Useful Equations that Deal with the Photoelectric Effect
The top equation is the standard one that describes the photoelectric effect.
In words, the kinetic energy of an ejected electron equals the energy of the photon that ejected it minus the amount of energy required to eject an electron (the work function.)
Note that you can use this equation regardless of which energy units you use... as long as you're consistent.
The bottom equation is important in a laboratory context. We can't measure the kinetic energy of an electron with a stopwatch and meterstick. How do we know with what maximum energy electrons are ejected from metals? We wire a circuit consisting of a pair of metal plates to an adjustable power supply and an ammeter. When the right wavelength of light is shone on one plate, it will emit electrons because of the photoelectric effect. Some of these electrons will strike the other plate, producing a small current. But if we apply a potential difference across the plates forcing the electrons back towards the plate they'd be ejected from, this will stop the current. We call this potential difference the "stopping potential," and it can be calculated by assuming that the kinetic energy of an electron calculated via the photoelectric effect is turned into potential energy, as seen in the bottom equation. This is all easiest to see if you play with this simulation of the photoelectric effect, seen here:
Useful Equations that Deal with the Photoelectric Effect
The top equation is the standard one that describes the photoelectric effect.
In words, the kinetic energy of an ejected electron equals the energy of the photon that ejected it minus the amount of energy required to eject an electron (the work function.)
Note that you can use this equation regardless of which energy units you use... as long as you're consistent.
The bottom equation is important in a laboratory context. We can't measure the kinetic energy of an electron with a stopwatch and meterstick. How do we know with what maximum energy electrons are ejected from metals? We wire a circuit consisting of a pair of metal plates to an adjustable power supply and an ammeter. When the right wavelength of light is shone on one plate, it will emit electrons because of the photoelectric effect. Some of these electrons will strike the other plate, producing a small current. But if we apply a potential difference across the plates forcing the electrons back towards the plate they'd be ejected from, this will stop the current. We call this potential difference the "stopping potential," and it can be calculated by assuming that the kinetic energy of an electron calculated via the photoelectric effect is turned into potential energy, as seen in the bottom equation. This is all easiest to see if you play with this simulation of the photoelectric effect, seen here: