Joule—Capitalize when not the unit: Joule effect, Joule's law, but 13 joules.
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