# Stopping Potential

1. Feb 5, 2012

### Tsunoyukami

1. The problem statement, all variables and given/known data

I'm doing some practice problems for an introductory quantum mechanics course and am unsure whether or not I'm solving this problem properly - I need confirmation if I'm doing it right and help if I'm doing it wrong! :)

"What is the stopping potential when 250 nm light strikes a zinc plate?" (Chapter 3, #18 in Modern Physics 2nd ed. by Randy Harris)

2. Relevant equations

K = E - $\varphi$ (where K is the kinetic energy, E is the energy of the incident light and $\varphi$ is the work function)

This can be written as:

$\frac{mv^2}{2}$ = $\frac{hc}{\lambda}$ - $\varphi$ (where m is the mass of a scattered electron, v is the speed of this electron, h is Planck's constant and c is the speed of light)

$\frac{mv^2}{2}$ = qV (where q is the electron charge and V is the stopping potential)

h = 6.626 x 10^(-34) Js
c = 3 x 10^8 m/s
$\varphi$ = 4.3 eV

3. The attempt at a solution

If my equations above are correct, I can write:

$\frac{mv^2}{2}$ = $\frac{hc}{\lambda}$ - $\varphi$ = qV
$\frac{hc}{\lambda}$ - $\varphi$ = qV
V = $\frac{\frac{hc}{\lambda} - \varphi}{q}$

I can then simply plug in my values (remembering to either convert h in eV*s or $\varphi$ into J) and this should give me the stopping potential, correct?

Last edited: Feb 5, 2012
2. Feb 5, 2012

### Delphi51

Yes, that all looks good. No quantum mechanics in there, though; its all high school physics.

3. Feb 5, 2012

### Tsunoyukami

Thanks! And you're right, there isn't really any quantum mechanics here; it's just included in part of the course and its part of the introduction leading into the actual quantum mechanics.