Joule—Capitalize when not the unit: Joule effect, Joule's law, but 13 joules.
journal titles—Use italics in all cases, including for news releases (deviation from AP style, since we frequently deal with journal/article distinctions). According to a new study in Nature, ..... Also use italics for book and magazine titles, movies, TV series, and any other work works for which Chicago specifies italics. (2016, updated 2017)
Jr. or Sr.—no commas: John Johnson Jr. will be speaking on . . . . Same for roman numerals: John Johnson III. In a format where names are in all caps the "r" is still lower case: J.J. JOHNSON Jr. [revised 09/00]
K—suffix meaning 1,000 (acceptable in computing context); symbol for planetary wave number
Kármán—Always include both accents