Understanding AC and DC: Key Facts and Differences

[YesIam]

I just wonder if the readers here realize a couple of things.

1. Basic Ohm's law is different than the Laboratory Ohm's law.
2. If a DC voltage is applied to a resister, an AC voltage is always generated across the resister. I hate to use 'always' but as far as I know it is this way.
3. If a DC voltage is applied to a resister in a vertical position it will generate a higher AC voltage than if it is in a horizontal position. Now this can be compensated for but left uncompensated for, it is true.

 

[Carl Pugh]

I hate to tell you this, But your meter is lying to you.

Welcome to the wonderful world of electronics where every component and every test instrument will lie to you if given the opportunity.

 

[vk6kro]

I just wonder if the readers here realize a couple of things.

1. Basic Ohm's law is different than the Laboratory Ohm's law.
2. If a DC voltage is applied to a resister, an AC voltage is always generated across the resister. I hate to use 'always' but as far as I know it is this way.
3. If a DC voltage is applied to a resister in a vertical position it will generate a higher AC voltage than if it is in a horizontal position. Now this can be compensated for but left uncompensated for, it is true.

I would like to know why you think these things are true. Did someone tell you?

As far as I know, statements 1, 2 and 3 are all false (unless you consider the rise in voltage when DC power is first applied, to be AC).

 

[Phrak]

Out environment is filled with 60 and 120 Hz AC noise (50 and 100 Hz in Europe and Japan, etc.) and other stuff. The vertical vs. horizontal distinction is a bit harder to swallow, and the coupling depends as much on how the probes drape.

As well, AC voltage measurements could be an VRMS measurement depending upon the construction of the meter. 5VDC has an RMS voltage of 5 volts.

 

[YesIam]

I would like to know why you think these things are true. Did someone tell you?

As far as I know, statements 1, 2 and 3 are all false (unless you consider the rise in voltage when DC power is first applied, to be AC).

First of all I saw some where in this forum to the effect that this forum is for the benefit of 'students' of physics. Basically we are all still students-there is always something to learn today and tomorrow. I have learned quite a bit from this forum in the last couple of days...and I thought maybe I could share a bit from all of those "TOMORROWS" that I have had. I like the part where vk6kro says, "As far as I know..." This is the way we all should think.
1. I think vk6kro has approached this better than most, better, but still lazily. I have an idea he/she did not try looking into it on his own. If he had, he would have probably seen 'why this is so'...and the most important part is that he would have learned a lot of new things along the the way. This is why term papers for a class are so important-it is because of all the information that one comes across during the writing.
2. I think this is important: Everyone should realize that that they know almost an infinitesimal amount about 'anything'. I don't like to be presumptuous and I try to be humble but in answering vk6kro's first question, "...why..." I have worked about 44 years in electronics with the last about 40 of that in Metrology Electronics. I know I should not have added this but vk6kro asked, okay. I would say that this may be one the first things that a person working in electronics should learn.
3. Vk6kro's second statement tells me that he is really thinking about it. But no, the AC is generated for the whole time the DC voltage is applied to a resistor. And once you figure it out, you may do one of those forehead slapping things...'I should have known that'.
4. You will notice that I didn't ask a question..so I am not looking for an answer. This is something that is around us all of the time-it is well considered in the design of these computers that we are using right now, and believe me, a lot of time and effort has been spent in coping with just this problem. If you look in most electronic equipment, you should see that most resistors are mounted in the horizontal position. By the way I helped build one the more bigger computers in about 1960...It took 3 railway box cars to haul it away and deliver it to the customer.
5. Is vk6kro a ham call sign-I have had a couple-I still have one.
6. You asked, "Did someone tell you?" To answer this, I will tell you...in about 1957 or so I had a coworker that was well versed in this and he took a few minutes to tell me what he knew about the basics. I still remember his name.
6. I don't know why the manager of this forum believes that all postings should be so short without explaining much.

