It can be hard for people which are not formerly educated in the field of physics to properly tag a question that they are asking. So I think there should be a tag like "should be tagged by a moderator". Because somebody can be curious about some phenomena without knowing which branch of physics it is a member of.
I just noticed that the untagged tag is blacklisted, and that is probably a hint that SE does not want such tags.
You only need one tag to pose a question. If in serious doubt, you could e.g. try if some of the words in your question exist as tags. (Example: Your Phys.SE question here contains the word electron in the title...)
The site is rather good (mostly through the efforts of Qmechanic) at re-tagging questions when the original tags are sub-optimal. If you're a new user and you're not sure quite how to tag something, just do your best effort and if the tags need editing then somebody will change them for you. (If you're really not sure, then just add a comment afterwards asking for help with the tags.) Don't sweat the tags too much - focus on the content.
As for an additional tag as you propose, it's (i) a meta tag, which we only use when they're really, really necessary, and (ii) unlikely to be discoverable by the bulk of the population that could use it, so it's unlikely to be helpful.