Photoelectric Effect Post Video Assessment

[Megan Cao 1I]

hello me again
so question 34 of the assessment asked
"If molybdenum is irradiated with 194 nm light, what is the maximum possible kinetic energy of the emitted electrons?"
with the given information of
"Molybdenum metal must absorb radiation with a minimum frequency of 1.09 x 1015 s-1 before it can emit an electron from its surface."
I know that the minimum energy needed to produce the effect is 7.22x10^-19J

what I did was
194x10^-9 minus 7.22x10^-19 =Ek
which turned out to be wrong so can someone help guide me through this?

[Rachel Yu 1G]

Kinetic Energy = Energy of photon - Work function. Your work function (7.22 x 10^-19J) is correct but for the energy of the photon, you have to convert the wavelength given (194nm) to energy first. You can use the equation c = vλ, since the speed of light is constant and the wavelength is known. First, convert 194 nm to meters since the speed of light is given as units of m/s. After plugging in the values, you can find the frequency of the wave to be 1.5 x 10^15 Hz. Then, use the equation E=hv to find energy: E(energy of photon)= (Planck's constant)(1.5 x 10^15) = 1.02 x 10^-18 J. Then we can calculate Ek;

Energy of photon - Energy required to remove e- = Kinetic energy
(1.02 x 10^-18) - (7.22 x 10^-19) = 3.05 x 10^19 J

[205296774]

Yup, the above reply is correct. You used the wavelength as the photon energy instead of first converting it with c=wavelength x v and E= vh equations. I find it easiest to organize what I know based on looking at the units, so maybe try that in the future to keep everything sorted.

[805394719]

Hi, we know that E(photon) - Work function = Ek(electron)
We know that the work function is 7.22x10^-19J. We also know that the wavelength of the incoming light is 194 nm which is 194 x 10^-9 m. To calculate the energy of the photon, we need to use the equation E = hv. So we need to know the frequency. To find the frequency, we need to use the equation c = wavelength x frequency. The frequency will then be 1.54 x 10^15. Substituting this value into E =hv will make the energy 1.02 x 10^-18J.

(1.02 x 10^-18) - (7.22 x 10^-19) = Ek(electron)
Ek = 3 x 10^-19J