How do we pick where to set electric potential equal to zero?

[jerro]

Sometimes we set potential to equal zero at infinity, other times at the origin. What's the general rule of thumb here-how do I know which to choose when doing a problem?

 

[aftershock]

You generally pick a point where the potential is zero. Infinity usually works, but the potential may not go to zero at infinity if the charged object is infinitely large. Think how the electric field does not diminish as you travel away from an infinite charged plane.

 

[Philip Wood]

You generally pick a point where the potential is zero.

With respect, the only reason for potential being zero at any point is because you've chosen it to be so. Specifically, you choose the arbitrary constant when you integrate the field strength due to the charge configuration in such a way as to make the potential zero at a point of your choice. Choosing that point to be a very way from your charge configuration ('at infinity') makes formulae for potential easier than otherwise.

The example of the infinite plate is interesting. A point 'at infinity' has no meaning in this case. Might as well call the plate's potential zero. But note that there are no infinite plates in practice!