 

[vk6kro]

So, are you talking about noise?

 

[sophiecentaur]

I just wonder if the readers here realize a couple of things.

1. Basic Ohm's law is different than the Laboratory Ohm's law.
2. If a DC voltage is applied to a resister, an AC voltage is always generated across the resister. I hate to use 'always' but as far as I know it is this way.
3. If a DC voltage is applied to a resister in a vertical position it will generate a higher AC voltage than if it is in a horizontal position. Now this can be compensated for but left uncompensated for, it is true.

I think you are actually "having a New Year's laugh" here. Reading your OP in the light of what you say about your past, I see that you are just drawing a distinction between theory and practice. The fact is, though, that practice in a vast amount of electronics work is actually pretty damn close to theory. That's why circuit simulations are so popular and successful.
It is true to say that there is always noise and there is always interference, wherever you go. These will affect the resulting signal from any circuit but, with care and good engineering, their effects can be made negligible in many / most practical cases or, as someone in metrology will know, they can be taken account of. But holding your finger on a well chosen resistor can make all the difference between a circuit working and not working. . . . . Magic?

 

[Phrak]

I just wonder if the readers here realize a couple of things.

1. Basic Ohm's law is different than the Laboratory Ohm's law.
2. If a DC voltage is applied to a resister, an AC voltage is always generated across the resister. I hate to use 'always' but as far as I know it is this way.
3. If a DC voltage is applied to a resister in a vertical position it will generate a higher AC voltage than if it is in a horizontal position. Now this can be compensated for but left uncompensated for, it is true.

Can you supply the conditions involved and an explanation for the results if you have one to advance?

 

[Averagesupernova]

YesIam, you have a very strange way of coming out with something. Most of the threads on this forum are started as a genuine question. While you didn't technically punctuation wise ask a question it is toned in such a way as to sound like: Hahahaha, I know something you guys don't, you aren't as smart as you think you are. Can you guess what it is? I bet not.
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It's interesting how you throw something out there (3 things actually) and when asked to support your claims you say something that amounts to: You should look it up yourself.

I think vk6kro has approached this better than most, better, but still lazily. I have an idea he/she did not try looking into it on his own. If he had, he would have probably seen 'why this is so'...and the most important part is that he would have learned a lot of new things along the the way.

Sorry, that's not the way things work here. If you expect everyone on this forum to look it up for themselves everytime someone throws out a claim you are mistaken.
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Most of us know about thermal noise, I personally have never heard of the horizontal vs. vertical resistor thing, and the ohms law claim I have a really hard time swallowing. So please do tell us how much smarter you are than the rest of us. And if this post sounds sarcastic and smart alecky, good. It was meant to. But seriously, I am waiting an explanation on all of it.

 

[YesIam]

Hi Phrak - Yes I can, but as I told VK6KRO there are just too many people on this forum that have 'used' basic Ohm's Law and then think that they are 'on top of it'...and then tell me that I don't know what I am talking about. I haven't seen any evidence of you thinking about this or looking into it...You just asked 'can I tell you'. As I said I thought this forum was for the benefit of students. That is okay and I looked back at some of your postings and I can tell (now don't take this wrong, it is just an observation and not an insult) that indeed you are a student. You do not have any or very little real life experience in electronics or physics-am I right? Be truthful. Everyone has to start some where, I understand that. The one statement you made about the light bulb is real sneaky, but true, where you said, "Did you know that in your house, current will flow in the bulb before the switch makes contact or arcs?"
No, I am not ''having a New Years Laugh" as the fellow above said. That is why I don't care if the Mentors kick me off this forum or not - I'll just have to beat them to it. If I wasn't retired and still working, I wouldn't put up with this. Now as for learning, you had better believe I can learn a lot from this forum, but it will be from reading and not writing-I can filter out the BS faster than your microsecond.
I'll make you a deal...if you can solve my 2 + 2 problem, I'll fill you in about what I said in my first posting.
And I do not understand why the management wants postings short with no explanations.

