Calculating the Speed of a 796eV Electron

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To calculate the speed of a 796 eV electron, it's essential to convert the energy from electronvolts to joules, as 1 eV equals approximately 1.602e-19 joules. The kinetic energy formula, Ek = 1/2 mv^2, can be rearranged to solve for speed, v = sqrt(2Ek/m). The mass of the electron is 9.11E-31 kg, and using the correct conversion for energy leads to the proper calculation of speed. Misunderstandings arose regarding unit conversions and the application of the kinetic energy formula, which were clarified in the discussion.
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Homework Statement



What is the speed of a 796eV electron?

Homework Equations



E = hc/lambda
E = hv

The Attempt at a Solution



I'm not sure how to get a value for meters... I know if I take h (plank's constant) and divide the electron, I will get 4.14E-15 ev*s / 796 eV = 5.20E-18 seconds, but that is just a time, i don't know how to get the distance for that time...
 
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That energy is entirely kinetic.

Ek=1/2 mv2
 
If 796 = 1/2 * 9.11E-31 * v^2, I get v to be 4.18E16 m/s which is wrong...i'm not sure what I'm doing wrong...
 
Watch your units.
 
I tried converting eV to joules, which gave me 1.27E-16 joules, which gave me 8.36E6 m/s but that is wrong too. I think the units for mass (kg) are correct, I'm not sure what I have wrong...
 
Check your math.
 
For:
796 = 1/2 * mv^2 (where m = 9.11E-31) I keep getting 4.18E16 for v

I don't understand what part I'm doing wrong, I've done it about ten times...
 
Show how you solved for v.

--
 
eV = 1/2 mv^2
eV / (1/2m) = v^2
v = sqrt (eV / 1/2m)

Am I doing this wrong?
 
  • #10
So far OK.

Seems to me like somehow you are misplacing 2 when calculating value.
 
  • #11
I'm using
m = 9.11E-31
eV = 796

796 = 1/2*9.11E-31 v^2
796 = 4.55E-31 v^2
1.748E33 = v^2
v = 4.18E16

Don't know where I'm wrong...
 
  • #12
Seems like my guess was wrong. You have ignored my earlier remark about using correct units. Sigh.
 
  • #13
I didn't ignore it, I don't understand where my units are wrong. eV, m = kg

If you would tell me which part I'm doing wrong I could try and fix it.
 
  • #14
Nevermind, I got it myself. I was using the wrong value to convert eV. Thanks anyway
 
  • #15
skibum143 said:
I didn't ignore it, I don't understand where my units are wrong. eV, m = kg

You have to convert eV to J, from what you wrote you were assuming 796 [eV] = 1/2 9.11E-31 [kg] * x2 [m/s]2. eV is NOT kg*m2*s-2, it is about 1.602e-19 kg*m2*s-2 (or 1.602e-19 J).
 

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