This is the 17988th most frequent Russian word.
физике
"Физике" is the dative or prepositional case form of "физика," meaning "physics" in English.
In this sentence, 'физике' is in the dative case, used as the object of interest ('заинтересован в').
Я заинтересован в физике, особенно в квантовой механике.
I am interested in physics, especially quantum mechanics.
Here, 'физике' is in the prepositional case, indicating the topic discussed ('на уроке').
Вчера на уроке физике мы изучали закон Ньютона.
Yesterday in the physics lesson, we studied Newton's law.
In this sentence, 'физике' is in the dative case, indicating the field of the referred lab work (following 'по').