Matter Questions

[HarveyD]

What wavelength is associated with a beam of neutrons of energy 0.030 eV?
Can you just use E = hf
then c = f x wavelength?

[mark_alec]

No, you will want to use de Broglie's equation, lambda = h/p
Together with the usual formula for kinetic energy, E = p^2/2m

[Lasercookie]

What wavelength is associated with a beam of neutrons of energy 0.030 eV?
Can you just use E = hf
then c = f x wavelength?

I doubt a beam of neutrons would be moving at the speed of light.

edit: also, when I work these questions (from the textbook anyway), I normally approach it in this method:
de broglie, solve for v
and then sub that into the
instead of the other one that mark_alec referred to (that way is probably easier)

edit 2: *facepalm* I didn't read that question properly either... I described how to find the energy. Also remember if you begin working with velocity (that includes the de broglie equation) and the regular kinetic energy equations you must work in Joules.

[HarveyD]

ah k, but you would need the mass yeah? to use the equation p = (2 x m x Ek)^0.5 and then sub into de broglie?
because the question didnt give the mass O.o

anyway does that mean that E = hf can only be applied to photon/light?

[Lasercookie]

Wait, yeah I think you can use E=hf. De broglie's equation was a rearranged version of compton's for the momentum of the photon. That one was related to E=hf, can't remember the exact relationship off the top of my head.
http://www.hep.man.ac.uk/u/roger/PHYS10302/lecture23.pdf also seems to suggest so (on closer reading, does it?).

I'm going to have dinner now, I'll double check later.

Edit:



(since we're talking about photons)
If you did relativity then you would be familiar with



Which gets us:
Compton's equation:
de Broglie: 

So that's how e=hf is related to de broglie's equation. I am not sure if it suggests that we can use e=hf though (since velocity of a particle wouldn't be the speed of the light for the majority of the cases). I'm still looking this up (trying to find a hard, reliable source that states it in simple terms).

[Lasercookie]

I'm going to say, no, you can't use E=hf. I can't find any other sources that seem to suggest it.

Either way, I don't ever remember having to use E=hf to calculate the energy for a matter wave. I always used the kinetic energy formula to find the energy.

[HarveyD]

yeah i think we only use e=hf for all the emission/absorption of photons stuff
and the rest is de Broglie
topic is light and matter, so they must be two seperate things/formulae i guess?

with the above question, you'd need mass yeah?

[Lasercookie]

yeah i think we only use e=hf for all the emission/absorption of photons stuff
and the rest is de Broglie
topic is light and matter, so they must be two seperate things/formulae i guess?

with the above question, you'd need mass yeah?

Yeah, e=hf is only for the energy of a photon. I just wanted to confirm it to myself.

The way I would work it out requires the mass. Where did it come from btw e.g. the heinemann book provided you with the mass of a neutron, the Nelson book provided you with the mass of a proton - do they expect you to assume the masses are the same?.

[HarveyD]

it came from heinemann and there were only masses given for protons and electrons lol
that was what confused me

[HarveyD]

you can use E=hf for gamma and X-rays yeah?

[Lasercookie]

you can use E=hf for gamma and X-rays yeah?

Seems so. I think we may have used E=hf in Unit 1 (can't remember - too long ago).
http://en.wikipedia.org/wiki/Electromagnetic_spectrum#Range_of_the_spectrum
It looks like e=hf applies to all forms of electromagnetic radiation.

[HarveyD]

ah yes, that would make sense
thanks heaps

[mark_alec]

The way I would work it out requires the mass. Where did it come from btw e.g. the heinemann book provided you with the mass of a neutron, the Nelson book provided you with the mass of a proton - do they expect you to assume the masses are the same?.

The masses are almost identical, so if you aren't given the mass of the neutron just use the mass of a proton and make a note of it.

[Lasercookie]

The masses are almost identical, so if you aren't given the mass of the neutron just use the mass of a proton and make a note of it.

Yeah, that's what I do. I wasn't sure where he got the question from. If it was the textbook that's what they definitely want you to do, but if it were one of the less-VCAA style trial exams or something they might expect something slightly different.