# Thread: Stupid electrical math question

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

2. Originally Posted by jonnyGURU
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.

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Originally Posted by jonnyGURU
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.

5. Originally Posted by gdjacobs
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.
Last edited by ashiekh; 07-05-2018 at 01:40 PM.

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Originally Posted by ashiekh
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.

7. Originally Posted by gdjacobs
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

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

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

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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.

11. LOL! Why does this 10+ year old thread continue to thrive!!!!????

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