View Full Version : Stupid electrical math question

Jon Gerow

10-01-2007, 04:39 PM

Question:

Equation for resistive loss is P = I^2 * R, where P is Wattage, I is Current and R is Resistance.

Since P = I * V (Voltage), wouldn't I divide both sides by I and come up with the equation of V = I * R? Wouldn't that be the same thing?

Oklahoma Wolf

10-01-2007, 05:15 PM

Since P = I * V (Voltage), wouldn't I divide both sides by I and come up with the equation of V = I * R?

Ohm's law thinks you're correct :)

Jon Gerow

10-01-2007, 05:28 PM

Right... because Ohm's Law is I = V / R which is the same as V = I * R. Damn... Why bother with the P = I^2 * R. Sheesh!

ianm2

10-02-2007, 03:07 AM

yes correct, its called algebra, you will find all sorts of relations if you keep on dividing and multiplying. but its been done before and the equations we have I think were all done that way, apart from the 'laws' which were experimentally derived from observation, and perhaps from 'first principles' by the clever people.

why bother? depends what 'variable's' you have available and where you want to get to.

the one's on the right are called the 'independant' variables, and the left one is 'dependant' as its dependant on the others, or sometimes called a 'function' of the others.

your favorite people;) the arabs, came up with al gabra, when some of them were enlightened.

Jon Gerow

10-02-2007, 08:34 AM

Thanks smart ass. I got an A in algebra, perfect score on the exam and was a programmer for some years. That's not the point of my question.

I was just wondering if I could apply it to the electrical equations because the paper I was looking at was clearly trying to differentiate P = I^2 * R and V = I * R and I can't for the life of me understand why.

ryan_975

10-02-2007, 12:51 PM

Thanks smart ass. I got an A in algebra, perfect score on the exam and was a programmer for some years. That's not the point of my question.

I was just wondering if I could apply it to the electrical equations because the paper I was looking at was clearly trying to differentiate P = I^2 * R and V = I * R and I can't for the life of me understand why.

If you don't know the voltage, but just need the power the circuit dissipates then you'd have to use two formulas to find the power without P=I^2 * R. Now, in what circumstance you'd not know or care what the voltage supplied to the circuit was... I haven't a clue. But it's good to have something to cover all bases I guess.

Jon Gerow

10-02-2007, 01:24 PM

Well, yeah. That's kind of my point. How would you actually know the wattage and not either the voltage or current (of course, if you knew either and the wattage than you can figure out the other.)

I just think some people make explaining electrical theory overly complicated just for the sake of it.

ryan_975

10-02-2007, 03:38 PM

Well, yeah. That's kind of my point. How would you actually know the wattage and not either the voltage or current (of course, if you knew either and the wattage than you can figure out the other.)

I just think some people make explaining electrical theory overly complicated just for the sake of it.

Well, say you have a circuit that has to stay under 500 watts (because you're using a shared 110v/15a branch circuit), and you have to need to maintain a constant current of 5 amps (for say a lead acid battery charger). You need to find out what the resistance needs to be so you can choose your components. You'd have to use P=I²R to solve the problem.

500 = 5²R;

500 = 25R;

500/25 = 25R/25;

20 = R.

Your circuit can have a resistance of no more than 20 Ohms.

fancyStufkiller

12-06-2007, 01:30 PM

I think you already wrote the answer to your question in your first post :)

The P = I² * R formula is nothing else then the short version of P=I*I*R which is nothing else then P=I*V | V = I * R (the trick is to view the V as an variable, where you can put in any equivalent therms).

Basically, this has nothing to do with Ohm`s law, it is pure algebra i would say ;)

Ripudio

04-08-2008, 02:31 AM

I am pretty sure the point of physics/science is to make things more complicated than they need to be, I'm pretty sure one of my old text books actually says that on the cover...

dish_moose

08-03-2012, 07:55 AM

^+1 If things were too easy people without iron rings on their fingers could do it too. ;-)

-Bruce

ashiekh

12-22-2017, 06:13 PM

Thanks smart ass. I got an A in algebra, perfect score on the exam and was a programmer for some years. That's not the point of my question.

I was just wondering if I could apply it to the electrical equations because the paper I was looking at was clearly trying to differentiate P = I^2 * R and V = I * R and I can't for the life of me understand why.

P = I * V is always true, V = I * R (Ohm's law) is not always; they are independent equations.

One can use Ohm's law to get variations

P = V^2/R

P = I^2 * R

but these depend on Ohm's law, and so are not always true; as such it is best to see the last 2 as derived, and not fundamental.

gdjacobs

07-04-2018, 11:49 PM

Right... because Ohm's Law is I = V / R which is the same as V = I * R. Damn... Why bother with the P = I^2 * R. Sheesh!

Usually because R is an end product of actually measuring voltage and current (and phase) through a circuit branch.. More often than not, components will be dynamic with different operating conditions and Requiv is not an invariable number.

ashiekh

07-05-2018, 10:02 AM

Usually because R is an end product of actually measuring voltage and current (and phase) through a circuit branch.. More often than not, components will be dynamic with different operating conditions and Requiv is not an invariable number.

