ek= workfunction + energy of photon

[Chem_Mod]

when do we know when to use the ek= workfunction + energy of photon? What are the clues that let us know we have to use this equation?

[Chem_Mod]

That equation is something you use whenever you are doing calculations with the photoelectric effect (i.e. an electron is being ejected from a metal surface using light). You should use that equation whenever you have 2 of the variables. For example if you have the velocity of the electron and the frequency of the light used to eject it you have 2 of those variables. You can calculate the kinetic energy of the electron (Ek=(1/2)mv^2) and using the frequency of the light you can calculate its energy (E=hv) and from those 2 you can find the work function.