In electronics voltage is often notated by the letter E (Electromotive force). In the automotive business most companies use V which is self explanatory. But the Europeans use the letter U for voltage. Such as a specification U=5. Or to denote battery voltage U[sub]batt[/sub]
One of my students asked me why the Europeans use the letter U. I asked a few of our technical people, and the only answer I got is that is what they learned in school.
Keeping in mind that Dopers know everything, I ask why / how did the letter U come to note voltage?
Man, good luck with that. I did an extensive search and couldn’t find anything. I did see several EE discussion boards ask the same thing and even the EEs in europe couldn’t say why they use it.
Well I don’t know about physics but I think this is done for the sake of clarity.
Because E denotes energy, it’s a bit overused, and since there’s also E in electric field strength (voltage per distance, typical unit V/m), it tends to get a little confusing. Traditionally in SI the quantity (here voltage) and its unit (like volt) are kept separate, which means using different characters for either: current (I) is measured in amperes (A) and resistance ® is measured in ohms (the upper-case omega). Having V to mean both quantity and unit sort of violates the spirit. Yet this use was apparently the norm early on. For this reason (and from here on, I might be wrong) some Germans took freedoms and started calling voltage “U”, probably since that letter was largely unused and so couldn’t be confused with anything else. They also came up with etymology: U is for Unterschied, which is German and means “difference”; very fitting since voltage is obviously the same as potential difference. Unfortunately, this German connection caused it not to receive worldwide popularity as most Romance languages and English didn’t follow suit. Thus the U for voltage is mainly used only in Central, Northern and Eastern Europe (and yes, the techies you asked were right, we write U here because that’s what we learned in school). Thankfully, this customary difference/Unterschied is unlikely to cause serious intercultural misunderstandings: if a Russian writes R=U/I while a Spanish person writes R=V/I the context should make it obvious that it’s the Ohm’s law that’s beeing talked about.
So my theory that Europeans use only Kosher electrons is right out the window. 
I’m not sure this was the norm in English from early on. “Volt” as the name of the unit was first formally adopted - by a British Association for the Advancement of Science committee - in 1873 and its recommendation was then followed by the first international electrical congress in 1881. It really wouldn’t surprise me if someone had proposed the name before that, but, on the other hand, such standards committees do often pluck names out of the air. My impression is that the name was then widely adopted without opposition.
On the other hand, a cursory survey of the small handful of late 19th and early 20th century electrical texts I have to hand suggests that, in Britain at least, the customary notation for the quantity was E, from EMF as suggested by Rick in the OP. At some point there must have been a shift, since physicists now do predominately use V, but this shift probably happened in the 20th century.
However, your suggestion that the continental U relates to Unterschied is almost certainly correct. But I’d guess that the sequence was more that that was a perfectly reasonable term for German physicists to use to describe the concept in the first place and that U was then just an abbreviation for that, rather than the name being invented after the notation.
Of course, both sequences are possibly testable hypotheses.
Thanks guys, needless to say you guys rock.
I now have an answer for my smart ass students who ask these questions.
I will pass this thread along to my coworkers and dazzle them with just how smart I am. 
While we’re on the subject, why is current represented by “I”? You’d think someone with an electrical engineering degree would know; apparently not.
I is for the french word “Intensitie” which is close enough to its english equivalent that you should be able to figure it out from there.
To illustrate, in the American usage, one will often see something like
V = 6V
which would mean “Electric potential is equal to six volts”. But it’s easy to see how such a notation could be confusing; it looks like the penultimate line of one of those fallacious “1=2” proofs. V as the symbol for the unit “volt” is internationally established and recognized, but there’s no official standard for what to call an electric potential difference, so one is free to relabel that for clarity.
Or a line of program code meaning "the new number in register “V” is equal to 6 times the old number in “V.”
I can’t think of any language where multiplication is notated by concatenation: If you want to have multi-letter variable names (and you have to, if you want any degree of readability) the parser can’t decide if “ab” is “a” times “b” or the variable “ab”.
That said, I think I have dim recollections of some kind of calculator program I’ve used that allowed multiplication to be notated by concatenation. But I can’t find any such thing in a quick Googling.
Picky, picky, picky. 
The operator would have to be specified but that’s a mere detail.