Photoelectric effect/photon stuff

In summary, the conversation is about solving various physics problems involving temperature, power output, kinetic energy, momentum, and gas temperature. The first problem involves finding the surface area of a blue supergiant star and solving for its radius using the Stefan-Boltzmann law. The second problem involves finding the maximum kinetic energy of photoelectrons using the photoelectric effect. The third problem involves finding the momentum and kinetic energy of neutrons and determining the gas temperature if they were an ideal gas. There are discrepancies in the answers given by the online platform and the methods used by the individuals.
  • #1
ProPatto16
326
0
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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  • #2
super star Q is easy
Psun=[tex]\sigma[/tex]4[tex]\pi[/tex]r2*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[tex]\pi[/tex]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 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.
 
  • #3
pat666 said:
except.
accept!
 
  • #4
Mastering physics is a bit stupid like that.
You can be wrong if your too accurate, and wrong if your not accurate enough.
 
  • #5
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.
 
  • #6
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:
  • #7
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?
 
  • #8
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.
 
  • #9
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.
 

Related to Photoelectric effect/photon stuff

1. What is the photoelectric effect?

The photoelectric effect is a phenomenon in which electrons are emitted from a material when it is exposed to light of a certain frequency. This was first observed by Heinrich Hertz in 1887 and was later explained by Albert Einstein in 1905 in his theory of the quantum nature of light.

2. What is a photon?

A photon is a fundamental particle of light that carries energy and momentum. It is considered as a particle of electromagnetic radiation and has no mass. Photons are responsible for the photoelectric effect as they transfer their energy to the electrons in a material, causing them to be emitted.

3. How does the photoelectric effect support the particle theory of light?

The photoelectric effect is a key piece of evidence that supports the particle theory of light. This is because it demonstrates that light is made up of discrete packets of energy (photons) that can transfer their energy to electrons, rather than being a continuous wave as previously believed.

4. What is the work function in the context of the photoelectric effect?

The work function is the minimum amount of energy needed to overcome the attractive forces between an electron and its parent atom and allow it to be emitted from a material. It is different for each material and determines the threshold frequency of light needed to cause the photoelectric effect in that material.

5. How is the photoelectric effect used in technology?

The photoelectric effect has many practical applications in technology. For example, it is used in solar panels to convert light energy into electrical energy, in photodiodes to detect light and in photocells to control the flow of electricity. It is also used in devices such as photocopiers, barcode scanners, and digital cameras.

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