I am majoring in physical chemistry at university.

One part is important.

He is majoring in linguistics.

He has a deep interest in astronomy.

Electromagnetism is a part of physics that studies the interaction between electricity and magnetism.

He is good at solving geometry problems.

He is an expert in toxicology and conducts many studies.

He is an expert in mythology and gave a lecture on the gods of ancient Greece.

He is an expert in bibliography and has many publications.

He is an expert in sociology.