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This question already has an answer here:

So I understand Alternating current like a battery, in which the Poles are constantly changed out, so the circuit stays completed, but the electrons change directions. But how can then in an AC plug one wire (neutral) always be 0V? The electrons always move from positive to negative right? If I´d be right the neutral wire had to be "+" 25/30 times a second, but that can't be possible if neutral alsways is at 0V.

Please explain, so an electronics noob could unterstand it :D

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marked as duplicate by John Rennie, GiorgioP, tpg2114 May 20 at 10:43

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Current always flows towards a lower potential, kind of like water always flows downhill. The only important thing here is the gradient of (i.e. difference in) potential, not the absolute potential, just like for the water only the slope of the surface matters, not its elevation.

(Indeed the absolute value of the potential is arbitrary in the sense that you can add some value $C$ to all the potentials in your system and the physics wouldn't change. In practice, we usually make a convenient choice for where the potential is $0$ (e.g. at infinity or electrical ground) and then work with that, just like we usually make a convenient choice for where the height is $0$ (e.g. at the floor or at sea level) and work with that.)

Now, ac cables are usually designed such that the potential in one wire is constant (and grounded, which we choose to be $0\,\mathrm V$) and alternating in the other one (in a $230\,\mathrm V$ power grid this will alternate between $-325\,\mathrm V$ and $+325\,\mathrm V$). In our water comparison this would be like a surface fixed with a joint at one end. You can imagine moving the other side up and down – sometimes being higher, sometimes lower than the fixed side – and water on that surface flowing from from one side to the other and back accordingly.


Just as a side note: Since electrons are charged negatively, they actually flow opposite to the current direction, from $-$ to $+$.

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  • $\begingroup$ Thank you so much!!! $\endgroup$ – Moritz May 19 at 19:40
  • $\begingroup$ A potential only exists across conductors. It doesn't make sense to talk of one conductor alternating and the other not, unless you are implicitly talking of the potential of each relative to ground, and not relative to each other. Obviously, if you ground one conductor, then that conductor will always nominally be at 0V relative to ground, but neither conductor is ever at 0V relative to the other conductor (except, at the moment during the AC cycle, when both are at 0V relative to one another). Put another way, if one conductor in a functioning AC circuit alternates, then so must the other. $\endgroup$ – Steve May 20 at 0:12

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