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### Photoelectric effect pre-module assessment #17-19

Posted: Mon Sep 26, 2016 4:22 pm
Hi :) Can someone explain what the answer to the question is and the reasoning behind it please?

Light hits a sodium metal surface and the velocity of the ejected electron is 6.61x10^5 m.s^-1. The work function for sodium is 150.6 kJ.mol^-1.
Answer the following 3 questions.
A. What is the kinetic energy of the ejected electron?
a. 3.01x10^25 J
b. 3.98x10^-19 J
c. 7.96x10^-19 J
d. 1.99x10^-19 J
e. none of the above

18. B. How much energy is required to remove an electron from one sodium atom?
A. 2.501 x 10-22 J
B. 1.506 x 105 J
C. 2.501 x 10-19 J
D. 9.069 x 1028 J
E. None of the above

19. C. What is the frequency of the incident light on the sodium metal surface?
A. 3.01 x 1014 Hz
B. 2.27 x 1038 Hz
C. 4.54 x 1058 Hz
D. 6.78 x 1014 Hz
E. None of the above

### Re: Photoelectric effect pre-module assessment #17-19

Posted: Mon Sep 26, 2016 10:32 pm

### Re: Photoelectric effect pre-module assessment #17-19

Posted: Tue Sep 27, 2016 12:41 pm
Thank you very much!

### Re: Photoelectric effect pre-module assessment #17-19

Posted: Wed Sep 28, 2016 8:45 am
How did you find the mass of the sodium e-?

### Re: Photoelectric effect pre-module assessment #17-19

Posted: Wed Sep 28, 2016 8:59 am
The mass of an electron is a constant.

### Re: Photoelectric effect pre-module assessment #17-19

Posted: Wed Sep 28, 2016 9:00 am
right! Thank you :)

### Re: Photoelectric effect pre-module assessment #17-19

Posted: Wed Sep 28, 2016 3:51 pm
Can you explain the concept behind #18? I thought that the work function was exactly the same as the threshold energy (energy needed to remove electrons) so I don't quite understand what is being done. Thanks

### Re: Photoelectric effect pre-module assessment #17-19

Posted: Wed Sep 28, 2016 6:28 pm
It is. The threshold energy given is in energy/mol. The question asks for energy required to remove an electron from one sodium atom, not a mol of sodium atoms.

### Re: Photoelectric effect pre-module assessment #17-19

Posted: Thu Oct 25, 2018 10:24 pm
For A, I used the formula for kinetic energy, which is 0.5*(mass of electron)(velocity)^2. The velocity is given, and the mass of an electron is the constant 9.109 x 10^-31 kg. After plugging these two values into the equation, you should get the result 1.989x10^-19 J, which corresponds to D.