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Author Topic: Ohm's Law  (Read 3527 times)

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Offline rockroboticsTopic starter

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Ohm's Law
« on: March 11, 2010, 11:25:09 AM »
Hi,

If you want to know the current of a 9V battery, then you would say:

I = V / R bu if there is no resistance than wouldn't the current be 9?

But my question is, is it 9amps or 9milliamps!

Please tell me what I'm doing wrong!

Thanks,

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Offline waltr

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Re: Ohm's Law
« Reply #1 on: March 11, 2010, 11:49:48 AM »
Not quite. No resistance is zero Ohm or a short across the battery.
If nothing is connected to the battery then R is infinite so that I = V divided by infinity. In practice I is then equal to zero.

An unconnected battery is just a voltage potential without any current flow. A circuit needs to be connected across this voltage for current to flow. So if you place a 1000 Ohm resistor across the battery 9mA of current will flow through the circuit.
« Last Edit: March 11, 2010, 02:02:29 PM by waltr »

Offline little-c

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Re: Ohm's Law
« Reply #2 on: March 11, 2010, 12:00:51 PM »


division by zero. infinity.

at that point P= VI, small number times infinity, = infinity, massive creation of energy, and you have solved the power issues for the universe.

 :D

current of a battery that isn't in a curcit is 9/(some number of)Gohms which is about a few nano amps. electrical conductivity of air and battery casing. which is so small as to be non existant, ecept in long term storage.


the current depends on the risistance of the curcit the battery is connected to.

and V=IR is in SI unit, Volts, Amps, Ohms.

Offline rockroboticsTopic starter

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Re: Ohm's Law
« Reply #3 on: March 11, 2010, 02:17:59 PM »
OHHH thanks for explaining it to me! :)
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Offline Soeren

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Re: Ohm's Law
« Reply #4 on: March 11, 2010, 03:13:51 PM »
Hi,

Once again, I'll have to protest...
Either you use V, A and Ohm or their symbolic representations U(or E), I and R - please be consequent and stop mixing real and symbolic names :)
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline rockroboticsTopic starter

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Re: Ohm's Law
« Reply #5 on: March 11, 2010, 03:24:33 PM »
sorry I don't get exactly what you mean
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Offline madsci1016

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Re: Ohm's Law
« Reply #6 on: March 11, 2010, 03:24:47 PM »
Hi,

Once again, I'll have to protest...
Either you use V, A and Ohm or their symbolic representations U(or E), I and R - please be consequent and stop mixing real and symbolic names :)


Perhaps i'm misunderstanding, but are you saying to write the equation 'V=IR' is a mixture?

Offline rockroboticsTopic starter

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Re: Ohm's Law
« Reply #7 on: March 11, 2010, 03:33:37 PM »
Wow,

I must be really stupid, but wat is a mixture?  I asked this question because lets say I have a 500milliamp LED and a 9V battery,  what resistor would I need?  Write me back!

Thanks,

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Offline Soeren

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Re: Ohm's Law
« Reply #8 on: March 11, 2010, 03:42:33 PM »
Hi,

Perhaps i'm misunderstanding, but are you saying to write the equation 'V=IR' is a mixture?
Yes.
V (Volt) has a symbolic representation called U (I believe you'd use E (for Electromotoric force) in the US, like we used to back in time)
I and R are symbolic representations for A(mpere) and Ohm respectively.æ

Ohms Law is R=U/I, but could be written W=V/A, when applied to specific numbers.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline little-c

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Re: Ohm's Law
« Reply #9 on: March 11, 2010, 03:45:07 PM »
680 ohm.

I have a lookup table! ;D
5v, 330ohm, 9v 680 ohm, 12v 1k ohm, 24v 2k2 ohm.

Offline waltr

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Re: Ohm's Law
« Reply #10 on: March 11, 2010, 03:46:36 PM »
Hi,

Once again, I'll have to protest...
Either you use V, A and Ohm or their symbolic representations U(or E), I and R - please be consequent and stop mixing real and symbolic names :)



Perhaps i'm misunderstanding, but are you saying to write the equation 'V=IR' is a mixture?


