Post by: HarveyD on September 20, 2011, 04:55:21 pm
Can you just use E = hf
then c = f x wavelength?
Post by: mark_alec on September 20, 2011, 07:10:26 pm
Together with the usual formula for kinetic energy, E = p^2/2m
Post by: Lasercookie on September 20, 2011, 07:20:09 pm
What wavelength is associated with a beam of neutrons of energy 0.030 eV?I doubt a beam of neutrons would be moving at the speed of light. :P
Can you just use E = hf
then c = f x wavelength?
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.
Post by: HarveyD on September 20, 2011, 07:41:54 pm
because the question didnt give the mass O.o
anyway does that mean that E = hf can only be applied to photon/light?
Post by: Lasercookie on September 20, 2011, 07:49:07 pm
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:
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).
Post by: Lasercookie on September 20, 2011, 08:44:33 pm
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.
Post by: HarveyD on September 20, 2011, 08:49:59 pm
and the rest is de Broglie
topic is light and matter, so they must be two seperate things/formulae i guess? :P
with the above question, you'd need mass yeah?
Post by: Lasercookie on September 20, 2011, 08:55:46 pm
yeah i think we only use e=hf for all the emission/absorption of photons stuffYeah, e=hf is only for the energy of a photon. I just wanted to confirm it to myself.
and the rest is de Broglie
topic is light and matter, so they must be two seperate things/formulae i guess? :P
with the above question, you'd need mass yeah?
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?.
Post by: HarveyD on September 20, 2011, 09:00:37 pm
that was what confused me
Post by: HarveyD on September 20, 2011, 09:32:34 pm
Post by: Lasercookie on September 20, 2011, 10:06:53 pm
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.
Post by: HarveyD on September 20, 2011, 10:43:58 pm
thanks heaps :D
Post by: mark_alec on September 21, 2011, 07:17:56 pm
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.
Post by: Lasercookie on September 21, 2011, 07:23:00 pm
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.