# Speed of a 796eV electron

#### skibum143

1. Homework Statement

What is the speed of a 796eV electron?

2. Homework Equations

E = hc/lambda
E = hv

3. 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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#### rock.freak667

Homework Helper
That energy is entirely kinetic.

Ek=1/2 mv2

#### skibum143

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...

Mentor

#### skibum143

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...

Mentor

#### skibum143

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...

#### Borek

Mentor
Show how you solved for v.

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#### skibum143

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

Am I doing this wrong?

#### Borek

Mentor
So far OK.

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

#### skibum143

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...

#### Borek

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

#### skibum143

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.

#### skibum143

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

#### Borek

Mentor
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).