True, a capacitor and inductor have a frequency dependent imaginary impedance, and together with resistance in a circuit they form a complex impedance; one needs to know about complex numbers, which have a real and imaginary part. Imaginary is a rather unfortunate name, as they are as real as real numbers.

Practical Electronics for Inventors, 4th Ed, by Scherz and Monk is a good place to start as well as being very affordable; but be forewarned about the many typos and errors in this text.

gdjacobs

07-05-2018, 06:41 PM

True, a capacitor and inductor have a frequency dependent imaginary impedance, and together with resistance in a circuit they form a complex impedance; one needs to know about complex numbers, which have a real and imaginary part. Imaginary is a rather unfortunate name, as they are as real as real numbers.

Practical Electronics for Inventors, 4th Ed, by Scherz and Monk is a good place to start as well as being very affordable; but be forewarned about the many typos and errors in this text.

Yes, in a general sense, you have to account for complexes of reactive components. Even for pure DC, though, components often have complex behavior that can't be modeled as pure resistance (although they usually have a resistive component). For instance, a DC motor will include resistances from the rotor coils, brushes, and field coils (although those are separately measurable) as well as back EMF generated on the rotor coil.

I prefer Electronics Principles by Malvino. His approach using successive approximations of components is brilliant and clear, I think.

ashiekh

07-05-2018, 07:17 PM

I prefer Electronics Principles by Malvino.

$136 is out of my budget, so I ordered the 6th edition to save money; thanks for the suggestion.

Electronics for Inventors is $21 new for the latest edition

gdjacobs

07-11-2018, 02:37 PM

I believe my copy is the 6th Ed as well. I hope you enjoy reading it.

ashiekh

07-11-2018, 05:00 PM

Came in yesterday, but I haven't really started it yet.

Batboy88

07-14-2018, 11:59 AM

That's where some got into trouble changing stuff and think that really matters...they designed it all for a reason and matching PWM, Impedances frequency dependent etc, as well stated it's not just resistance.

Jon Gerow

07-14-2018, 01:58 PM

LOL! Why does this 10+ year old thread continue to thrive!!!!???? :D :D :D

Batboy88

07-14-2018, 04:02 PM

Well We've seen quite a few rendition's and some a little different stuff going on recent years and we know who to turn to and who probably already took a Peak at :)

And always these same question oh I have this unit or that or why should I choose it over the next one stuff :)

ashiekh

07-29-2018, 05:07 PM

I believe my copy is the 6th Ed as well. I hope you enjoy reading it.

Have started, and while it looks like a good book, it has annoyed me from the start; numbers without units, talk of 'proving' Coulomb's law through experiment; and this is just page 3

gdjacobs

08-03-2018, 11:17 PM

You have to look beyond it's roots in engineering.

ashiekh

08-04-2018, 06:58 AM

You have to look beyond it's roots in engineering.

Hard for me; he has electrons in planetary orbits... that's not Engineering, that's just plain wrong.

Batboy88

08-04-2018, 10:10 AM

It is a Science and methodology that the math works and it works....and you can transfer over it all to other things etc.

ashiekh

08-04-2018, 02:39 PM

et cetera et cetera

Jon Gerow

08-07-2018, 09:20 PM

Have started, and while it looks like a good book, it has annoyed me from the start; numbers without units, talk of 'proving' Coulomb's law through experiment; and this is just page 3

This actually has me more intrigued than anything. :D

ashiekh

08-07-2018, 10:12 PM

You would certainly benefit greatly from this book

Jon Gerow

08-07-2018, 10:39 PM

Sadly, newest revision at Amazon is fourth edition.

Should I care?

ashiekh

08-07-2018, 10:44 PM

Sadly, newest revision at Amazon is fourth edition.

Should I care?

* Latest edition of Malvino and Bates, Electronic Principles on amazon is the 8th (Bates joined with the 7th edition)

https://www.amazon.com/Electronic-Principles-Albert-Paul-Malvino/dp/0073373885/ref=sr_1_1?s=books&ie=UTF8&qid=1533696377&sr=1-1&keywords=electronic+principles

This is the one that annoyed me (I have the 6th edition) and had you intrigued; even so it looks like a good book to start with.

* Perhaps you were looking at Practical Electronics for Inventors, 4th Edition by Paul Scherz and Simon Monk (Monk joined with the 3rd edition)

This is also a good book to start with; much cheaper, but it has a LOT of typos. I sent what I found to Monk, about 100 and others have made lists (for the 3rd edition); I estimate about 200 found so far that are still in the latest (4th) edition. Regardless of this, I like this book; 200 errors in 1000 pages means just one error every 5 pages or so, some are trivial, some serious.

* Everyone should eventually have a copy of Horowitz and Hill; The Art of Electronics; but I would not start there.

I'd start with Practical Electronics for Inventors; the price is right even if the title is misleading.

$20 for the 4th edition (latest)

ashiekh

08-18-2018, 09:33 PM

Finally getting into Malvino's book; I like it.

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