# How much energy is carried by one quantum of these electromagnetic waves?

1. Apr 28, 2004

### mustang

Problem 6.
Radiation emitted from human skin reaches its peak at wavelength=960um.
How much energy is carried by one quantum of these electromagnetic waves? Answer in eV.
I have found that the frequency is 3.125*10^11 Hz.
Note: What do I do?

Problem 8. Light of wavelength 350 nm falls on a potassium surface, and the photoelectrons have a maximum kinetic energy of 1.3 eV.
What is the work function of potassium? Answer in eV.

Problem 17.
Light of wavelength 3*10^-7 m shines on the metal lithium, iron , and mercury which have work functions of 2.3 eV, 3.9 eV, and 4.5eV, respectively.
For those metals that do exhibit the photoelectric effect, what is the maximum energy of the photoelectrons?
where do I start? :happy:

2. Apr 29, 2004

### ShawnD

Use Plack's Constant.

Calculate the frequency, use planck's constant to find the energy. Subtract 1.3eV from the energy you calculated, that is the work function.
Work function is the energy required to pull an electron from an atom and have 0 kinetic energy. For these equations, it goes like this:
light energy = work function + photoelectric kinetic energy.

Calculate the frequency, calculate the energy with planck's constant. Take that energy and subtract 2.3 eV for lithium. Subtract 3.9 eV for iron. Subtract 4.5 eV for mercury. Do each of those subtractions separately. If the resulting number is negative, photoelectrons do not come from that metal. Due to the wording of the question ("For those metals that do exhibit the photoelectric effect"), I would suspect mercury does not let go of electrons. That's just suspicion though.

Last edited: Apr 29, 2004
3. Apr 29, 2004

### mustang

In regards for problem 6.

So for problem 6.
I would use the Plack's formula: E=hf
Where the f=3.125*10^11Hz and the h=6.63*10^-34 and solve for E?

4. Apr 29, 2004

### swansont

Yes. And then convert the answer, in J, to eV.

5. Apr 29, 2004

### mustang

Problem 6.

For problems 6 I multiplied f with h and got 2.071875*10^-22 .If this is right what is the number (1.6*10^-6, I think) to convert this answer into eV.

6. Apr 29, 2004

### swansont

1 J = 1.6e-19 eV

7. Apr 29, 2004

### gnome

Correction:
$$1 eV = 1.6 \times 10^{-19}J$$