Power and Ohm's Law

Oh, dear.

For power to be constant then voltage and current are inversely proportional to each other. You are forgetting which attribute, if any, is a constant and which are variables.

Taking copyrighted material (without permission) from one of the textbooks published by the Kris-Del Institute for Electrical Engineering:

for voltage think of how fast an arrow is traveling
for current think of how heavy an arrow is
for resistance think of how dense the straw is packed behind the target
for power think of how far the arrow goes into the target and straw pack

No, in his Power = V * I, V is inversely proportional to I if power is constant. Whether power is constant is another matter, covered in various posts.

do you see R anywhere?

The "R" is resisting

Another way of looking at it:-
"In power equation voltage is inversely proportional to current"
This isn't really right because you are looking at the equation:-
V=P/I and in the real world if you increase I then P would increase as well and thus V would remain constant!

You can' independently adjust all the variables.
Del

Ohm's law equation (formula): V = I × R and the power law equation (formula): P = I × V. P= power, I or J = Latin: influare, international ampere, or intensity and R = resistance. V = voltage, electric potential difference Δ V or E = electromotive force (emf = voltage).

Think of it in hydraulic terms: Voltage=pressure, Current=flow. Resistance impedes current flow. With more resistance, you have more pressure but less flow.

Power is pressure x flow. You have to have both pressure and flow to get any power.

That's as simple as I can make it.

Somewhere is going:

Has anyone mentioned that Power is in Watts, is that what is confusing you?

Tony

P = U·I = (∂A₁/∂q₁)·(∂q₂/∂t₁) = ... = ∂A / ∂t = F_i·∂s / ∂t = m·a · ∂s / ∂t = m·(∂s / ∂²t) · ∂s / ∂t = ... = m · ∂²s / ∂³t

R = U/I = (∂A₂/∂q₃) / (∂q₄/∂t₂) = ... = ∂A·∂t / ∂²q = !!! = k·(F·∂s·∂t) / (F·∂²s) = !!! = k · ∂t / ∂s

the same "mistake" again :

P = m · ∂²s / ∂³t , I = ∂q / ∂t

R = k · ∂t / ∂s , I = ∂q / ∂t

Shortly put - it´s because the P and R represent different statistical factors

I suppose this is a joke?

Many years ago, teaching a group of fresh graduates the fundamentals of electrically powered centrifugal pumps for the oil industry, one of my bright sparks of a student then decided that Ohms Law did not and could not be applied to a submersible pump and its operation, despite the whole pumping system being powered by a rather large HP motor.

When asked how he had come up with this and pointing out that Ohms Law applied to all thing electrical (the motor) he was still steadfast in his statement.

In return to rebuke him I then told him the following....

"Son, the laws of physics have been about the dawn of time and Ohms Law is one such law. If it now doesn't work or cannot be applied then you will have to find an alternative or rewrite Ohms Law.

If you decide to rewrite Ohms Law as you will not find an alternative, then you better think of rewriting the others too, as they will also be wrong."

"BUT.... before you lock yourself away in a darken room for the next thirty years, rewriting the Laws of Physics in an attempt to get a Noble Peace prize, I would like you to remember one thing.... and that is what Scott would always say to Captain Kirk on the bridge of the Starship Enterprise....

"Ye canna change the Laws of Physics Cap't!!"

Our OP is that bright spark..... there is one in every generation!!

I have a certain sympathy with your student. Ohm's law, strictly speaking, relates only to DC voltage, DC current and resistance. As soon as you discuss an AC-driven pump you introduce further complications, including magnetic effects (back EMF) and frequency-dependent impedance rather than straight resistance.
I also have to point out that your assertion that the laws of physics have been around since the dawn of time is a logical postulate which itself requires verification:)

Ohms law may be relative to DC when discovered, but the students today(including mine) are not taught that.

And while you may introduce back EMF and all the other lovely other things we know and associate with AC supplies, for simple power and other calculations, Ohms Law works.

Lets not try to over think it, says me... KISS!

As for the Laws of Physics being around since the dawn of time.. of course they have!

They were not invented, only discovered!

Take Newton, an apple and gravity! Always there, but never discovered.. need I go on?

I now have even more sympathy with your student, as you appear unwilling to listen. Allow me to repeat my message: it is an article of faith among scientists that the laws of physics have existed since the dawn of time, but there is no proof that this is so. The evidence may be excellent within our observable range, but our observable range is limited in both time and space.

Listen to what?

Physics.. it is all around us, governs EVERYTHING we do, from the internal combustion engine to your refrigerator.

You can't change them only use them... so you might have sympathy for a numpty student that doesn't know is arse from his elbow, I wonder if you fall into the same box as him.

If you can prove to me that the laws of physics were not around at the dawn of time, then I am happy to hear you out! Until then.. I wait!

If you can prove to me that the laws of physics were not around at the dawn of time, then I am happy to hear you out! Until then.. I wait!
Sorry, you are still not listening hard enough. You make the assertion. I query the assertion as unprovable. It is up to you to prove the truth of the assertion. It is up to me to continue to query it.

