We consider two non-statistical definitions of entropy for dynamic
(non-stationary) black holes in spherical symmetry. The first is analogous to
the original Clausius definition of thermodynamic entropy: there is a first law
containing an energy-supply term which equals surface gravity times a total
differential. The second is Wald's Noether-charge method, adapted to dynamic
black holes by using the Kodama flow. Both definitions give the same answer for
Einstein gravity: one-quarter the area of the trapping horizon.Comment: 3 pages, revte
