"Common knowledge" -- you don't need to cite where you found "common knowledge". But what is "common knowledge"? Guess what -- it varies. Here is a safe definition, because you don't want to be accused of plagiarism.
Did you know it without looking it up? If you are just learning about your field, you should err on the side of caution. Better to cite something that didn't need a citation, then to be accused of plagiarism.
Would other people in the class know this? Common knowledge is based, to some extent, on what the "general practitioner" in the field would know. And what would be general knowledge for a practicing electrical engineer MAY NOT be common knowledge for a freshman engineering student.
Do you find the information in many PUBLISHED (not web pages) resources? Some experts say if you find it in 3 reference tools, no need to cite. Others say 5. If you don't have time to check that the information has been used in many different places, cite it. Play it safe. The citation can be easily removed later and until the paper is in it's final version, you will know where to go back and verify the information.
This issue, at the University of Arkansas, is addressed as reusing your work from one class in another class. Doing so without both faculty members approval is considered a level one violation of the academic honesty policy.
Most publishers do not allow redundancy or self-plagiarism. The statements to authors specifically prohibit the reuse of previously published works.
"Most current discussions of plagiarism fail to distinguish between:
Such discussions confuse plagiarism with the misuse of sources." For the student: it is better to be marked down for inadequate citations than to be accused of cheating. Citing incorrectly is better than not citing at all.