What really is a sinusoidal wave current (AC)?

[patjk]

What really is a sinusoidal wave current? What exactly is happening to the electrons that is different from a DC? Are there any readings you suggest that will give me more insight into this?

Thanks in advance.

 

[mikeph]

They move back and forth rather than in a single direction, the motion is entirely different.

 

[patjk]

If you have a wire, and we are looking at it like this with current flowing to the right:
---------a>--------------------b>----------------------------->c

Are the electrons moving through the wire by going from a to b, b to a, a to c, etc. as they move along the wire? If so, why exactly are they doing this? If not, what motion exactly are they moving in and why?

 

[mikeph]

No, they go a to b to a to b... there is no net direction of electron travel for AC current. This is because the potential difference is alternated, so the force on the electrons is always oscillating back and forth.

 

[patjk]

Ah. Assuming a is the beginning of a wire and b is the end of the wire, do all the electrons alternate between a and b, or just some?

And for DC, the current continues just from a to b, and all the electrons move from a to b, correct?

 

[mikeph]

For DC that is correct.

For the AC, you need to consider that in most circuits, the drift speed (actual mean velocity of electrons) is extremely slow, perhaps of order of millimetres a second. To put that in context, the alternation of an AC circuit is usually 50Hz, so 50 times a second. So the electrons on average in an AC circuit oscillate over length scales which are usually significantly shorter than the length of the overall circuit lengths. It depends on a number of factors which you haven't given, but in a circuit in your home it is easy to say that the electrons oscillating close to a wall switch will never get anywhere near the light bulb.

At this stage I might pre-empt your next question... if the electrons travel so slowly, why is electricity so efficient for transferring energy? The signal travels extremely fast, but the actual signal carriers, the electrons, are slow moving.

 

[patjk]

For DC that is correct.

For the AC, you need to consider that in most circuits, the drift speed (actual mean velocity of electrons) is extremely slow, perhaps of order of millimetres a second. To put that in context, the alternation of an AC circuit is usually 50Hz, so 50 times a second. So the electrons on average in an AC circuit oscillate over length scales which are usually significantly shorter than the length of the overall circuit lengths. It depends on a number of factors which you haven't given, but in a circuit in your home it is easy to say that the electrons oscillating close to a wall switch will never get anywhere near the light bulb.

At this stage I might pre-empt your next question... if the electrons travel so slowly, why is electricity so efficient for transferring energy? The signal travels extremely fast, but the actual signal carriers, the electrons, are slow moving.

Thanks for the detailed explanation. That certainly helps answer my question.

For the bold text, what exactly is the signal that is traveling fast? Are they waves of some sort?