help black body radiation!!! (1 Viewer)

[haboozin]

hey i cant do these questions can someone please do them with working out!!
THANKS


3
A beam of light of 6.9 x 1014Hz, imparts 4.5mW to a cold black body. Calculate the number of photons striking the black body from the beam each second.
Answer:



4
A beam of UV light of 1.1 x 1015 Hz shines onto a metal surface. Electrons are emitted from the surface of the metal with a maximum Kinetic Energy of 2 x 10-19 J.

Given 1 eV = 1.06 x 10 -19 calculate the potential required to stop these electrons from being emitted.

5

A beam of UV light of 6.9 x 1015 Hz shines onto a metal surface. Electrons are emmitted from the surface of the metal with a maximum Kinetic Energy of 2.8 x 10-19 J.

Given 1 eV = 1.06 x 10 -19 and Planck's constant = 6.629 x 10-34, calculate the work function of the metal surface.

6

A beam of UV light of 8 x 1015 Hz shines onto a metal surface. Electrons are emmitted from the surface of the metal with a maximum Kinetic Energy of 5.9 x 10-19 J.

Given 1 eV = 1.06 x 10 -19 and Planck's constant = 6.629 x 10-34, calculate the threshold frequency (F0) of the metal surface.

 

[Xayma]

3.

P=W/t
W=Pt
W=ΔE
Since we want to calculate it per second:
W=4.5*10^-3*1
=4.5*10^-3 J

Now the energy of each photon is given by

E=hf
=6.629*10^-34*6.9*10¹⁴
=4.574*10^-18


To get the amount of photons we want to divide the work done per second by the energy of the photons:

Ie
No. of photons=W/E
=4.5*10^-3/4.574*10^-18
=9.84*10¹⁴ photons per second

or 984 Terraphotons a second.

4,5 you need the differnce between the energy of the beam and the energy of the electrons.