There's more to it than "It's on or it's off". Since you just installed an alternator, the belt was off and it can be installed and not be aligned correctly, like one groove off, which that damage looks like. That belt is a tough customer and a simple wire rubbing on it did NOT do that damage. I'm not trying to give you a hard time, I'm just want to make sure that you don't do it again. We've all made our share of mistakes. Good luck.