3. Professors of physics classes, the longer they've known you the better.
Professors of physics classes are much better to get a letter from than "pretty much anyone." Here's the deal, you can get a letter from anyone you want. You could get one from your grandmother, and it is allowed. The impact of your letters on your admissions chances, however, is going to depend on
a) how well-suited your letter writers are to evaluate your research and academic potential in graduate school (how long they've known you and in what capacity) and
b) the letter they write you.
Getting a letter from a prof that's only known you one semester will lack strength in a, but if they'll still write you a good letter and if that's the best you've got, it's better than nothing.