Yes it's totally different than a resistor, but it still follows Ohm's law. PN junctions due to their molecular interactions cause otherwise conductive materials to exhibit 'virtual insulation' behaviors due to internally created electric fields which prevent electron flow between the two layers under various circumstances and to difference degrees. No they don't behave linearly, but at any given instant of time a semi conductor has definable ohmic state. Basically in a semi conductor the V and I parts of the circuit cause a variable R. It's dynamic, not fixed, but in the end it still follows ohms law, there's just a whole lot of other math tacked on with it.

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hi sceadwian,
You are quite right, I am sure your explanation makes it clear to a student.

I was concerned that a student/newbie would think he could use a ohm meter
etc, to measure the 'resistance' of his semi's.

Once had a univ student on a mid summer work experience, he was trying to find the labs 'megger' [insulation tester].
Asked why, he said he wanted to check the gate input resistance of some FET's he was using, he had been
told it could be megohms!.

Regards
EricG

 

That's a good way of turning a mosfet into a smoke generator, single shot of course =)

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hi all
v=IR is always valid for any material for instance. that is
V(t) = r I(t) hold for any material : r is a function of time
in other words
ΔV = r ΔI

 

Nice JavaScript Ohms Law Calculator

http://ourworld.compuserve.com/homep...en/ohmslaw.htm

 

Removed,
10 pages for formula and not a Thankyou

 

To the Ineffable All,
There seems to be some confusion about what Ohm's law is. The formula V=IR or V=IZ is NOT Ohm's law. It is the resistance or impedance formula. Ohm's law is a property of a material, not a method of calculating current,impedance, or voltage. Read what the physics books say about this.

"We stress that the relationship V=IR is not a statement of Ohm's law. A conductor obeys Ohm's law only if its V--I curve is linear, that is, if R is independent of V and I. The relationship R = V/I remains as the general definition of the resistance of a conductor whether or not the conductor obeys Ohm's law. ..... Ohm's law is a specific property of certain materials and is not a general law of electromagnetism, for example like Gauss's law."
The above snippet is from Physics, by Prof David Halliday, University of Pittsburgh & Prof Robert Resnick,Rensselaer Polytechnic Institute, 1967 , page 780.

And the following.
"Ohm's law states that for many materials (including most metals), the ratio of the current density and electric field is a constant, which is independent of the electric field producing the current.
Materials that obey Ohm's law, and hence demonstrate this linear behavior are said to be ohmic. The electrical behavior of most materials is quite linear for very small changes in the current. Experimentally, one finds that not all materials have this property. Materials that do not obey Ohm's law are said to be nonohmic. Ohm's law is not a fundamental law of nature, but an emperical relationship valid only for certain materials."
The above is from Physics for Scientists and Engineers, Raymond A Serway, James Madison University, Third edition, 1990, page 745.

There you have it. Ohm's law should not be confused with the always correct resistance or impedance formula. It is a property of a material, not a method of calculation. Materials like semiconductors with their bent V--I curves do not obey Ohm's law. Ratch

 

very interesting post on ohms law

 

Observe carefully:
1: R=V/I
2. V=R*I
3. I=V/R

 

Basically, it only applies in metals, not in semiconductors, or more appropriate, devices that current and voltage are non-linear. But it is ABSOLUTELY Ohm's Law. Does it apply in every instance, with every material? No. But it is still Ohm's Law, and is ABSOLUTELY correct to refer to that relationship: V=IR, as Ohm's Law.

Are you debating actual engineering concepts, or semantics?

Most any university engineering (electrical) professor (Ph.D) will tell you, and explain that it is in fact Ohm's law, fundamentally, when applied to metals. Once you get into semiconductors, and other materials, with a non-linear relationship, you will in fact start to see that it does not apply, but it is still HIS LAW. It is applied through observation.

"The greater the voltage, the greater the resulting current. For a large class of conductors, the current increases in direct proportion to the voltage. Physical experimentation leads to the following equation: i = v/R, or, v=Ri, which is know as Ohm's Law. "

Source: Foundations of Electrical Engineering J.R. Cogdell

So, is the law wrong when concerning metals, or as it has been applied here in this thread?

