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Topic: Ohm's Law (Read 2675 times) previous topic - next topic

kf2qd



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 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.


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...

outofoptions


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.

Shpaget

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.

KeithRB

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.

Shpaget

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

JimboZA

Quote
In any case, it strikes me as beneficial not to use same letter for physical quantity and the unit of that physical quantity.


Yes, but abbreviations aside, it is a peculiarity of the quantity Voltage that its measure is Volts; Power isn't measured in Powers, nor do we measure mass in masses. Although we do... damn.... atomic mass units. Oh and to complicate things further, we do sometimes talk of amperage. and Wattage. Although the latter usually refers to a capacity not the actula value at an instant.

This is more complicated than I thought, so as soon as my son comes in from night-shift I'm going back to bed....
My hovercraft is full of eels.

No PMs for help please.
DO NOT power servos from Arduino 5V: give them their own power and connect the grounds.

Grumpy_Mike


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.

How can you get the answer wrong by confusing E and V, all the other quantities in the formular are the same?

Shpaget


it is a peculiarity of the quantity Voltage that its measure is Volts


Not in all languages. For example:
In Croatian quantity is "Napon", unit is still "Volt". Also, in Croatian, current (the thing measured in A is called "Jakost", which would translate to "Strength".
In German "Spannung" / "Volt"
In French "Tension / "Volt"
In Chinese, according to Google Translate, ?? (prononced something like "tiamya") / ?? (prononced something like "fut h")

Grumpy_Mike

I think William Shakespeare said:-

A volt by any other name would still cause an amp to flow through a resistance of one ohm. 

MisterResistor

Writers are into poems not ohms. :smiley-roll:

Krupski


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.


Me too. Voltage to me is "E", not "V".
Gentlemen may prefer Blondes, but Real Men prefer Redheads!

michinyon

I use V for voltage  and E for electromotive force.

MisterResistor

If you found yourself across 440 3 phase, do you think about V or E?

KeithRB

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How can you get the answer wrong by confusing E and V, all the other quantities in the formular are the same?


They initilialized E, and used V for the Loop. Since it was FORTRAN, declaration was not required.

68tjs

Have a look at :
http://www.metricationmatters.com/docs/MetricationTimeline.pdf

This book was writen in english and explain how the actual international system (SI in french and IS in english) was create.
1) At the begining was american scientists (Benjamin Franklin, Thomas Jefferson, Georges Washington) and some French scientists.
2) The american scientists fail to convince the U.S people  to use their system.
3) French-Revolutionary scientists continue to develop it and succeed to apply it.

The first main international système unity was named  "mksA" :
m for meter
k for kilo gramme
s for second
A for ampères

All others unity derivate from this four mains.
Now the name of the official system was Système International (SI) ( IS in english ) and head-quater was always in "Pavillon de Sèvres" (France).

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