# Ohm's Law

On these forums I frequently see Ohm’s Law stated as V = I * R,
however all my life I have been thought that it is U = I * R.

I realize this may be considered nitpicking but in all of my books V != U.
V is unit
U is physical quantity

What follows is that if V is used in the equation, A and ? should be used instead of I and R.

Could this be region (I live in Croatia) specific? Have I been thought wrong?

My US college text books in the early 80's used V=IR, and that's what I've used ever since.

http://en.wikipedia.org/wiki/Ohm's_law

I can't find an explanation of why I for current tho. Like many things, it was defined well before my time 8)

Must be a language thing. It was always E = I * R to me. E (Electromotive force [emf]) equals I (represented by current in amperes) * Resistance (in ohms). It's all good.

Wikipedia has this for I, seems reasonable to me

“The conventional symbol for current is I , which originates from the French phrase’ intensité de courant’, or in English ‘current intensity’. This phrase is frequently used when discussing the value of an electric current, but modern practice often shortens this to simply current. The symbol was used by André-Marie Ampère, after whom the unit of electric current is named, in formulating the eponymous Ampère’s force law which he discovered in 1820. The notation travelled from France to Britain, where it became standard, although at least one journal did not change from using C to I until 1896.”

Also goes along with P=IV http://en.wikipedia.org/wiki/Electric_power

& by substition P = (I^2)R = (V^2)/R

Hey, I remember when Hertz was what it did after your took an inside pitch to the head. It was Cycles Per Second, up until the 70's. ;)

I think you are reading too much into the letters being used.

From my experience at looking at datasheets, most of the world uses V or E for "volts" and European countries tend to use "U".

CrossRoads: My US college text books in the early 80's used V=IR, and that's what I've used ever since.

http://en.wikipedia.org/wiki/Ohm's_law

I can't find an explanation of why I for current tho. Like many things, it was defined well before my time 8)

I was told that "i" was already used for "imaginary" number in mathematics and since that comes into play in AC circuits another symbol was chosen. The wiki thing sounds reasonable too. Who knows?

Learning: Hey, I remember when Hertz was what it did after your took an inside pitch to the head. It was Cycles Per Second, up until the 70's. ;)

Evidently there was a lot of resistance to changing from CPS to Hz. I remember a tongue-in-cheek article in QST magazine about how to convert cycles-per-second to Hertz, complete with nomographs.

outofoptions:

CrossRoads: My US college text books in the early 80's used V=IR, and that's what I've used ever since.

http://en.wikipedia.org/wiki/Ohm's_law

I can't find an explanation of why I for current tho. Like many things, it was defined well before my time 8)

I was told that "i" was already used for "imaginary" number in mathematics and since that comes into play in AC circuits another symbol was chosen. The wiki thing sounds reasonable too. Who knows?

EEs use "j" for the square root of -1 instead of "i" to avoid confusion with current.

I had an older electronics teacher in the 70's who insisted that Hz was "cycles" so it was correctly "Hz per sec". When I pointed out that my physics text said Hz was cycles per sec he informed me that "This was electronics, not physics". :roll_eyes:

Define older. :astonished:

Well, he was pushing 60 in the '70s so he learned electronics pre-WWII.

Well as a measure of the rate of change of frequency, Hz / sec would make sense 8), just like m/s2 for linear acceleration.

Could this be region (I live in Croatia) specific?

It could be. I know a lot of continental types have difficult with distinguishing between the two.

most of the world uses V or E for "volts" and European countries tend to use "U".

I knew it, the UK is not really in Europe. :P

We had real problems with an EE professor who was Greek. He kept confusing the Greek and English letters, so we never knew if when he said "beta" he meant "beta" or "b"

Learning: Hey, I remember when Hertz was what it did after your took an inside pitch to the head. It was Cycles Per Second, up until the 70's. ;)

Evidently there was a lot of resistance to changing from CPS to Hz. I remember a tongue-in-cheek article in QST magazine about how to convert cycles-per-second to Hertz, complete with nomographs.

outofoptions:

CrossRoads: My US college text books in the early 80's used V=IR, and that's what I've used ever since.

http://en.wikipedia.org/wiki/Ohm's_law

I can't find an explanation of why I for current tho. Like many things, it was defined well before my time 8)

I was told that "i" was already used for "imaginary" number in mathematics and since that comes into play in AC circuits another symbol was chosen. The wiki thing sounds reasonable too. Who knows?

EEs use "j" for the square root of -1 instead of "i" to avoid confusion with current. [/quote]

Except that i, j & k are the imaginary part of vectors and match up with X, Y & Z, a vector was commonly written as (X + i, Y + j, Z + k).

So you have to be careful how you use them as you might just be sending some other message. Like trying to speak English with an English man, and Irish man, and American, an Aussie & an Kiwi. Lots of talk but little comprehension...

JimboZA:
Well as a measure of the rate of change of frequency, Hz / sec would make sense 8), just like m/s2 for linear acceleration.

Well, as you age, hurts per second makes perfect sense.

What do you know… I’m absent for a day and entire page of answers greets me.
Interesting to hear others having come across different notation, too.

I don’t know why this region decided to use U; it probably comes from notation for potential energy (which, by the way, I was always thought to be Ep and nothing else).
In any case, it strikes me as beneficial not to use same letter for physical quantity and the unit of that physical quantity.

Why?

In "Elements of Programming Style", Kernighan and Plauger point out many example programs (from textbooks!) that confuse E and V and get the values wrong accordingly.

I would sooner attribute that to the lack of properly established standard, than to usage of any particular letter.