# Speed of a 796eV electron

• skibum143
In summary, the conversation discusses finding the speed of a 796eV electron using the equations E = hc/lambda and E = hv. The participants of the conversation struggle with unit conversions and solving for the correct value of velocity. After some attempts and suggestions, they eventually figure out the correct conversion and solve for the velocity to be 4.18E16 m/s.

## Homework Statement

What is the speed of a 796eV electron?

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...

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...

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...

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?

So far OK.

Seems to me like somehow you are misplacing 2 when calculating value.

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...

Seems like my guess was wrong. You have ignored my earlier remark about using correct units. Sigh.

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.

Nevermind, I got it myself. I was using the wrong value to convert eV. Thanks anyway

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).