Karl Friedrich Alexander (born 01.05.1925) is a German physicist and a member of the GDR Academy of Sciences (1973). He was born in Berlin. He graduated from the University of Göttingen (1950). In 1956—66 he worked at the Central Institute for Nuclear Research of the GDR Academy of Sciences (from 1961 as head of a laboratory), in 1961—66 he was also professor at the University of Leipzig, and in 1966—69 he was at the Joint Institute for Nuclear Research (Dubna). From 1969 he has been at the Central Institute for Electron Physics of the GDR Academy of Sciences in Berlin (from 1970 as director).
His main research concerned nuclear physics, plasma physics, transport processes in liquids and thermal diffusion. He studied the short-lived isomeric states formed in reactions with neutrons and with heavy ions, and the acceleration of ions by a high-current relativistic electron beam.