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### Topic: Photoelectric effect problem  (Read 1107 times)

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#### Carol_mrinn

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##### Photoelectric effect problem
« on: September 16, 2021, 02:30:47 PM »
Hi everyone! I've got a question. I have this problem and I want some help because I couldn't with it.

The threshold frequency for shedding an electron from the cesium surface
metallic is 5.18 x 10¹⁴ s⁻¹. Calculate:

• The energy of a photon of this frequency
• The energy of a photon of light of wavelength 400 nm
• The kinetic energy of an electron that gives off cesium from the surface
a beam of light with a wavelength of 400 nm.

For the first one I used the equation E=hv, for the second I used E = h*c/λ and the last one kinetic energy = h (photon frequency - threshold frequency)

But I get negative kinetic energy (-3.93x10⁻¹⁹ to be exacts). Am I doing something wrong? How would it be the right way to solve this problem?

#### mjc123

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##### Re: Photoelectric effect problem
« Reply #1 on: September 16, 2021, 03:30:09 PM »
Method looks correct, don't know how you got a negative energy! Show your calculation.

#### Corribus

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##### Re: Photoelectric effect problem
« Reply #2 on: September 16, 2021, 03:34:57 PM »
Make sure to pay attention to units.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

#### Carol_mrinn

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##### Re: Photoelectric effect problem
« Reply #3 on: September 20, 2021, 04:03:34 PM »
Method looks correct, don't know how you got a negative energy! Show your calculation.

• E=hv > E = (6.63x10⁻³⁴ J*S) * (5.18x10¹⁴ S⁻1) = 3.43x10⁻¹⁹ J
• E = h*c/λ > E= (6.63x10⁻³⁴ J*S) * (3.00x10⁸ m/s) / 4x10⁻⁷ m = 4.97x10⁻¹⁹ J
• kinetic energy = h (photon frequency - threshold frequency) > kinetic energy = (6.63x10⁻³⁴ J*S) * (4.97x10⁻¹⁹ J - 5.18x10¹⁴ S⁻¹) = -3.93x10⁻¹⁹
Maybe I did something wrong but I don't know what?[/list]

#### mjc123

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##### Re: Photoelectric effect problem
« Reply #4 on: September 20, 2021, 04:23:54 PM »
The photon frequency is not 4.97 x 10-19 J. That is the photon energy, as you calculated. What are you doing subtracting a quantity in s-1 from a quantity in J? Have you no sense of units?

#### Carol_mrinn

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##### Re: Photoelectric effect problem
« Reply #5 on: September 21, 2021, 09:25:39 AM »
The photon frequency is not 4.97 x 10-19 J. That is the photon energy, as you calculated. What are you doing subtracting a quantity in s-1 from a quantity in J? Have you no sense of units?
You're right. I wasn't paying attention to that. Thank you so much! Despite you being rude (jk)
« Last Edit: September 21, 2021, 09:41:39 AM by Carol_mrinn »

#### mjc123

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##### Re: Photoelectric effect problem
« Reply #6 on: September 21, 2021, 04:29:52 PM »
Sorry, didn't mean to be rude. But it's very important to get your units right (as Corribus said in an earlier post). "Paying attention to that" will save you from a lot of errors in the future. You would have realised at once that what you wrote wasn't right.