line of force vs field line

line of force and field line both are nouns.

Word	Noun	Adjective	Verb	Adverb
line of force	Yes	No	No	No
field line	Yes	No	No	No

As nouns, line of force and field line are synonyms defined as:

line of force and field line: an imaginary line in a field of force; direction of the line at any point is the direction of the force at that point

line of force (noun)	field line (noun)
an imaginary line in a field of force; direction of the line at any point is the direction of the force at that point	an imaginary line in a field of force; direction of the line at any point is the direction of the force at that point