Then Sir.. you are a fool!!

Thank you. Clearly what you are lacking in logic you more than make up for with abuse.

I too have some sympathy with your student.

There aren't many physical laws that are universally applicable. Conservation of mass and energy are pretty good, but following E = m.c² it's only the total that's conserved. Newton's laws of motion and gravity are fine for most purposes and once thought of as universal, but now known to have limits.

Ohm's law is no exception. It applies to (most) metallic conductors at constant temperature, but there are many situations where it doesn't, especially if the system considered is viewed as a "black box". Eg an incandescent lamp bulb, where at low voltage (hence low power and temperature, the resistance is low, but higher at design voltage. Giving rise to a high inrush current. Then there's diodes (solid state) including Zener diodes, LEDs, electrolytic cells etc.

With AC motors, within allowable voltage limits eg 380 - 420 volt 3-phase, at constant power higher volts means lower amps, as can be seen in manufacturers' data. When driving eg a centrifugal pump, increased volts causes a slight increase in speed, hence power drawn, but not enough to negate the reduced amps required (and as a thought experiment power could be kept unchanged, by adjusting the pump discharge throttling, and anyway the data is at fixed rated output). I don't know much about DC motors, it's probably similar, but needing more twiddling to maintain constant shaft power with changed voltage. Taking account of back-EMF, I'd guess Ohm's law applies to the windings, but it's different looking at the motor as a whole. That's maybe what your student had in mind.

Codemaster...

Fresh out of their protective shell of college, the students I had, while bright, had no practical experience in engineering, no skill sets to speak of and no common sense and no idea on how to apply what they had had been taught in college.

My job and that of the other instructors was to teach them baby steps in learning the practical side of some of the theory they had learnt in college, which was extremely difficult. These guys had been programmed in college, and if what we taught them did not fit in with that they thought they knew (taught in college) we ended up taking them back to Basics 101.

As to what was in my students mind at the time?.. who knows, as he didn't.

As for all the lovely stuff like back EMF, changing loads, frequency variants, power factor, torque output, AC theory etc, etc.... before anyone can understand that, they MUST have a firm grip of the basics.
These students did not understand the basics, so a comment like "ohms law doesn't apply" is so left field, and based on nothing, it's plain stupidity!

Brich

I don't think you read my post. Your student may have been somewhat bumptious, but he was right in thinking you cannot directly apply Ohm's law to the pump motor, submersible or otherwise, or any other motor. Ohm's law would imply current proportional to voltage, and power to volts². If you measure the static resistance with a multimeter, and calculate current from supply voltage, it would be much higher than actual FLA. Motors don't behave like that. There's no option but to bring back-EMF into it, and inductance etc if it's AC. If that's too advanced for the class, better IMHO to stick to ordinary resistors for the time being. But let's not fall out about it, we've had an interesting chat .

I still say Ohm's is not a universal law (though it probably applies near enough to DC motor windings if used correctly). Plenty of other scientific laws are in the same category eg the ideal gas laws - OK for most everyday purposes, but not for any old pressure and temperature.

There is a phasor form of Ohms Law that handles AC very accurately. This is still considered to be Ohm's Law. The power, current and voltage do still have the same proportionality relationships. With one of these variables set to a constant, the other two will have the same proportional relationship (inverse or not) as when one works with a simple DC source. The difference between these two will just be that a complex, frequency dependent impedance (Z) sets the ratio and that the current and voltage wave forms will likely not be in phase.

I know both of you know this. Now you and brich should shake hands and agree that you were both just failing to communicate.

What surprises many students is how quickly this seemingly simple relationship gets very complicated when a non-linear device in the load produces multiple frequencies (harmonics) in a circuit.

In general the diameter of a tree trunk is proportional to the height of the tree.

Where the volume of wood in the trunk is proportional to the diameter squared x the height.

why In the volume equation height is inversely proportional............?

I really wish I hadn't started this, at first it seemed like a really simple way of pointing out the mistake in the OP's thinking. I still hold that it makes about as much sense, but when I realised that the volume is a function of the square of the diameter I began to wonder if the height would normally be directly related to diameter or cross sectional area or somewhere in between. I started trying to look it up and found that most measures of tree diameter girth are done "at breast height": which immediately begs the question whose breast?.....

Energy can neither be created nor destroyed; rather, it transforms from one form to another.
As an Electrical Engineer i always consider the above statement whenever i am crossing other discipline this way i do not encounter difficulties in relating electrical parameters to other discipline. I hope this helps

Hello,

P=U*I watt

W =Joule/sec

U =Joule/Coulomb

I= Coulomb/sec

Then U*I =Joule/sec (watt)

P=I^2 * R

if you using the just dimensions for I and R then the answer is in Watts

Not sure what that's meant add to it, but you left off P = U²/R

That reminds me of reply #20. You left out the water wheel that explains power, too.