What Do We Measure?
There’s this very common measurement equiptment, found in almost any electronics lab, used to measure voltage. Sometimes it is called a potentiometer, sometimes fluke, sometimes otherwise; the idea, in all cases, is to know the potential difference between two points. The question is, what potential?
Potential is primarily a mathematical concept, used in calculus to derive fields and easily describe various natural phenomena. In physics, it can be regarded as sort of an abstraction level: we don’t want to deal with the actual forces, fields, or flaws, so we hide them behind the relatively simple ‘potential function’. In electricity, for example, one can think of an ‘electrostatic potential’ that describes the electric forces in space; in mechanics you can think of a ‘grativational potential’, describing the gravitational field; in thermodynamics, one often thinks of a ‘thermodynamic potential’ (usually involving the concentration difference of a species and some temperature effect); in chemistry, a notorious ‘chemical potential’ can conceptualize the probability of reactions to take place, and so forth.
Now, one issue that recently arose in the lab was: what potential does the potentiometer measure?
The answer is not very simple, and is somewhat surprising. To my best understanding, a common potentiostat does not measure the electrostatic potential. It probably measures something much more complicated, called an electrochemical potential. One part of this potential is the electrostatic field; but other parts take into account mobility and diffusion constants, and temperature. Shortly, all the forces that usually cause movements to a particle in the matter are taken into acount.
This is somewhat surprising, because on my first degree I studied only about the electrostatic part, but an important part of being a research student is to actually try and understand what’s going on; this is usually much more complicated then initially grasped.
To demonstrate this, I just give here a nice picture, from a different field, that I found recently. This is a model of how a cell membrane looks. It is a bit more complicated then the ‘chop suey’ model I wrote about a while ago, and I will not go into the details (just note, that there are much more proteins in the membrane than most people usually think) - just enjoy, for the moment, this nice picture.
