maximum Frequency

• Nov 22nd 2010, 05:50 AM
gbenguse78
maximum Frequency
1. The problem statement, all variables and given/known data
Hi
I am solving the june 2009 Physics 1 paper (Syllabus A). The mark scheme answer has got me slightly confused on 2 questions:
1. Calculate the maximum frequency of the photon emitted when the 9.0 ev
electron collides with an atom.
2.Calculate the specific charge of an isotope with 143 neutrons and 92 protons

2. Relevant equations
1) E= hf
2) Specific charge= charge/mass

3. The attempt at a solution
1)f= E/H
Therefore
f= 9 x 1.6x 10^-19/6.63 x 10^-34
But the schemes answer is very different to mine.
2)In the schemes answer, the mass was calculated as both 143 and 92. I presumed that the nucleon number constituted the mass, while the electrons (or number of protons) constituted the charge. (Multiplied by 92 in this case).
Anybody? Thanks.
• Nov 22nd 2010, 06:48 AM
Unknown008
1) Well, you have to take into account the work function...

$\displaystyle E = hf_o + hf$

$\displaystyle 9.0 \times 1.6\times10^{-19} = Work\ function + hf$

2) The total mass of an atom is given by the sum of relative mass of neutrons and protons, that is 143+92 = 235. The charge is given by the sum of the relative charges of all the protons, that is +92

Hm... you're dealing with Uranium-235!
• Nov 23rd 2010, 01:11 AM
gbenguse78
Hi
Thanks for the usual prompt reply. Okay, lets go!

Question 1
So, the specific charge will be
92 x 1.6 x 10^-19/235 x 1.67 x 10^-27. Please confirm this

Question 2
I cant seem to work out the work function. How can I do this?

And yes I am dealing with Uranium! I have 8 modules in January. Sso maybe one of its higher isotopes??
• Nov 23rd 2010, 04:57 AM
Unknown008
1. Yes

2. It's normally given in the question as the threshold frequency, or worked out by using other values before. I can't give you the answer here, because different metals have different work functions.
• Nov 23rd 2010, 06:23 AM
gbenguse78
Thanks once again.