I believe in Europe the letter U is commonly used for voltage in (eg.) Ohm's law \$U = I × R \$. I think I understand where the letter V came from, commonly used in North America. But what's the story with U?
The best reason I've heard is to avoid this: -
V = 2 V (which of course is meant to say "voltage = 2 volts")
U = 2 V sounds more sensible after all we use a different symbol for current (I) and also amps. Voltage is a bit on its own - we wouldn't say "amps = 2 amps" or "current = 2 currents".
It seems to me this is the sensible reason for choosing U over V but having said that I never use "U"! Maybe I should?
I found another explaination here:
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.
So it's U for Unterschied (which means "difference")
Voltage is a difference.
In German "difference" is "Unterschied".
The chain of thought is that there is a difference in number of free electrons between two places.
Electrons that can move are free to move. If there are many such free electrons we call that a "charge".
Analogy: Imagine a train with two carriages full of people, say 40 + 40. If a school class (20 pupils) leaves one of the cars, people will move to use empty space and spread evenly in the train.
So voltage tells us the difference in number of electrons that can move and spread evenly between two places.
Since electricity goes back to Georg Ohm in Germany the explanation fits. Sadly, it is too late to ask late Mr. Ohm if it is true or not.
But I have noticed that my students find it helpful.
I my teaching I use E for “voltage rise” i.e. source of electrons that are free to move (battery, capacitor) and U for “voltage drops” (resistors).
This gives an advantage in analyzing circuitry as now I can compare electrical circuits to things my students are already familiar with like water circulating in a fountain or even income and payments.
Doesn't answer where U comes from but here is a similar discussion:
Q: voltage symbol u or v? In German Physics books: I = U/R means I[A] = U[V]/R[Ohm] It seems to be that in English you would write: I[A] = V[V]/R[Ohm] Right or wrong?
I liked these three comments
PhD., Eng.; R&D Magnetic and Power Electronic Engineer, Project Leader at ABB PL Corporate Research Center
Both of voltage description "U" and "V" are proper, however it must be mentioned that in European notation "U" describes voltage source while "V" describes rather voltage potential. It means that U = V1 - V1 (voltage is a difference between voltage potentials). I agree that in IEEE and American standards voltage is described by "V" letter.
Very similar situation is with other electric symbols also (e.g. resistors, capacitors, current sources, etc.), where European and American standards are different.
Supervisory Board Member at JP Energetika Maribor d.o.o.
Based on experience with writing articles I can conclude the following: For European scientific space U and I are signs for average value of voltage and current respectively and u, i are signs for instantaneous values of voltage and current. U is more appropriate to use not to mix the parameter U with its value in V (volts).
I don't know if US or IEEE standards, or any other standards for that sake, are more rightful than other regional standards. However, I learned to use U for voltage at school and personally I think U = 5 V makes more sense than V = 5 V, but I'm flexible
Some German textbooks claim that the origin of the symbol U is unknown. One possible explanation is that it comes from the Latin word urgere which can mean
- press/squeeze/bear hard/down
- tread/traverse continually