Or, are you just debating semantics?

Seriously, no one in this thread is wrong in stating that those equations are Ohm's Law..... An Ohm, is in fact, a Volt/Ampere.

 

Ohm's "Law" see what I am saying, there is a law for each section of Electronics..

If it were easy everyone would be doing it. You have found the perfect place here my friend. Everyone here is Excellent! Ask the proper questions get the proper answers.

Your other questions need to be answered by:

Lenz's "Law"

No I can't forget Mr. Maxwell!

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Error[888] "while trying to load":[reality.sh] kernel: [Panic!]...{Universe has been Modified!}...

 

hi i'm tom and i'm new here-- and i dont "tipe"too well!
but i have fun learning from others and giving what i can to spread knowledgs.
i have often joked that "laziness" is the mother of invention or creation of new ways of doing stuff!
i dont like using ohms law bwcause it makes me do a bunch of math with sometimes odd numbers.
i know that in elex we have some leeway and dont need to be too exact but my methode is usually exact or as they say "close enuf for govt. work.
we often lite up leds with pic chips with 5v. and a max of 25 ma from the chip.
i see that most leds get 2v at reasonable currents so if the resistor varies its always about 2v which is close enuf. soo, how to get the max of 25 ma just for fun?
first what res. gives 25 ma? well 25 v on 1000ohms is 25 ma without thinking , right?
that means 5v on 200 ohms is the same since 200 is 1/5 of 1000.
now how to get 25 ma with the led using 2v, we have 3v on res. 5v on 200 is like 1v on 40 ohms in our head right? that means the 2 v is on 80 ohms of the led and 3v is on 3x40 or 120 ohms . so 120 ohm +led = 25 ma. or we could say 3v on 120 is like 6v on 240 or then 12 v on 480 and 24 v on 960, theres 40 ohms left with 1 v so its the same as 25v on 1000 where we started .
10 v on 100k ohms is the same as 100v on 1million ohms si we have 100 microamps!

how about 16v on 4700 ohms . thats almost like 32 v on 10000 (9400) so call it 33 v on 10000 or 3.3v on 1000 or 3.3 ma.
with some practice it becomes so ez you dont need to use a calculator or even your head!
it all starts with iv,iohm, and 1 amp--1v on 10ohm=10v on 100 or 100 on 1000 or 100 ma

6v on 17 ohm=12v on 34=36v on 100=360v on 100 or 360 ma or 1/3 amp appx, since the "old way is 6 over 17 or appx 1 over 3 or appx 1/3 amp
now if your'e a purist and fussy about being exact you should realize that this is probably as close as the tolerance will be anyway!!!
hope you have fun with this

tom

 

Learn Ohm's Law, along with a little English, spelling, grammar, and vocabulary.

 

Ohms law is true for resistors sort of by definition. If it does not follow the law it is not a resistor. ( at least in some definitions ) Semiconductors do follow ohms law, but semiconductor junctions do not. ( which is probably what was meant by saying semiconductors do not follow it ) It is not worth arguing much over these issues, you can move definition and laws around to variously make things true or false. Someone can probably find something wrong in almost any statement unless the statement is made unbarebly tedious.

 

Semiconductor do obey the ohm's law. That's why it is a law, not theory. I guess everyone knows the difference between the two. But, there are additional parameters that is needed to consider in using ohm's law with the semiconductors. This is due to the fact of the non-linearity of semicon with respect to certain parameters (say, temperature). Example, a diode, if 100mA flows through it, it will have 0.7V drop. If we slightly change the source current, the voltage drop also change. This is normally affected by the ambient temperature as well as self-heating. But then, if you're interested this with the resistance of the diode at 100mA current and 0.7V voltage drop, Of course, you can use V=IR. The point here is, if I increase the current, it doesn't mean the voltage stays at 0.7V.

The voltage across the resistor is directly proportional to its resistance. True. so you need a constant. In the case of ohm's law, V=IR, I is the constant. But in reality, is it constant? Even the ideal resistor has a slight non-linearity. That is why there is a tolerance. I hope this clears up things.

If say something wrong, please do comment. BTW, I'm new in this forum. And I have shared my ideas in other threads. I hope its okay.