# Photoelectric effect/photon stuff

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!

## Answers and Replies

super star Q is easy
Psun=$$\sigma$$4$$\pi$$r2*T4
from the text book 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$$\pi$$r2
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 wont 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.

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.

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 wont 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 whats 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.