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Question: The kinetic energy equation E=hv-(work function) can be written E=hv(photon)-hv(ejected). Why is it that if a problem states "Radiation of X Hz is required to eject an electron...," you plug X into the ejected half, whereas if it states "Radiation of X Hz strikes Cs and ejects an electron..." you plug it into the photon side? What is the difference between these statements?
Answer: The main equation is that E(photon)=Phi+KE, where phi is the work function which is the minimum energy required to remove an electron and KE is the kinetic energy of an electron. Basically the equation is using conservation of energy. The energy given by the photon is used to remove an electron then any remaining energy goes in to giving it velocity as it is ejected. E(photon) talks about the total energy of the photon which would be hv (where v is frequency). Phi is h(vo) (where vo is the minimum frequency required). Your first statement about radiation X Hz is required to eject would need to be plugged in as vo in the work function. The second statement that radiation of X Hz strikes and ejects would have to be plugged into hv for the energy of the photon. The first statement tells you the minimum required frequency but the second statement tells you the total energy supplied to the electron which will be used to first liberate it and then give it velocity.
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