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# energy and de broglie question watch

1. Hi everyone,

I'm revising for a QM exam but some of this question is A-level-ish

I'm simpley not getting the answers provided, which are after each question in brackets

A1: Caluclate the wavelegnth of the following:
a) a 10eV photon ()
b) an electron with a kinetic energy of 10eV ()
c) a thermal neutron of average kinetic energy in a neutron gas of temperature 300K ()
d) a tennis ball with mass of 57g travelling at 50m/s ()

Does anyone actually get these answers?
2. I make (a) 1.2 x 10^-7 m; I can't see where their answer comes from.

I agree with (b). First work out the velocity of the electron, then use the de Broigle equation (ignoring relativistic effects).

I don't have the knowledge to comment on (c).

I agree with (d); it's a straightforward application of the de Broigle equation.
3. for c) you can find the average translational kinetic energy of the neutron, given by (3/2)kT where k is the boltzmann and T is temperature in kelvin. plug that into KE=(p^2)/(2m) to find momentum of the neutron and voila
4. I'm not 100% sure if I did correctly, but I agree with C.

Use kinetic theory of gases - .

Express mean momentum in terms of mean kinetic energy. Then use the Broglie relation of wavelength and momentum.
5. I have to disagree that is Stefan-Boltzmann constant. The Stefan-Boltzmann constant is used in Stefan-Boltzmann law: .
The in kinetic theory of gases is just Boltzmann constant.

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