an observation on Ohms Law

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curious, at what point in time did E transpose to V?

I saw E expressed as V in Ohms law recently and I thought it was a mistake. But when I googled I came across a few sites that express it like this.

I learned that voltage was expressed as E

E= Voltage or (E)nergy
I= Current or (I)ntensity
R= Resistance (obvious)
P= Power (obvious)

P=IE and E=IR or Pie and Eir, Pioneer, get it? snicker snicker (yes my electronics teacher was a super dork). Actually he was smart, he pointed out that its important to not look at E as voltage per se, but as energy, sighting a cross reference to water and air flow. His point was to teach us not to think of Ohms Law as an "electrical" formula, but as a physical law of nature.

I know its just academic, but this was one of those tiny things that helped me to think independently.
 
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as I recall, E usually means "potential" (what it is relative to, I don't recall). so voltage between two points is really the difference between the two points' potential.

or V = delta E.

Potential itself will not cause electrons to flow. But he difference in potential will.
 
millwood said:
as I recall, E usually means "potential" (what it is relative to, I don't recall). so voltage between two points is really the difference between the two points' potential.

or V = delta E.

Potential itself will not cause electrons to flow. But he difference in potential will.


Interestingly, in some countries like Sweden and Germany,
we use U for voltage and V for potential. To add to the
confusion, E is often used for the voltage of a voltage source.

Just don't ask me where the U comes from originally, but I
suspect it has its origin in Germany.
 
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Christer said:



Interestingly, in some countries like Sweden and Germany,
we use U for voltage and V for potential. To add to the
confusion, E is often used for the voltage of a voltage source.

Just don't ask me where the U comes from originally, but I
suspect it has its origin in Germany.

YES, thats rite 'U' is incarnation of German Engineering.

Compliments to Christer
ampman
 
here in SG for physics class...we have this crappy ohms law wheel thingy...basically it's based om P=VI and V=IR...we dp use E and V interchangeably though...but for clarity...V is used most of the time during theory and test papers...wonder why the E and V thingy...perhaps originally it was like Current...represented by I and has a unit of A...so Potential Difference would be represented by E and measured in V...I dunno..just speculating...
 

rif

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I believe it's a difference between physicist's and engineer's conventions is all. Same thing with imaginary numbers: physicists use i, engineer's use j but they're both sqrt(-1).

What else is different -- hmm...tough to remember so far back to university.

**Maybe vector notation--putting the arrow above or below the variable?

**mhos vs Siemens?
 
Since I am a physicist AND an engineer I might be able to add to the confusion.

There is actually a well established notation for most of these things. However the notation depends on the subject (which litteraly means I change the notation depening on what I am calculating).

*E is usually energy, this is an absolute number measued in Joules (J)
*U is potential energy which is the relative energy.
Since voltage is the potential energy of an electron (except for a constant, e=1.6022e-19) you also use it to denote electric potential.

In electromagnetic theory E is the electric field and V is used to denote voltage. In order to avoid confusion E is not used to denote energy, usually Q is used instead.

Finally E can also be the be used to denote the electromotive force of a battery, when you say you have a 12V battery you are really talking about E. Approximately U=E-RI where R is the inner resistancs of the battery.

and mho has never been a "real" unit AFAIK, the SI unit is Siemens and that is the only "legal" unit.
 
Tobbe_L said:
Since I am a physicist AND an engineer I might be able to add to the confusion.



and mho has never been a "real" unit AFAIK, the SI unit is Siemens and that is the only "legal" unit.
:

It takes an expert to totally confuse us.:D

The mho was real in the uk. It was one of the words on "Call My Bluff", a gameshow where a team has to pick the correct definition of a word taken from the Oxford English Dictionary, against two bogus definitions. The contestants didn't believe that Mho was the inverse of Ohm.:D
 

rif

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I remember now: E (cursive font) is for emf (also called electromotance) is the integral around a closed loop of the force per unit charge. After doing some simple calculus, it equates to IR.

V is potential difference between 2 points. Often they're used interchangably.

BTW: this is related to another thread I've got in Pass forums, which I still ahven't answered 100% yet.
 

rif

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Tobbe_L said:
mho has never been a "real" unit AFAIK, the SI unit is Siemens and that is the only "legal" unit.

don't know either, but I prefer mho, not that I use it often!

Why? Well Mho has some relevance to the actual physical meaning -- Siemens is a name. Same thing for celsius vs. centigrade.

I guess now that I think of it, I'm a unit elitist! Get there first for a non-derived unit and you get to name it after yourself! :D Or does the naimng only happen after death :dead: ?

