Photoelectric effect/photon stuff

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The discussion revolves around three main physics problems related to the photoelectric effect and astrophysics. The first question involves calculating the radius of a blue supergiant star based on its power output and surface temperature, with participants noting discrepancies in their calculations. The second question focuses on finding the maximum kinetic energy of photoelectrons ejected from a silver target using 265 nm light, with users experiencing issues with the accuracy of their constants and values. The final question addresses neutron diffraction and the calculation of neutron momentum and kinetic energy, with participants expressing confusion about deriving the gas temperature from the results. Overall, the thread highlights common challenges in physics problem-solving and the importance of precision in calculations.
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got a couple of questions...

firstly,

A blue supergiant star has surface temperature 29kK and has total power output 100000 times that of the Sun.

where is the wein peak of this stars balckbody curve?

I got lambda=9.99*10^-8m, which i know is right.

now, radius of sun is 6.96*108m, find radius of supergiant..

so ratio here should give the supergiant a surface area of A=100000A of sun.. solving for that gives r = 2.11*10^11m... but it tells me its wrong?





secondly,

A photoelectric experiment uses 265 nm light and a silver target.
Find max kinetic energy of photoelectrons.

Kmax=hf-W0
h=6.626*10-34
f=c/lambda=1.13*10^15
W0 for silver = 7.43*10^-19

subbing in gives Kmax = 5.738*10^-21J... and it also tells me that is wrong?




thirdly,

Suppose you want to observe neutron diffraction in a crystal with 0.28 nm spacing between lattice planes.

Find the momentum of neutrons having wavelength equal to this spacing.
found p=2.4*10^-24 kg*m/s

Find the kinetic energy of neutrons having wavelength equal to this spacing.
found K=1.7*10^.21 J

If these neutrons constituted an ideal gas, what would be the gas temperature?
How the heck do i do that :S??



thanksss so much for any help!
 
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super star Q is easy
Psun=\sigma4\pir2*T4
from the textbook temp of sun is about 5800K
solve for Power (also in the text - could just look up)
then 100000*3.91*10^26=5.65*10-8A*(29*103)4 or whatever your temp value was.
A=9.73976*1020=4\pir2
solve for r

the second question I got wrong twice, you have to use a more accurate value for c then 3*10^8 or it won't except- mine was out by 0.04 and it told me I was wrong

as for the last question I don't have a clue yet.
 
pat666 said:
except.
accept!
 
Mastering physics is a bit stupid like that.
You can be wrong if your too accurate, and wrong if your not accurate enough.
 
for Kmax i got 6.59427*10^-21 J and its still telling me its wrong. and i tried 0.04116 eV and its telling me its wrong too.
 
ProPatto16 said:
for Kmax i got 6.59427*10^-21 J and its still telling me its wrong. and i tried 0.04116 eV and its telling me its wrong too.
I had a wavelength of 264*10^-9 m and w_o of 7.43*10^-19 (silver)
my first answer was 1.0*10^-20J and it finally accepted 1.04*10^-20J

not sure why it won't accept your answer, procedure is the same as mine.

btw MP always seems to like SI units (J).
 
Last edited:
howcome you used 264 nm? when it says 265?

the way i did it was K= hf - W and f=c/265nm = 299792458/265*10-9 = 1.131292294*1015 Hz
i used all exact values for constants so...

K = (6.62606821*10-34*1.131292294*1015) - 7.43*10-19

= 6.60*10-21 J what did i do different?
 
my value was 264! not sure why it won't accept that answer - it was screwed up for me too - maybe go see him and ask what's going on.
 
dont worry about it, i used your answer and it still told me it was wrong, which was my last attempt...

so the answer it gave me was 7.57*10^-21 J so go figure. i sent the uni an email so we'll see. thanks anyway.
 

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