Avogadro's number: 6.02X10^23 ions per mole. The Faraday constant: 96,500 coulombs per mole. The charge on an electron: 1.6E-19 coulombs per electron.
Why this post? A small illustration. The background is this: A college physics exam, "cheat sheet" allowed. I brought my cheat sheet, but it had an omission that resulted in consternation during the exam. For a particular question, I _needed_ to use the fundamental charge on the electron. Sorry, I don't recall the exact question, maybe because it was over 40 years ago. But I clearly recall the process I used to address this particular problem because I had neglected to write down this fundamental constant on my cheat sheet. Panic! Woe! At least, initially. Then I thought about it. I did have some information that would allow me to calculate the charge of an electron. Yes, I was fairly certain that I knew the value: but I was in a bit of a panic at the time. How to verify? I happened to have two other important constants on my cheat sheet (but I was certain of them anyway). One was Avogadro's number, the number of atoms per mole of a substance: 6.02X10^23. And the other number, Faraday's constant, which gives the number of coulombs ( a measure of charge) per mole. Dividing Faraday's number by Avogadro's number gave me the charge on a single ion AKA electron: 1.6X10-19.
I used the calculation to solve that particular test question.
Why would anyone be interested in Faraday's constant, other than someone with a peculiar memory for odd physical constants?
Two industries come to mind. One is the electroplating industry. If we know the surface area of something we want to plate, and the thickness we wish, we can use Faraday's constant to calculate the current * time needed to get that thickness. Oh, yeah: Q (coulombs) = current * time. Why is this important? What if you're plating something expensive, like silver. Electroplaters made a shitpot of electroplated silver pieces for folks because it was relatively cheap: but they needed to very precisely manufacture the pieces, including the thickness of that precious silver layer...too thick and they lost money, too thin and the pieces wore out too soon. Faraday to the rescue.
Another industry: the infant electric utilities. They needed to know how much electricity they had delivered to each customer. If they diverted a small percentage of the delivered current to an electroplating cell, they could determine the amount of power consumed by weighing the amount of silver that had been plated.
In both cases, capitalism demanded it: and physics delivered.
While it might seem that physics and the real world have significantly parted ways lately, that's far from the truth. The LED light bulb is a great illustration (pun intended) of this. It is a story of many different disciplines. I will likely elaborate on this in another post.
Quiz time. Here's another useful "constant" I use regularly: .301030. What is it? No, it's not found in physics or chemistry.
.301030 is the logarithm of 2 to the base 10 or log10^2
ReplyDeletehow do you use this regularly? Just curious.
Hi Robert,
ReplyDeleteI use it as a way of quickly finding the (approximate) magnitude of powers of 2. What's 2^24? 24*.3 = 7.2, so it's a little over 10 million. The old HP15 sez it's 16,777,216 so not too bad for a quick approximation.