Yep, I'm also guilty and should know better.

 V, A and Ohm are the units of the measurement.
U (E), I and R are the symbolic representations of the qualities.

An analogy would be the velocity equation:
velocity = distance / change in time or v = d/Δt
        where the units of measure could be feet, meter, hours, seconds
so that the equation v =  m/s (or velocity = meter/second) is wrong.

Unfortunately, it is becoming more and more common to see V use in Ohm's law. I have seen this in college text books as well as many, many places on the web including Wikipedia.
http://en.wikipedia.org/wiki/Ohm%27s_law

I know you are correct Soren but this is going to became the standard form of Ohm's Law.

Offline rockroboticsTopic starter

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Re: Ohm's Law
« Reply #11 on: March 11, 2010, 03:49:35 PM »
Ok cool!

But I have on more question off the subject.

I was thinking about building the $50 robot when I saw the servos cost like $9.  But since you modify the servo and disconnect the potentiometer, than wouldn't it be the same thing and cheaper to use just a regular motor?  Please answer this question!

Thanks,

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Offline Soeren

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Re: Ohm's Law
« Reply #12 on: March 11, 2010, 03:50:36 PM »
Hi,


I must be really stupid, but wat is a mixture?

If you want to know the current of a 9V battery, then you would say:

I = V / R
That is.

I asked this question because lets say I have a 500milliamp LED and a 9V battery,  what resistor would I need? 
Information missing!
You need to know the voltage drop of the LED.
The formula is:
  (U_batt - U_LED) / I_LED

Eg.
 (9V - 3.6V) / 0.5A = 10.8
(for an LED that drops 3.6V)
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline little-c

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Re: Ohm's Law
« Reply #13 on: March 11, 2010, 03:53:25 PM »
more or less 680 ohm resistor.  ;)

Offline madsci1016

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Re: Ohm's Law
« Reply #14 on: March 11, 2010, 03:54:40 PM »
Well, i can only guess this is a fundamental culture difference between US/UK, like the spelling of color/colour.

We use E for electric field vector. I was taught V=IR all throughout school.

I tried googling for R=U/I but couldn't find any information on it.

Offline Soeren

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Re: Ohm's Law
« Reply #15 on: March 11, 2010, 03:54:49 PM »
Hi Waltr,

Unfortunately, it is becoming more and more common to see V use in Ohm's law. I have seen this in college text books as well as many, many places on the web including Wikipedia.
http://en.wikipedia.org/wiki/Ohm%27s_law

I know you are correct Soren but this is going to became the standard form of Ohm's Law.

Yep, all the more reason to correct it whenever possible, or we'll all live in the Tower of Babel soon  ;D
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline madsci1016

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Re: Ohm's Law
« Reply #16 on: March 11, 2010, 03:57:21 PM »
Unfortunately, it is becoming more and more common to see V use in Ohm's law. I have seen this in college text books as well as many, many places on the web including Wikipedia.
http://en.wikipedia.org/wiki/Ohm%27s_law

I know you are correct Soren but this is going to became the standard form of Ohm's Law.


Ahh, makes sense, it's one of those 'times are changing' things. I just checked all my old text books and they are all V=IR with publish dates near 2000, so I guess it's been in a state of flux for a while. In the 4 years of EE degree, I never saw the use of U or E for volts. It was always V.

Offline Soeren

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Re: Ohm's Law
« Reply #17 on: March 11, 2010, 04:01:38 PM »
Hi,

I was taught V=IR all throughout school.

And school is allways right, like we all know ;D


I tried googling for R=U/I but couldn't find any information on it.

It's allways possible to find some bad search terms, try Googling "Ohms Law" and just check the first few links (Wikipedia etc.).
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline madsci1016

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Re: Ohm's Law
« Reply #18 on: March 11, 2010, 04:11:18 PM »
I think neither usage is wrong at this point. Googling Ohm's Law and the first few links, (wikipedia and NASA) both say V=IR; and I was taught by some very smart people.

