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Topic: maximum velocity of electrons?  (Read 14116 times)

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Offline eq123

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maximum velocity of electrons?
« on: May 12, 2010, 10:12:25 AM »
i've been trying to figure out this problem..

The longest wavelength of light that causes electrons to be ejected from the surface of a copper plate is 243 nm. What is the maximum velocity of the electrons ejected when light of wavelength 200. nm shines on a copper plate?

the answer is 6.22 × 10^5 m/s

any ideas?

Offline Borek

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Re: maximum velocity of electrons?
« Reply #1 on: May 12, 2010, 12:08:21 PM »
You have asked the same question at physicalforums and you were already given several hints - have you tried to use them?
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Offline MrTeo

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Re: maximum velocity of electrons?
« Reply #2 on: May 12, 2010, 02:37:22 PM »
You have asked the same question at physicalforums and you were already given several hints - have you tried to use them?

Spotted!  ;D
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Offline eq123

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Re: maximum velocity of electrons?
« Reply #3 on: June 09, 2010, 09:13:37 AM »
thank you for nothing!

Offline sjb

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Re: maximum velocity of electrons?
« Reply #4 on: June 09, 2010, 12:59:52 PM »
What equations do you know that may be relevant (perhaps look for Photoelectric effect, or similar)

Offline Prof. Bohr

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Re: maximum velocity of electrons?
« Reply #5 on: June 13, 2010, 05:47:32 PM »
you could work that out using De Broglie's postulate - All matter has an associated wavelength, and the wavelength is inversely proportional to the mass x velocity.
in brief, Lambda = (Planks const)/(mass x veloc.)

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Offline opti384

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Re: maximum velocity of electrons?
« Reply #6 on: June 15, 2010, 08:36:45 AM »
 I think this problem is related to the photoelectric effect.

"The longest wavelength of light that causes electrons to be ejected from the surface of a copper plate is 243 nm. What is the maximum velocity of the electrons ejected when light of wavelength 200. nm shines on a copper plate?"

Here the wavelength of 243nm indicates the minimum frequency of light required for the electrons to be emitted from copper plate. frequency = the speed of light/wavelength. Energy = Plank constant x frequency

Therefore, the maximum KE = the energy of the 200nm wave - work function(which equals to the energy of 243 nm wave).

From the maximum KE, you can determine the velocity.


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