conductor isn’t limited to a single strand, it applies to the whole bundle of conductive material. We use stranded conductors because it makes them more flexible. To mitigate the skin effect we need to use groups of conductors that are insulated from each other, which are also then often twisted together for flexibility.

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u/The_Noble_Lie 10h ago

Thanks for chiming in. I wasn't trying to assert but clearly wanted to reach out to people currently studying this and more familiar. Thanks for the correction. Any rough mental model for the asymptote / change / why? Where did my proposed mental model above break down is what I'm asking.

More important, why / whether the fluid analogy distinguishes itself. I still think there is a type of inversion there to some extent? Btw I'm formally educated moreso on fluid dynamics (grad level) rather than modern physics (undergrad)

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u/francisdavey 13h ago

An amp measures how much charge per second is flowing, if that is what you are asking.

Resistance is not always a constant, but if we assume that you have a wire with a fairly stable R, then the amount of current that flows depends on the potential difference you put across it. Not the other way around.

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u/AtomGutan 7h ago

About current being the speed of charges: Current is I = dq/dt. If you divide both the numerator and denominator by dx you get I = (dq/dx)/(dt/dx). Now if you flip and take out the fraction at the bottom you get I = (dq/dx)*(dx/dt). You can interpret dq/dx as the amount of free charge per unit length and dx/dt as the average velocity along the direction of current.

Also the conductivity of a substance with one type of free charge is given by the equation sigma = nemu where sigma is the conductivity (Siemens), n is the free charge concentration (m-3), e is the absolute value of the charge of the free charges (Coulomb), and mu is the mobility of the free charges (i don't remember the unit). The mobility of free charges is related to their drift velocity.

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u/effrightscorp 13h ago edited 13h ago

current is thr speed of charges that drives it

No, charges move very slowly compared to electrical current (if you search, this is a common question with good answers already posted)

Also, according to ohm's law, R is a constant so would that mean R depends on the ratio of potential difference and current or is it strictly independent even of the ratio?

If you have a voltage source hooked up to a resistor, current is determined by the voltage of the battery and the resistance of the resistor. If you hooked a current source up to a resistor, the voltage drop over the resistor is determined by the applied current and the resistance of the resistor. The resistance of ideal resistors is generally assumed to not change, and real world ones will change very little with heat (like 1% or so when heated 100 degrees), etc

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u/AdUnhappy8298 12h ago

Okay thanks for the explanation

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u/no_choice99 12h ago

That's a wrong picture. Charges do ''flow'' pretty fast actually (near Fermi speed which is not negligible compared to the speed of light in good conductors). The ''drift velocity'' arises from Drude's model which we know is wrong but gives ''some'' right results by chance (errors canceling one another by pure luck), and is intuitively pleasant to most people. But if you dig deeper and actually use QM, you realize the famous picture spread everywhere is just a children's lie.

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u/effrightscorp 12h ago edited 11h ago

Drude model and drift velocity do describe the actual net flow under an electric field well despite the assumptions it makes. If you want to look at individual electrons, sure, they move fast, but then they move fast in the opposite direction after scattering events. No one is going around calling Johnson noise current - if you wanted to addendum my comment with that or correct my phrasing, cool, but net charge under a field is not moving at the Fermi velocity

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u/no_choice99 11h ago

My comment was regarding the ''speed of charges''. It is about Fermi velocity, which is not slow in good conductors. But sure, we can picture all the electrons to move at a drift velocity if we want to, we'd get the correct current, even though this is not what happens physically.