# Potential - When and when not to use infinity as a reference point?

I'm reading Griffiths' Electrodynamics and having a bit of a problem to understand when we can use infinity as a reference point and when it cannot be used.

One question asked to calculate the potential a distance s from an infinitely long wire that carries a uniform line charge. According to the solution manual, infinity cannot be used as reference point because as he puts it "the charge itself extends to infinity". What does that mean? And is there a general rule for when we can use infinity as a reference point and when it cannot be used?

A previous post (Potential when charge distribution is to infinity) explained it somewhat from the point of view that the potential blows up if we use infinity as a reference point, but Griffiths draws that conclusion before he calculates the potential, and that is what I don't understand how he sees it.

• Depends upon the equations. If the evaluation of the equation at $\infty$ blows up - or becomes infinite - it becomes a physical singularity - which is a bad thing. Jul 13, 2019 at 20:55
• If you think about, e.g., an infinite line of charge in spherical coordinates, can you imagine the potential on a surface of constant radial coordinate $r$ tending to a constant as $r \rightarrow\infty$? Jul 13, 2019 at 21:29