Offline Soeren

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Re: Ohm's Law
« Reply #19 on: March 11, 2010, 04:18:59 PM »
HI,

Ahh, makes sense, it's one of those 'times are changing' things. I just checked all my old text books and they are all V=IR with publish dates near 2000, so I guess it's been in a state of flux for a while. In the 4 years of EE degree, I never saw the use of U or E for volts. It was always V.
No, it's one of this "some people don't wanna keep to common sense", if you pardon my french.
In DK, there's a widespread us of an incorrect term which translates to "open up". It really goes hard on my ears, as in Danish, it's either to "open" or (this may sound illogical to you, but is correct Danish language) to "close up" (meaning to "open"). Each week, I hear it at least 10 times, some of it from the official Danish TV/Radio.
I have plenty of examples of how people ditch the language, but while it is annoying to hear, I still find engineering terms so much more important to be correct - we probably all know disasters like air plane "fuelers" that were forced to use the metric system with a plane crash as the sad result (whether it's just an urban legend or what).

If one wants to use V, it makes sense using W and A as well - I would think it to be logic to all with any kind of electronics education, as it is in "everything" electronics, like calculating energy or power or...

You wouldn't use specs for an "ideal" op-amp in a real world circuit I guess (/hope) and that is in the same category.


The bottom line is that it isn't important what we call things, but it IS important that we agree on the names - and since the names have allready been established and is known throughout the engineering society, no reason to throw a stick in the machinery by inventing new terms.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline Soeren

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Re: Ohm's Law
« Reply #20 on: March 11, 2010, 04:26:15 PM »
Hi,

I think neither usage is wrong at this point. Googling Ohm's Law and the first few links, (wikipedia and NASA) both say V=IR; and I was taught by some very smart people.

When I Google Ohms Law, the first hit it this page at Wikipedia stating I = U / R
(which isn't Ohms Law, as that is R=U/I  ;))

Whether you think it's wrong or not doesn't make a lot of difference in whether it is, but I'm a bit surprized that you cannot follow the logic of not mixing symbolic names and real world names.

I know it can be hard to admit a mistake and I have no reason to hold you to accept my words or not, but it's a poor reason to further a mistake just because lots of other people do it.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline little-c

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Re: Ohm's Law
« Reply #21 on: March 11, 2010, 04:35:49 PM »
E=IR, V=A(insert Ohm symbol) correct notation. V=IR incorrect, but can be understood. therefore use correct notation, and put small corrections on formula which is incorrect when reviewing work or reading textbooks. as you would when making annotations.
problem solved.

Offline rockroboticsTopic starter

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Re: Ohm's Law
« Reply #22 on: March 11, 2010, 05:15:50 PM »
Hi guys!

Thanks for all your opinions and stuff, but I was just asking the answer to a simple question.  This turned into a huge debate! LOL

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Offline madsci1016

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Re: Ohm's Law
« Reply #23 on: March 11, 2010, 06:03:42 PM »

When I Google Ohms Law, the first hit it this page at Wikipedia stating I = U / R
(which isn't Ohms Law, as that is R=U/I  ;))



This is what i see when i click your link.



So i don't know if you are getting a UK version of wikipedia but there's no U there, just a V.       I=V/R



And i see all your arguments, however i think the engineering community (at least in the US) is evolving to accepting V=IR as correct. Four years of college, taught by very respectable people, all my text books and now working in industry all tell me V=IR is correct (and wikipedia and google, for what that's worth); forgive me for not changing my way because a few anonymous people on the internet tell me i'm wrong. It's not about mixing symbolic and real names, I was taught V is the symbolic for Volts.

I'm open-minded enough to accept both ways as an acceptable in discussion, as it does seem to be changing (much as languages evolve), but personally i'm going to stay with V=IR, as it is what i was taught and have come to know. (much like you were taught and know R=U/I). I hope this doesn't keep you from giving me any advice in the future.  :(



Offline dunk

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Re: Ohm's Law
« Reply #24 on: March 11, 2010, 06:25:40 PM »
Either you use V, A and Ohm or their symbolic representations U(or E), I and R - please be consequent and stop mixing real and symbolic names :)
are you sure V is not a correct symbolic representation for voltage?
in high school physics class in Scotland 20 years back Ohms Law was always I=V/R.
Madsci1016 was apparently taught the same.
admittedly that's a physicist's terminology rather than an engineers.

i just asked an E&EE what his university taught back in the day and it was also I=V/R.

this may be a language issue rather than a representation issue.


dunk.