 

[YesIam]

Hey - I forgot to include the opening quote about Phrak's question in my posting (Yes I can...)...And Averagesupernova - a question for you. Can you guess how long you will have to wait?

 

[Averagesupernova]

Ummmmmmmmmmmmmmmmmmmmmm, not sure who you are referring to in post #10 but I would like to see the link containing: "Did you know that in your house, current will flow in the bulb before the switch makes contact or arcs?" regardless of who posted it. Until you show some evidence of anything I won't try to solve your 2+2 problem. Whatever the heck that is.
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Never mind, I found it. I know how long I will have to wait because I suspect you won't be posting here much longer.

 

[Phrak]

And I do not understand why the management wants postings short with no explanations.

They are preferable to long with no explanation.

 

[sophiecentaur]

@YesIam
"I know something you don't know"
Let us all into the secret then. From what you say, you are old enough and have enough experience to know better than this.

 

[Jobrag]

YesIam
Ohms law V=I*R
What is the expression for your laboratory Ohms law?

 

[vk6kro]

I had an email from him.

He reckons Ohm's Law doesn't apply because of Johnson-Nyquist Noise.

I wish Johnson-Nyquist Noise was the worst of my problems.

When you have to deal with unmarked ICs and PC boards with no documentation, a bit of Johnson-Nyquist Noise wouldn't be too hard to cope with.

 

[sophiecentaur]

YesIam
Ohms law V=I*R
What is the expression for your laboratory Ohms law?

Actually, that isn't Ohm's Law. Ohm's law says that I is proportional to V, in a metal at constant temperature. And it's not as much a fundamental law as a description of behaviour of certain materials.
It's a common misuse of the term to say that a light bulb doesn't 'follow Ohm's Law'. It doesn't get the chance to follow Ohm's Law because its temperature varies 'quite a lot'! Take a low voltage, fat, filament with a good thermal inertia and it will do quite a good Ohm's Law with fast varying voltages.

 

[gnurf]

So after all those words (and pretentious drivel), is this what YesIam was trying to say:

The open-circuit rms noise voltage produced by a resistance is V_t = sqrt(4KTBR), where K is Boltzmann's constant, T is abs temperature, B is noise bandwidth and R is the resistance. The thermal noise in a resistor can be represented by adding the thermal noise source, V_t, in series with the resistor.

In the http://en.wikipedia.org/wiki/Johnson%E2%80%93Nyquist_noise" example, V_t for a 1 kΩ resistor at room temperature and a 10 kHz bandwidth, is 400 nV.

Sophie: was this also the objection you had to Jobrag's post about Ohm's law?

 

[YesIam]

Hi Jobraq - Okay, for right now let this be the way that I think it is. Your basic Ohm's Law as you pointed out is an everyday working formula, a tool to do things-it is rounded off and it is close enough to understand and teach electricity and 'stuff'. It says that if one runs one amp of current through one ohm, one volt will be dropped across the one ohm resister. But they have known that this is in error slightly. Just look on the web for this history. But as measuring abilities had gotten better over the years they realized that they should change the value of something to get things closer. Notice I said better, just better not absolutely right. Of the three things involved in this relationship, which can be changed physically to get things closer...the ohm. As I said they keep changing the value of the ohm over the years and I think it is now at 1.00049. This means that it now takes 1.00049 ohms of resistance to drop one volt when one amp is run through it. So this means that your basic Ohm's Law if used as being one ohm, one volt, one amp can be used and it really doesn't make too much difference. Can you imagine using 1 Volt = 1.00049 Ohms x 1 amp for everyday use. Now you can if you wish, work around this by saying, "An ohm is a ohm," and the premise is still the same, and nobody will get too upset. I won't. Just like our 'time'-we don't use an exact time for our day-to-day use, it is just close. When the error exceeds a half a second, they just change it one second and call it a 'leap second'. I think the Earth is slowing down now at about one second every two years or so. And yes, at this rate it will just stop turning 'soon'.
Hey, all of this information is out there on the web. Why does everyone want to just take the easy route and have someone else tell them. They forget how much of this information on this forum can be bogus-I see it every time I look at it. But really everyone has the same problem. They don't know if what they read is the straight scoop or not...and the same goes for this. As well they shouldn't. Check into it yourself.
Jobrag, I didn't use the word 'expression' you did. Now someone will say that 'I' said the expression is different. That is just the way people are-I am not long for this forum. Good luck.

