To elaborate on Djenthallman's comment:
In Russian, the verb "учить" сould mean both "to teach" and "to study". If the direct object (i.e. the one that is in the accusative case) is an academic subject (i.e. "физику"), the meaning is unambiguously "to study".
If the direct object is either a person or a group (i.e. "тебя", "его", "студентов") or is absent/implied, the meaning is unambiguously "to teach". You would then use the dative case for the subject being taught - "физике" (it's an indirect object here).
I'm not confident on this by any stretch but I'm starting to get the impression that this is a thing about verbs ending in -вать. This is a suffix given to some perfective verbs to create an imperfective version. It seems to me (from a very limited sample size!) that these cases don't include the в in the present tense forms.
Another very similar sentence in this lesson emphasizes the dative "я преподаю физике". I understood the explanation to be that the accusative here was for the group being taught (students), and that dative was used for the subject. In English you could say either "I teach students" or "I teach physics"; you could also say "I teach physics to students," which suggests physics in the accusative and students in the dative. So i can see the use of физику lining up with the English usage, but am having trouble reconciling this with the discussion in the other lesson using физике.