Offline Soeren

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Re: Ohm's Law
« Reply #25 on: March 11, 2010, 06:43:39 PM »
Hi,

This is what i see when i click your link.
[Snip]
So i don't know if you are getting a UK version of wikipedia but there's no U there, just a V.       I=V/R
Oh well, too bad I didn't make a pic of it, but I didn't think that someone would stoop so low as to make a redirect or whatever, just to prove a missing case, but the edit date ("This page was last modified on 11 March 2010"), sort of give it away ;)
Let's end this discussion, apparently it's fruitless.


I hope this doesn't keep you from giving me any advice in the future.  :(
Not at all, even if you probably won't take it  ;)

I know some people find me a bit harsh from time to time, but don't let my winning personality fool you... We actually share the passion for 'bots and construction :)
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline Daanii

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Re: Ohm's Law
« Reply #26 on: March 11, 2010, 06:57:37 PM »
Here's my two cents. Soren is right. The correct formula is E=IR. That is, electromotive force equals current times resistance. If you use V for volts, you are using units, so you should use A for amperes and a capital omega for ohms.

But the current practice in the United States, even more today than before, is to use V=IR. Right or wrong, that is what you see.

Myself, I'm fine with V=IR. Conventions should change, I think, to make things easier to remember. Why we still use I for current is silly. That's because Andre-Marie Ampere called it "electrical intensity." That term did not last long. But we still use I, just to confuse everybody.

And E instead of voltage? Confusing, for no real benefit. (I've never seen U, what does it stand for?)

No reason to stay in the past, in my opinion.

Offline madsci1016

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Re: Ohm's Law
« Reply #27 on: March 11, 2010, 07:00:47 PM »
Oh well, too bad I didn't make a pic of it, but I didn't think that someone would stoop so low as to make a redirect or whatever, just to prove a missing case, but the edit date ("This page was last modified on 11 March 2010"), sort of give it away ;)


Lol, apparently there has been edits today between U / V, but if you track the history (they store everything), It was V, then someone from Sweden changed it to U, then someone in Seattle changed it back to V. In case you were wondering, I'm in Florida. Another reason not to trust Wikipedia.

I will always seek the advice of a seasoned engineer, especially someone with the ability to design circuits as you do. (though, yes you are kinda harsh sometimes).

To Robots!

Offline Soeren

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Re: Ohm's Law
« Reply #28 on: March 11, 2010, 07:28:53 PM »
Hi all,

Let's leave it at that, since there's no reason debating after it's clear it won't change anything.
Whether we call it language, culture or whatever, I still find it strange to mix things up, but hey, most Danes talks abouts "pears" when they want a lamp (probably the equivalent of when you say "bulb" in English) and about "lamps" when they want an armature (don't know if it's the correct term - what's called a lamp shade sometimes) and I have given up on that  ::)

Anyways, I'd hate to start a "religious war", so let's just agree on having "cultural specialties"(TM) and leave it at that, the 'bot's will run as long as The Force is with them, no matter what names we attach to The Force  :P


Daanii <- U is used in Europe (at least in some places) and I don't know what brought that along - probably that it's close to V in shape or something like that.
I don't see a conflict with using E/U, R, I and P as they are used different than V, W, A and W and even though their attachment to the real world of today may be obscure, they serve a purpose.
A similar debate pops up in Denmark every now and then, where "Guardians of the Language" want to make Danish words for every computer related term to make people better able to learn and my argument against it is akin to the Tower of Babel, it hinders global communication, without providing any benefits, since most terms doesn't give any added comprehension if translated - Random Access Memory, to give an example is still a strange term if translated and I thing it actually helps to not have the burden of comparing the meaning of the words, as it just slip into your mind as an entirely new term.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline Razor Concepts

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Re: Ohm's Law
« Reply #29 on: March 11, 2010, 07:30:53 PM »
Who the hell cares? It's the same formula either way  :P

 


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