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I have been looking in to alternating current and I am confused. If the voltage reverses doesn't the flow of electrons also reverse? I am aware of another fair answer on this site here. That answer, however, confused me just a bit. I gathered that the electrons do not per se "flow." Is this true? How is the electrical current moving?

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  • $\begingroup$ Yeah you have it right. When the voltage reverses so does the flow of electrons. An approximate analogy would be like rowing a boat. The paddles go forward, then backwards, over and over. Rowing only uses the paddles when they go in one direction but don't focus on that, there are tricks with electricity that usually let you use it both directions. $\endgroup$ – Brandon Enright Jul 1 '14 at 2:35
  • $\begingroup$ possible duplicate of In an alternating current, do electrons flow from the source to the device? $\endgroup$ – Brandon Enright Jul 1 '14 at 2:35
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There is a commonly used analogy for electric circuits called the hydraulic analogy. This imagines the electrons as water and the wires as pipes. The voltage is equivalent to the water pressure and the current is equivalent to the water flow rate.

Start with a DC current and imagine the water is doing work by flowing through a water wheel:

DC

This is all very straightforward: the water/current flows steadily through pipe/wire and turns the waterwheel/electric motor, which can then do work.

But now suppose we pump the water from left to right, then reverse the pump and pump the water from right to left.

AC

This is exactly what an alternating voltage does. The water just moves to and fro within the pipe so there is no net flow. However the water can still move the waterwheel, but it moves it to and fro rather than at a steady rotation speed. We can still get the waterwheel to do work - if you're using it to grind corn it doesn't matter whether the wheel is rotating steadily or moving to and fro. The corn still gets ground.

Likewise, with an alternating voltage the electrons oscillate to and fro in the wire so there is no net flow of electrons. But the electrons can still do work e.g. heat a light bulb or run a (suitably wired) electric motor.

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If the voltage reverses doesn't the flow of electrons reverse?

It depends. If the alternating voltage is across a diode then, no, the current through the diode doesn't (effectively) reverse but is instead unidirectional.

However, a genuine alternating current periodically reverses direction - the electric charge 'sloshes' back and forth within the conductors and other circuit elements.

Electric current is the flow of charge so, technically, the electric current doesn't move (or flow), electric charge does. If the mobile charge carriers are electrons, then there is an associated alternating flow of electrons.

There's nothing particularly odd about a flow of something that periodically reverses direction.

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At a beach the waves carry energy and momentum from the sea to the shore, even though the water in the waves moves back and forth. It is the same way with alternating current: what matters is the energy flow carried by the electric and magnetic fields, not the movements of the charges.

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  • $\begingroup$ Doesn't the water in a wave move vertically up and down, as it transports energy horizontally? $\endgroup$ – DJohnM Jul 1 '14 at 4:05
  • $\begingroup$ Water in an ocean wave moves in a circle: up and down and forward and back. Energy in an electromagnetic field always moves perpendicular to both the electric and magnetic fields, along the Poynting vector $\vec S = \vec E \times \vec B$. $\endgroup$ – rob Jul 1 '14 at 14:43

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