Maxwell's equations are the most important thing out there re E&M and none of the units are named after him!
 
Ohm is a name as well so I don't really see your point. Almost all units are named after some famous scientist.

The thing with units is that is is not really notation. You MUST use the "real" unit in many (official) situations since they are a part of SI. You are free to use whatever notation you want in most cases (even though there might be recommendations) but you have to use the proper units.

In physics the recommendtaions coming from the big publishers (APS, IOP) tend to set the "rules", since you have to use the notation and units they specify when publishing papers.

The most confusing part of physics when it comes to units is magnetism, there are have dozens of units that are still in use for some strange reason.
The SI unit is Tesla, Gauss is common (and easy to translate to Tesla) but there is also Oerstedt and a few others, sometimes the conversion factors are really odd. Where I work the poularity of the units vary between the buildings.
 
rif said:


don't know either, but I prefer mho, not that I use it often!

My impression is that mho is frequently used in english, but
very rare, if used at all, in languages like german and swedish.


Why? Well Mho has some relevance to the actual physical meaning -- Siemens is a name. Same thing for celsius vs. centigrade.

Well, you just reverse Ohm, which is anyway the name of
a person. To be consistent maybe you should write it
entirely backwards BTW, that is 17 S should be written as
mho 71. :)

Most units are named after scientists and have no obvious
alternative name. I agree Centigrade might make some sense,
but since Celsius was swedish I feel obliged to use his name
for the unit. :) BTW, what would you call Kelvins if you want
a similar logical name. Absolute centigrades maybe?


I guess now that I think of it, I'm a unit elitist! Get there first for a non-derived unit and you get to name it after yourself! :D Or does the naimng only happen after death :dead: ?

No, I recently suggested the name Dieckmann for a new unit
in another thread, and AFAIK Fred survived his recent nose
surgery. :)

Otherwise, there are at least people who have been honoured
to get a constant named after them and who are still alive.
 

rif

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OK, I was only trying to be a little lightheated
:cannotbe: You all are correct, I really didn't think it all through :drink: :c_flag:



Christer said:

BTW, what would you call Kelvins if you want
a similar logical name. Absolute centigrades maybe?


Hmmm. I dunno -- that's a good one. It's called Kelvin b/c of his work in defining absolute zero? If so, that's a ways off from the water range:) IIRC, is the proper unit Kelvin (without "degree"). So a temp is 200 Kelvin.

We could always make up a new system!
 
E=-grad(V)

Well, difficult to express oneself without a scientific word processor ;), but I will try to contribute to ambient noise with my school memories :D

In EM theory, E is the electromagnetic field (Volts/meters), and is a vector quantity. It should be written with an arrow above it :)

E is derived from a scalar quantity V, which is the scalar potential (in Volts) with the vectorial operator gradient (roughly a space derivative) through the relation
E=-grad V

But there's also a vector potential (noted A) :D

The uppercase E without the vector's arrow often refers to the modulus of the vector.

But as Tobbe_L previously said, the letters use often depend on what part of physics you are referring to...

To go on with EM, and to avoid confusion with the E field, the energy is often noted as W (Joules), and the power as P in Watts (Joules/seconds). Note also that P, the polarization of a medium, which is a vector quantity, is used at the same time in EM...

The famous U is (in France at last) the difference of potential (Volts) between two points. So the correct french translation for Ohm's law should be U=RI

where... I is the current in Amperes, or more precisely in Coulombs/seconds :D So the Ohm unity should be a Volt.Second/Coulomb :D:D:D

Lowercase e is often used in electricity to detone the electromotive force, apart from the exponentiation and the electron's charge....

And last, Ohm's law can be derived from Maxwell's equations (which themselves can be derived from thermodynamics...), where E, B, D, H, J all mix up to reduce to scalar quantities in Ohm's law...

So don't forget your bow, and watch your "shift" key...

Sorry to be so unhelpful :confused:
 

rif

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Re: E=-grad(V)

CheffDeGaar said:

And last, Ohm's law can be derived from Maxwell's equations (which themselves can be derived from thermodynamics...), :confused:


That doesn't sound familiar to me at all from my stat mech/thermo classes. Are you sure? I'll try to find my old notes too
:xeye:

I know he did some significant work in the field (thermo/stat mech) but didn't think his fundamental EM derived from it.

(BTW -- Maxwells equation are the only physical laws that hold true in the "classical", quantum, relativistic, etc. limits :yes: and are in that sense mroe fundamental than all others!!)
 
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