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.

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