 

[YesIam]

VK6KRO - Well. son of gun...What you quoted me as saying is an OUTRIGHT LIE. I didn't say that and you know it. I don't REACKON any such thing. Why you would want to muck things up worse than it is, I don't know.

 

[sophiecentaur]

So after all those words (and pretentious drivel), is this what YesIam was trying to say:

The open-circuit rms noise voltage produced by a resistance is V_t = sqrt(4KTBR), where K is Boltzmann's constant, T is abs temperature, B is noise bandwidth and R is the resistance. The thermal noise in a resistor can be represented by adding the thermal noise source, V_t, in series with the resistor.

In the http://en.wikipedia.org/wiki/Johnson%E2%80%93Nyquist_noise" example, V_t for a 1 kΩ resistor at room temperature and a 10 kHz bandwidth, is 400 nV.

Sophie: was this also the objection you had to Jobrag's post about Ohm's law?

My objection was that Ohm's Law is not a 'formula' connecting V,I and R. It states that a relationship is constant under constant temperature. The statement V=IR is not a law, it is just a formula. The 'clever' thing about Ohm's Law is that it applies to metals for a massive range of currents.
You can take a non-ohmic conductor and assign it a resistance value by measuring V and I but it is not 'obeying' any particular law - certainly not Ohm's Law. Merely calculating its resistance doesn't ensure that the resistance will be the same for other currents.
I think my objection to an over-simplification of a sophisticated observation is quite valid.

I agree with what you write about YesIam. The effects of noise or interference on a measurement are not relevant to the quantity (if your measurement method is correct) they just represent possible errors in the result. We. of course, try to reduce these errors as much as possible - which is something that I would have expected someone experienced in metrology to have appreciated.

I think he is just out for a rant. Hope he will go away until he can come up with something more interesting and valid.

 

[YesIam]

Holy Mackerel - How people can draw conclusions and assumptions the way they do is beyond me, especially sophie and VK6KRO. I didn't connect noise with ohms law in any way-they did.

 

[sophiecentaur]

Well tell us what it's all about then. With or without the saintly fish.

 

[YesIam]

sophie - I will not, just 'flounder' around on your own and maybe pick something up. It looks like it has went from, "Hope he will go away..." to, "...tell us..." That's a funny one. Really, I saw somewhere that this forum is for the benefit of 'students'. I was trying to get some students to think about some things. But all I got back was a bunch of 'noise' and crying. I sure wasn't this way when I was a 'student,' officially. I consider myself still a student. I will learn something new tomorrow, but I'm not counting on much from this bunch...So it's good-bye, sayonare and sale la vie.

 

[sophiecentaur]

Actually, it's "sayonara" and "c'est la vie".

Should we take what you say about Science on the evidence of your knowledge of foreign languages?.

Happy floundering.

 

[Averagesupernova]

YesIam, I was wondering 'what you are' but I have a pretty good idea now. Since this thread started I have never just wished you would 'go away' as you stated in your last post. I always wanted more info. If you don't wish to be misquoted like you claim in post #20 maybe you should be sure to say whatever it is yourself in the forum instead of making someone guess and then whine because they (most likely) accidentally misquoted you from a PM. I'm not into riddles and I never had a teacher pull the crap you have here.