Finding energy of electron, given lambda

[schnappy]

Hey all,
I've done this question before but lost my formula sheet i had at the time.

All you're told is that de Broglie lambda = 0.2 nm.

Then find momentum, and then the energy in eV of the electron.

I've found the momentum to be 7.32E-24.

But I'm having trouble getting the correct energy.

Edit: could someone please upload the answers to 2010 Lisachem unit4 physics exam? Thanks.

[Linkage1992]

Use hc / lambda.

see below! 

[schnappy]

Use hc / lambda.

No. That's fundamentally wrong since electrons do not go at the speed of light...

Every method I've tried i get 2.34E20. It sounds wrong.... but is it?

[3Xamz]

Could you upload the question?
I'd have a crack at it

[Linkage1992]

woops sorry, wasn't thinking straight.

What you have to do work out the velocity of the electron. You have the mass and the momentum so you can use P = mv.
Then use E = .5mv^2 to get the kinetic energy which will be in joules.
Then convert this to eVs.

[schnappy]

Arr I see what I did wrong, thanks. Work in joules, until the end convert to eV. It's been way to long since I last did this!

[schnappy]

Actually, I'm still getting it wrong. Can someone PLEASE do it?

[Whatlol]

lambda = h/p
therefore p = 6.63x10^-34 / 0.2x10^-9
             p=3.315x10^-24 kgms^-1
             v= p/m
             v = 3.315x10^-24 /9.1x10^-31
             v = 3642857ms^-1
             Ek =1/2mv^2
             Ek = 1/2 x 9.1x10^-31 x 3652857 ^2
             Ek = 6.038x10^-18 J
            and that is equivilent to 37.7 eV

[schnappy]

Thanks. I asked someone for the answers to the question... I used a result to a previous question. My answer was correct there, but misread a 3 as a 7.

[Whatlol]

Thanks. I asked someone for the answers to the question... I used a result to a previous question. My answer was correct there, but misread a 3 as a 7.

heres the exam with answers at the back

[shokstar]

lambda = h/p
therefore p = 6.63x10^-34 / 0.2x10^-9
             p=3.315x10^-24 kgms^-1
             v= p/m
             v = 3.315x10^-24 /9.1x10^-31
             v = 3642857ms^-1
             Ek =1/2mv^2
             Ek = 1/2 x 9.1x10^-31 x 3652857 ^2
             Ek = 6.038x10^-18 J
            and that is equivilent to 37.7 eV

Or just  Ek=(p²)/2m, and from Ek to p=root(2mXEk)