# Ohm's Law or not Ohm's Law... That is the question

Discussion in 'General Electronics Discussion' started by CDRIVE, Sep 19, 2012.

1. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
The point is that Ohms Law is and can be used to return apparent E, I & R in the course of general design in simple DC or AC circuits, including Inductors.

In retrospect I believe this thread is going to be much shorter than I originally anticipated.
I see you're a Massachusetts resident. This explains everything. :roll eyes:

Chris

2. ### Laplace

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Apr 4, 2010
The real problem here seems to be determining just what is Ohm's Law. Georg Ohm published his work in the 1820's whereas alternating current was not invented until the 1830's. So Ohm's Law as originally understood only applied to DC and the proportionality constant between voltage and current was resistance 'R'. But in modern times we use a reformulation of Ohm's Law that applies to both DC and AC current.

BobK may hold to his ancient beliefs if he wishes, but the rest of us should move on.

3. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Well, actually the original Ohms Law looked like this: x = a/(b + l')

It related the reading on a galvanometer (x) to a term dependant on the temperature of a thermocouple (a), and constant (b), and the length of a piece of wire (l')

4. ### BobK

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Jan 5, 2010
That is like saying Einstein's general relativty equations are the same as Newton's law of gravitation.

Bob

5. ### BobK

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Ah, so now you have stooped to Ad-hominen via geography. By the way, what is it people don't like about Mass? Is the anti-intellectual response to to the fact the we have the highest scores on national tests, or is the anti-family values crowd that hates the fact the we have among the lowest divorce rate and teen-pregnancy rate of all states?

Bob

6. ### CocaCola

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Apr 7, 2012
Is stuff like this necessary or relevant in any way?

As Bob touched upon once you resort to using blatant and intentional logical fallacies to support your claims, you lose a vast amount of credibility in your argument even if the rest of your argument has footing...

What's next the "Hitler Card"?

The 'theory' of Ohm's law is solid, but the dumbed down application of "The Law" is flawed in some instances...

7. ### BobK

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Except that even that is not true. Here is a plot of your inductor circuit simulated:

Note that the current is at the max when the voltage is zero. Let's take Ohm's Law:

E = I R

and rearrange it to:

E / R = I

Now, what is the R that will give you the correct I when E = 0? If it obeys Ohm's law, there must be such an R.

Bob

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Last edited: Sep 20, 2012
8. ### Laplace

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In what way is saying, "Ohm's Law also applies to reactance as well as resistance." equivalent to saying, "Einstein's general relativty equations are the same as Newton's law of gravitation."?

9. ### BobK

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I am using the definition of Ohm's law that you will find on Wikipedia. It has 3 terms, I, E and R and R is a constant. It does not involve reactance which involves frequency. It deals with instantaneous I and V, not averages over a cycle like the generalized reactance equation. It does not involve complex numbers, which reactance must have in order for you to be able to apply it to circuits having a mix of the 3 different kinds of reactance. To call these two equations the same IS like calling Newton's simplified version of gravity the same as Einsteins. It is just not true.

Bob

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Except at absolute ZERO −459.67° correct?

11. ### Laplace

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Apr 4, 2010
That same entry on Wikipedia makes the following statement:

"This form of Ohm's law, with Z taking the place of R, generalizes the simpler form. When Z is complex, only the real part is responsible for dissipating heat."

I am talking about that form of Ohm's Law.

12. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
Yes, you're correct. I stated at the outset that Bob went out of his way to not merely question my original post but to insult me. So I came out of the starting gate p!ssing fire! You may want to read all of it from the beginning. I gave Bob an opportunity to tell me that his post was something other than what I perceived it to be. He didn't flinch! In fact, he confirmed it.

Personally, I find no pleasure in this kind of ugly banter. If it's going to go down like this then I prefer up close and personal, not on a keyboard.

My apologies to the mods and membership but I will not apologies to Bob. I post here because I enjoy sharing a lifetime of experience, not to send my blood pressure through the roof! I'm livid with Bob because he single handedly destroyed my happy and peaceful relationship with EP.

