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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?

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Sloppily drawn V? –  Olin Lathrop Feb 12 at 17:42
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In Physics, U is used for potential energy, so that's....something. –  dext0rb Feb 12 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. –  Ignacio Vazquez-Abrams Feb 12 at 18:14
    
@IgnacioVazquez-Abrams I think it was way before ISO and IEC times that the letter was introduced. –  jippie Feb 12 at 18:19
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My teacher told me U is used because V was already taken by velocity –  Tom Jul 2 at 11:42

4 Answers 4

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?

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Kind of the same reason why we use j instead of i for imaginary numbers (\$\sqrt{-1}\$). –  Ricardo Feb 12 at 18:37
    
Note that in any properly typeset document it should be well enough distinguishable: V = 2 V, or I = 3 A. –  leftaroundabout Feb 12 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!! –  Andy aka Feb 12 at 22:33
    
As you know, many lazy typists have taken to using "u" to avoid typing two additional letters to write "you" - so maybe using it for voltages would be confusing to them. On that note, please go right ahead! –  JYelton Feb 13 at 2:11
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Also, the U in Physics was traditionally used for "Potential Energy", which is exactly what "voltage" is. –  jose.angel.jimenez Feb 19 at 8:37

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

Radoslaw J.
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

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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
  3. tread/traverse continually
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The reason is that U is a Greek letter whilst V is a Latin letter. But both are correct if we refer to Voltage...

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Considering this is an old answer while I'm not 100% sure your answer is incorrect backing it up with further information may be a good idea. –  PeterJ Jul 2 at 11:17
    
U is definitely not a Greek letter. –  Matt Young Jul 2 at 12:15
    
U and V originally were one letter "V". They were separated in the late antique and gained their late usage during renaissance. –  Ariser Nov 24 at 13:28

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