# Where does U for voltage come from?

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?

• Sloppily drawn V? Feb 12, 2014 at 17:42
• In Physics, U is used for potential energy, so that's....something. Feb 12, 2014 at 18:12
• I've tracked it down to ISO 31, IEC 60027, and ISO/IEC 80000, but I don't have thousands of dollars to spend on something that may or may not be useful in answering the question. Feb 12, 2014 at 18:14
• Just to add further confusion: I originally learned Ohm's law as E=IR (in the US in the 1960s)
– Tut
Feb 12, 2014 at 18:44
• My teacher told me U is used because V was already taken by velocity
– Tom
Jul 2, 2014 at 11:42

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?

• Kind of the same reason why we use j instead of i for imaginary numbers ($\sqrt{-1}$). Feb 12, 2014 at 18:37
• Note that in any properly typeset document it should be well enough distinguishable: V = 2 V, or I = 3 A. Feb 12, 2014 at 22:22
• @leftaroundabout I think the point is that for V to equal 2 V, V must equal zero i.e. it is an algebraic nonsence that also doesn't look good for the uninitiated!! Feb 12, 2014 at 22:33
• Also, the U in Physics was traditionally used for "Potential Energy", which is exactly what "voltage" is. Feb 19, 2014 at 8:37
• @jose.angel.jimenez No, voltage is Potential Energy Per Unit Charge, not just Potential Energy. Mar 9, 2015 at 15:22

I found another explanation 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")

• Interesting find, although for German I would expect differenz to be used. May 2, 2015 at 10:30
• Over the decades I have heard lots of explanations for U, but never that one. I find U for urgere the most plausible, since back in the days, people did love to use latin. May 2, 2015 at 19:53

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.

Dejan K.
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).

Per L.

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

• So from IEEE, should we use V instead of U, even if U seems a bit better? Should we use always V since it's in standards? Jan 8, 2021 at 19:34

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.

• In math, you would say "differenz." "Unterschied" is more when discussing things or people.
– JRE
Feb 9, 2018 at 16:11

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

1. press/squeeze/bear hard/down
2. push/shove/thrust

Using V for Voltage would be problematic when working with both units and dimensions. We have a dimension of "length (s,d or l)" with a unit "meter [m]", but having a dimension of "voltage (V)" with a unit of "voltage [V]" would not be fun to work with.

• Why is it not fun?
– Bort
Feb 9, 2018 at 15:08

We have to use U because V was already taken. V stands for Volume and to avoid confusion in calculations we use U.

Simple as that.

• Do you have a source to back this up. Seems kind of illogical to me since volume is not an electrical unit, but a mechanical one. There's no situation in which volume will be in an equation with voltage. Apr 29, 2015 at 21:38