Question: current on transmission lines is AC and I understand AC current changes direction. But I am having trouble visualizing this. I read all kinds water analogies but it is still not clear to me. At which point does current change direction on a transmission line? Say electrons start flowing from source, when do those electrons change direction? Do they change direction before reaching destination? How do they change direction - if power station is the source and a transformer near my house is the destination, should not source become destination for direction to change? But that does not make sense as transformer does not produce current. Please help me understand this concept. As you can tell, I am pretty confused about it.

Answer: 
The ac voltage and current on transmission lines are described by wave equations which have the exponential solutions $$V=V\_0exp{[i(\omega t±kz)]}$$ and $$I=I\_0exp{[i(\omega t±kz)]}$$ where $\omega$ is the angular frequency and $k$ is the wave vector of the waves. The (complex) voltage and current amplitudes are connected by $$V\_0=Z\_0I\_0$$ where $Z\_0$ is the characteristic impedance of the transmission line. Thus the current and voltage on (one conductor of) the transmission line change phase and polarity periodically with the wave length $\lambda=\frac{2\pi}{k}$.

Note: You can see a video of the time variation of voltage and electron current on a transmission line here <https://en.wikipedia.org/wiki/Transmission_line> .
> 
> Say electrons start flowing from source, when do those electrons
>  change direction? Do they change direction before reaching
>  destination?
> 
> 
> 

On can loosely think of the mobile electrons sloshing back and forth and, on average, do not change position (otherwise, there would be a DC component to the current).

Perhaps [this visualization](http://www.sciencebuddies.org/science-fair-projects/electricity-magnetism-electromagnetism-tutorial#dcvsac) will help:

[![enter image description here](https://i.stack.imgur.com/gtw8Q.gif)](https://i.stack.imgur.com/gtw8Q.gif)

Since the drift velocity of electrons in a conductor is quite small, the displacement of an electron from it's average position can be [quite small](https://en.wikipedia.org/wiki/Drift_velocity#Numerical_example), e.g., $2\mu m$.