Help with 4 Physics Problems at Room Temperature

[SharpTJ]

Last test coming up and I need help with these last 4 problems please...

1.) A beam of ultraviolet radiation, with frequency of 2.57x1015 Hz is directed on a clean metal surface. The work function of the metal is 3.80 eV. Calculate the maximum kinetic energy for an electron dislodged from the surface by the radiation

(6.6e-34*2.57e15)-(3.80*1.6e10-19) to get 10.88 J what am i doing wrong here?

2.) A powerful laser can emit in 0.51x10-2 s an intense pulse of light having an energy of 5960 J. If the frequency of the light is 6.08x1014 Hz, how many photons will be emitted in this pulse.

Number of Photons = Energy of pulse / Energy of one photon...so Ephoton is 4.01e19...how do I get energy of pulse?

3.) A characteristic momentum for a particle in an ideal gas can be found by the relation p2/(2m)=(3/2)kT where k is Boltzmann's constant. For a gas of helium at 293 K (room temperature), what is the characteristic momentum?
DATA: k=1.38x10-23 J/ K.

I have no idea...I'm lost on this

4.) What is the characteristic de Broglie wavelength at this temperature?

need anser to #3 to solve this...

 

[CompuChip]

how do I get energy of pulse?

Read the question again. Carefully. Might it be given?

3.) A characteristic momentum for a particle in an ideal gas can be found by the relation p2/(2m)=(3/2)kT where k is Boltzmann's constant. For a gas of helium at 293 K (room temperature), what is the characteristic momentum?

You'll have to solve the given equation for the momentum, p. You are given the temperature T and the value of k. So only the mass is still unknown. What other information is given in this question that would allow you to determine (in the broadest sense: looking it up is also determining ) that mass?

 

[SharpTJ]

Read the question again. Carefully. Might it be given?


You'll have to solve the given equation for the momentum, p. You are given the temperature T and the value of k. So only the mass is still unknown. What other information is given in this question that would allow you to determine (in the broadest sense: looking it up is also determining ) that mass?

Well, I though it was given also but when I 4.01319/5960 J...the computer still says it is the wrong answer

 

[Astronuc]

In

Prob 1. Please check the units and orders of magntiude. 6.6e-34*2.57e15 = 1.6962e-18 J and 3.80*1.6e-19 = 6.08e-19.

Prob 2. "how do I get energy of pulse?" - that is given in the problem statement "intense pulse of light having an energy of 5960 J". The pulse width is superfluous with respect to energy, but would be a factor in a calculation of power.

Prob 3. See - http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/kintem.html

Think of how to relate momentum of a particle, p = mv, to kinetic energy KE or T = 1/2 mv².

Prob 4. It appears one knows the answer.

This appears to be a homework problem. If so, I will move it to Introductory Physics HW forum.

 

[SharpTJ]

Read the question again. Carefully. Might it be given?


You'll have to solve the given equation for the momentum, p. You are given the temperature T and the value of k. So only the mass is still unknown. What other information is given in this question that would allow you to determine (in the broadest sense: looking it up is also determining ) that mass?

1.) stupid mistake
2.) stupid mistake
3.) using p2=(3/2)(1.38e-23)(293)([2][8])=4.32e-20 with p=2.0797e-10 kg m/s can someone verify that it is correct...computer still says wrong but I don't know what else I am missing

 

[dynamicsolo]

Argh! I edited what was here instead of adding a new reply... see below

 

[SharpTJ]

What is that [2][8] supposed to be? If you're using the mass of a helium atom, shouldn't you express it in kg.?

4) As for your last question, the deBroglie wavelength of the helium atom will be given by (deB wavelength) = h/p .

i thought so too...that is sqr(4.85e-22) which is 2.20e-11 and still incorrect

 

[dynamicsolo]

3.) using p2=(3/2)(1.38e-23)(293)([2][8])=4.32e-20 with p=2.0797e-10 kg m/s can someone verify that it is correct...computer still says wrong but I don't know what else I am missing

You started from

KE = (3/2)kT ,

but KE is also

KE = (p^2)/2m .

so you should have

(p^2) = 3mkT = 3 · (4 · 1.67·10^-27 kg) · (1.38·10^-23 J/K) · (293 K) (kg·m/sec)^2

 

[SharpTJ]

You started from

KE = (3/2)kT ,

but KE is also

KE = (p^2)/2m .

so you should have

(p^2) = 3mkT = 3 · (4 · 1.67·10^-27 kg) · (1.38·10^-23 J/K) · (293 K) (kg·m/sec)^2

ah...got it thanks a lot!

WL=h/v -->6.6e-34/8.98e-34=7.35e-11 nm is that correct?

 

[dynamicsolo]

ah...got it thanks a lot!

WL=h/v -->6.6e-34/8.98e-34=7.35e-11 nm is that correct?

I am getting 9.00·10^-24 N·sec for the linear momentum of the helium atom.

The deBroglie wavelength is given by h/p , for which I am getting

(6.63·10^-34 J·sec) / (9.00·10^-24 N·sec) = 7.37·10^-11 m or 0.074 nm.

So I basically agree with your result, but not with the way you've presented it.

Be careful about what units the computer accepts your entries in.

 

[SharpTJ]

I am getting 9.00·10^-24 N·sec for the linear momentum of the helium atom.

The deBroglie wavelength is given by h/p , for which I am getting

(6.63·10^-34 J·sec) / (9.00·10^-24 N·sec) = 7.37·10^-11 m or 0.074 nm.

So I basically agree with your result, but not with the way you've presented it.

Be careful about what units the computer accepts your entries in.

it is was a computer error...i emailed instructor and he fixed it...thanks