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### Topic: Ohm's Law (Read 10346 times)previous topic - next topic

#### Shpaget

##### Mar 27, 2013, 08:21 pm
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?

#1
##### Mar 27, 2013, 08:27 pmLast Edit: Mar 27, 2013, 08:34 pm by CrossRoads Reason: 1
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
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

#### Learning

#2
##### Mar 27, 2013, 08:30 pm
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.

#3
##### Mar 27, 2013, 08:36 pm
Wikipedia has this for I, seems reasonable to me

http://en.wikipedia.org/wiki/Electric_current

"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."
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

#4
##### Mar 27, 2013, 08:43 pm
Also goes along with P=IV
http://en.wikipedia.org/wiki/Electric_power

& by substition P = (I^2)R = (V^2)/R
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

#### Learning

#5
##### Mar 27, 2013, 09:00 pm
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.

#### cmiyc

#6
##### Mar 27, 2013, 10:02 pm
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".

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#### outofoptions

#7
##### Mar 27, 2013, 11:08 pm

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
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?

#### Jack Christensen

#8
##### Mar 27, 2013, 11:28 pm

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.

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
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.

#### KeithRB

#9
##### Mar 28, 2013, 12:29 am
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".

#10
Define older.

#### KeithRB

#11
##### Mar 28, 2013, 03:59 pm
Well, he was pushing 60 in the '70s so he learned electronics pre-WWII.

#### JimboZA

#12
##### Mar 28, 2013, 04:10 pm
Well as a measure of the rate of change of frequency, Hz / sec would make sense  , just like m/s2 for linear acceleration.
Johannesburg hams call me: ZS6JMB on Highveld rep 145.7875 (-600 & 88.5 tone)

#### Grumpy_Mike

#13
##### Mar 28, 2013, 05:56 pm
Quote
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.

Quote
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.

#### KeithRB

#14
##### Mar 28, 2013, 06:08 pm
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"

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