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# "de broglie" question watch

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1. hi,

I need help with the following questions:

An electron has kinetic energy 3.00 eV. Find its wavelength?

i thought i would use 1/2mv^2 = 3 to find v which = 2.57x10^15.

then using that, put it into h/mv so it = 2.83x10^-19. But this answer is wrong! i got no idea how to do this now...please help!!
2. Remember that the equations work in joules. You need to convert the 3eV into joules before you subsitute it into ke=0.5mv^2 to get the right answer.
3. oh so divide by 1.6x10^-19??
4. No 1 eV=1.6x10^-19 Joules. So 3 eV would be 3 times that wouldn't it? ^^
5. does m = rest mass of electron(9.11*10-31)
6. At AS, yes you would have to use that.
7. (Original post by malolis)
No 1 eV=1.6x10^-19 Joules. So 3 eV would be 3 times that wouldn't it? ^^

ye sorry!

8. I get 1x10^-9 but I could be wrong. Check the answer - you said you have it!
9. sounds about right, i normally get something to the ^-10, when working out the de broglie wavelength
10. thank you very much
11. (Original post by malolis)
I get 1x10^-9 but I could be wrong. Check the answer - you said you have it!
it was 7.09*10^-10 thanks for your help!!
12. (Original post by Alevelstudent123)
it was 7.09*10^-10 thanks for your help!!
Yes thats right, I forgot to multiply by 2 when working out the velocity from ke=0.5mv^2 ^^

I fail...
13. i dont suppose you no how to do this do you?

"A photon has energy 2.00 eV. Find its wavelength."

cos i got no idea!
14. if you convert the energy into joules, the use the equation p=E/c, to find the momentum, then use h/p=de broglie wavelength, i think that'll give you the correct answer

(Original post by Alevelstudent123)
i dont suppose you no how to do this do you?

"A photon has energy 2.00 eV. Find its wavelength."

cos i got no idea!
15. (Original post by Alevelstudent123)
i dont suppose you no how to do this do you?

"A photon has energy 2.00 eV. Find its wavelength."

cos i got no idea!
You could just use E=hf and c= fλ to get E= hc/λ i.e. λ = hc/E
16. thanks a lot you really are a great help!

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Updated: November 16, 2008
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