Chris

13. ### john monks

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Mar 9, 2012
Cdrive, let me get into the fray a little bit.
Just like you I spent many years working in electronics And i spent 20 years of academia and helping many engineering students with their homework I have drawn the conclusion that it is a big mistake to study electronics by using ohms law or any other law unless it has been factually verified by the science or by experimentation. I certainly agree that their are many parallels in ohms laws with reactances and nonlinear components. But to study electronics by studying ohms laws or any other laws simply because it is in a book is a very big mistake. I believe it far better that when a student is doing his homework that ohms law simply not be used unless it can be derived on the spot based on physics and definitions. In fact this is what I did in my final circuits class for engineering. I simply memorized the characteristics of resistors, capacitors, inductors, transistors, and op amps. And I got an A in the course even though I had not studied electronics previously for 20 years.
So I hope you don't mind me saying so but electronic problems should be solved only by using physics and relevant definitions. Any formula should only be used only if it can be derived on the spot. That includes ohms law.

14. ### BobK

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Please show me the post where I insulted you. I did no such thing. I questioned your statement that "all components obey Ohm's law". Your very first post after that came across as combatative.

Bob

15. ### BobK

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And that Generalized Ohm's law (which is not Ohm's law) applies only in the case the voltage is a sine wave of a specific frequency, that is why w (or f) is a part of that equation. How does The inductor obey the generalized Ohm's law in response to a step function? What is the constant R that defines the ratio of the current to voltage in the response to a step function?

And, even if we agree that an inductor sometimes can be characterized by a generalized form of Ohm's law, there are many other components that do not.

Any negative resistance component cannot even be said to obey a generalized form of Ohm's law where the constant R is replaced by a function of the voltage, since it the current is multi-valued and depends on the history.

Bob

16. ### BobK

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Jan 5, 2010
And that Generalized Ohm's law (which is not Ohm's law) applies only in the case the voltage is a sine wave of a specific frequency, that is why w (or f) is a part of that equation. How does The inductor obey the generalized Ohm's law in response to a step function? What is the constant R that defines the ratio of the current to voltage in the response to a step function?

And, even if we agree that an inductor sometimes can be characterized by a generalized form of Ohm's law, there are many other components that do not.

Any negative resistance component cannot even be said to obey a generalized form of Ohm's law where the constant R is replaced by a function of the voltage, since it the current is multi-valued and depends on the history.

What about a microcontroller chip. What is the generalized for of Ohm's law that charcterized the current it draws versus the voltage?

Bob

17. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012

I asked you.. No response posted.

dis·dain   /dɪsˈdeɪn, dɪˈsteɪn/ Show Spelled[dis-deyn, dih-steyn] Show IPA
verb (used with object)
1. to look upon or treat with contempt; despise; scorn.
2. to think unworthy of notice, response, etc.; consider beneath oneself: to disdain replying to an insult.

Ridiculous:
Origin: 1540–50; < Late Latin rīdiculōsus laughable, droll, and Latin rīdiculus (adj. derivative of rīdiculum ridicule);

See chronology of events above.

Chris

Last edited: Sep 21, 2012

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19. ### Laplace

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"That Generalized Ohm's law (which is not Ohm's law)" BobK still clings to his ancient religion.

"applies only in the case the voltage is a sine wave of a specific frequency" Not a true statement when the circuit analysis is conducted in the complex frequency domain using the complex frequency 's' variable.

"How does the inductor obey the generalized Ohm's law in response to a step function?"
Ohm's Law for an inductor in the s-domain would be I=E/(sL). In the case where the excitation voltage is a step function of V volts, the unit step in the s-domain is given as 1/s so substitute E=V/s and the current becomes I=V/((s^2)L). Now if you want the current as a function of time, apply the inverse Laplace transform to go from the s-domain to the time domain.

"What is the constant R that defines the ratio of the current to voltage in the response to a step function?"
This is a nonsense question because when all you have is an inductor the constant is L and not R.

20. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
Just in case you missed it I not only agreed with you but apologized as well. That said the definition from your link, as it's applied here, is greatly flawed.

In what way did I negate the truth? I posted an example of what I taught as Ohms Law For AC Circuits. I didn't invent the term. It was a chapter title in my students text books. The title never surprised me though. The same term was used in my own student text books 46 years ago. If you're assuming that I believe I've lost this argument. You thought wrong.

Chris