Whom do I have to thank in the Acknowledgements section of a Bachelor thesis I will soon finish writing my Bachlor thesis in physics and I don't know whom I "have to" put in the acknowledgements.

There are two professors who will grade the thesis, one the head of the work group I'm writing the thesis at and the other one from another work group. I didn't have much personal contact with any of the two other than getting their signatures for some paperwork and talking about half an hour about the subject. They appear on the thesis as advisors.

The actual advisor is a PhD student from the work group; a practice fairly common, at least at the physics department. He has been really involved and helped me a lot.

So I was thinking to thank the head of the work group for the possibiliy to write the thesis there and one or two sentences for the actual advisor.

But most of my fellow students seem to mention a lot more people like the second  advisor (Professor), friends, boy-, girlfriend, family...

My problem with that is that the second advisor is not involved at all in the process (he works in a related field, but the workgroups don't really collaborate), but I am concerned that it might be seen as an insult. I can see why one thanks close friends and family in a PhD thesis where one works on for years but I think it is a little melodramatic in a Bachelor thesis (which takes four months, not full time). But I'm afraid this might come off as ungrateful or heartless.

Is it expected or recommended to put second advisor, friends and family in the acknowledgements?
The absolutely crucial thing is that you clearly acknowledge and delineate any work, original ideas, results, images, figures, tables, charts, quotations, prose, etc., that were contributed by someone other than the primary author (i.e., you).  Failure to do so could taint your academic reputation, and in extreme cases, might be construed as plagiarism (or some other form of academic misconduct).

Beyond that somber admonition, the remainder of your acknowledgements are a pleasantry which can be doled out entirely as you like.  I agree that extensive acknowledgements in a bachelor's thesis are a bit misplaced.  On the other hand, it's a good opportunity to think about the people who actually made it possible to complete your BS research—e.g., lab techs who helped run and maintain equipment, or administrators in the department who made sure all the paperwork went smoothly.  These people are extremely important and are often overlooked (even in PhD theses).

As for secondary members of your thesis committe, it's up to you.  You can easily say something simple and honest like, "Thanks to Professors X and Y for agreeing to be on my committee."  But I doubt they'll be insulted if you don't acknowledge them for a deep and lasting impact on your research career.  They've signed off on BS theses before.  They know it's no big deal (for them).