# Homework Help: Is this a trick question?

1. Nov 29, 2011

### WeeBey

The problem statement, all variables and given/known data

Compare a 2.2eV photon with a 2.2 eV electron in terms of energy, rest mass, speed, wavelength, and momentum

The attempt at a solution

So...

E = (2.2eV) x (1.60 x 10-19 J/eV) = 3.52 x 10-19 J

Wouldn't 3.52 x 10-19 J be the energy for both the photon and electron? If so, wouldn't that make mass, speed, wavelength, and momentum equal for both particles?

2. Nov 29, 2011

### DaveC426913

Well, hopefully it wouldn't make mass the same...

3. Nov 29, 2011

### Staff: Mentor

Tabulate all the values for each particle. What's the rest mass of a photon?

4. Nov 29, 2011

### WeeBey

Isn't it:

m = E / c2

?

Or is the rest mass of a photon always zero.

5. Nov 29, 2011

### Staff: Mentor

The rest mass of a photon is always zero.

6. Nov 29, 2011

### WeeBey

Okay, so the rest mass for the photon is 0 and for the electron it is 9.11 x 10-31.

Wavelength would be:

E = hc / λ, or rather, λ = E / hc

Momentum would be:

p = h / λ

I assume because their energies are the same, the results for wavelength and momentum will be equal for the electron and photon.

Speed is:

p = mv, or rather, v = p / m

In the end: their energies, wavelengths, and momentum are equal while mass and speed are different. Is that right?

7. Nov 29, 2011

### Staff: Mentor

Wavelength and momentum will not be the same. Look up DeBroglie wavelength, photon momentum.

8. Nov 29, 2011

### WeeBey

Hmm,

So I first calculate velocity with:

v = √2eΔV/m

And then use:

λ = h / mv

That gives wavelength

For momentum I use:

p = mv

Because their masses are different, I should get different results.

So only the energy is the same for both?

9. Nov 29, 2011

### Staff: Mentor

As I said, you should tabulate all the values for each particle. You will have to be sure to use the appropriate formulas that apply to each. Then compare results.