## Sunday, November 20, 2011

### Units

I needed to calculate a free energy today. I knew the equation I needed to use. And it involved Faraday's constant. I looked at a table of F values (with various units) and none of them seemed to make sense. I figured out how to get free energy in units of (mv*C*kcal)/mol, which seemed fairly unreasonable. So finally, after thinking through several possible values/units of F, I gave up and e-mailed my PI. Turns out the solution to my problems is knowing that volt=joule/coulomb. Somehow I don't think I EVER knew this.

So of course, I head to Wikipedia, where I find out that not only can joules be defined as kg*m2/s2 i.e. Newton meters,  they can also be defined as the work required to move a charge of one coulomb through one volt potential difference! Obviously my physics teachers failed to prepare me for real life, because I swear I've only ever heard the Newton meter definition.

#### 1 comment:

1. I just had the same issue come up in my undergrad class, where the kids were supposed to convert eV into Joules, and the key is knowing that e=1.6e-19 C, and that CV=J. When I asked how they would write 1 J, I only got the kg*m^2/s^2 definition, nobody knew the electrostatic version of a Joule (throught the work done to move a Coulomb of charge across a potential difference of 1 V). So you are not alone! In my class of nearly 70 nobody knew (and these are techy kids).