electromagnetic field vs electromagnetism

electromagnetic field

noun
  • The field of force that is due to the interaction of electric and magnetic forces of charged bodies 

electromagnetism

noun
  • Electricity and magnetism, collectively, as a field of study. 

  • A unified fundamental force that combines the aspects of electricity and magnetism and is one of the four fundamental forces. (technically it can be unified with weak nuclear to form electroweak) Its gauge boson is the photon. 

How often have the words electromagnetic field and electromagnetism occurred in a corpus of books? (source: Google Ngram Viewer )