Talk:Power (physics)

Formula

In the Tao of Jeet Kune Do, Bruce Lee defines power as speed times force. — Preceding unsigned comment added by 108.0.237.58 (talk) 19:44, 9 November 2016 (UTC)

Discussion

Can the power consumed by a DC circuit with changing voltage (as the battery is depleted) and changing current from times $t_{i}$ to $t_{f}$ be defined as follows?

$P=\int _{t_{i}}^{t_{f}}\!\left[v(t)\cdot i(t)\right]\,\mathrm {d} t$

If not, what do I need to change to make it work? --Oren Hazi 03:34, 29 Jan 2005 (UTC)

I think what you are actually defining here is the energy consumed by the circuit

$E=\int _{t_{i}}^{t_{f}}\!\left[v(t)\cdot i(t)\right]\,\mathrm {d} t$

To get the average power consumed, you could use instead:

$P_{avg}={\frac {E}{t_{f}-t_{i}}}$

-- Pgabolde 14:59, 31 Jan 2005 (UTC)

alternatively the instantaneous power is p = v*i

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Who is so sure that use of power in physics is commoner than that in sociology or mathematics? -- Taku 02:15 May 9, 2003 (UTC)

Take a look at its What links here, and decide for yourself. -- John Owens 02:24 May 9, 2003 (UTC)

No, not in wikipedia but generally speaking, of course. -- Taku 03:46 May 9, 2003 (UTC)

The physicists clearly have the primary use of this one. That's the meaning that you would normally assign to "power" unless the context indicated otherwise. Tannin

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I think there should be a picture to along with the AC power thingy. The phi is sort of useless without illustrating it. dave 04:35, Feb 22, 2004 (UTC)

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The books I have on electrical power use lowercase "i" and "v" for (varying) instantaneous current and voltage, capital "I" and "V" for the constant time-average RMS current and voltage. If no one objects, I'm going to make the article consistent with this. --DavidCary 20:46, 20 Aug 2004 (UTC)

power, frequency, amplitude, energy

two things that i know that seem to conflict in my mind:

1. power of a sinusoid is related directly to amplitude and unrelated to frequency.
   • an electrical wave of any frequency will have the same power if passed into a load, as long as the RMS or peak-to-peak amplitudes are the same
     • Yes. To prove this, you calculate the instantaneous power (V²/R) at each point on the sine wave over a complete cycle, then integrate over time. If you double the frequency then you get half the energy per cycle (because the cycles are half as long), but twice as many cycles per second. --Heron
2. higher frequencies have more "energy"
   • this is true in electromagnetic waves, right? gamma rays have more energy than microwaves
     • Not exactly. A quantum of gamma ray energy has more energy than a quantum of microwave energy. However, if these rays were generated by some processes involving voltages of equal amplitudes, then the average power over time would be independent of frequency, as above. The microwaves would contain more quanta than the gamma rays to make up for the difference in the energies of the quanta. --Heron
   • also true for vibrating strings and things like that, since the string is stretched more for a high frequency and intuitive because high frequencies decay more quickly.
     • Not true, at least in an ideal medium (linear and lossless) for the same reason as above - for 'voltage' (V) read 'displacement', and for 'electrical resistance' (R) read 'mechanical resistance'. A nonlinear or lossy medium could favour either high or low frequencies, depending on its properties. --Heron

can we explain why these two things seem intuitively conflicting? - Omegatron 18:54, Sep 2, 2004 (UTC)

I think the paradox you are describing is similar to the problem called the ultraviolet catastrophe. This was the problem that, according to classical (pre-quantum) wave mechanics, waves of infinitely high frequency would have infinite energy, leading to an infinite amount of energy being emitted by any radiating body. It took quantum mechanics to explain why this would not happen. --Heron 20:12, 2 Sep 2004 (UTC)

Actually, my knowledge of the energy vs frequencies was just incorrect, i guess.  :-) so then why do the high frequencies die off in a vibrating string more quickly than the low? i've heard people say that higher frequencies have more energy many times in different contexts, so we should make sure we address that in whichever relevant article. - Omegatron 21:53, Sep 2, 2004 (UTC)

oh wait you just explained why they die off more quickly. i missed that sentence - Omegatron 21:54, Sep 2, 2004 (UTC)

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I'd like to add the definition of peak power (of a periodic signal) somewhere. Is this article the right place or should it be a page of its own? -- Pgabolde 18:36, 15 Nov 2004 (UTC)

Go ahead, add it here. Don't worry about it being the wrong place - stuff gets moved around Wikipedia all the time. --Heron 09:50, 16 Nov 2004 (UTC)

Merge from Power (disambiguation)#Physics

I've proposed that the majority of the content in Power (disambiguation)#Physics be merged to this article. Nothing more is needed on a disambiguation page than enough information to distinguish one possible meaning of a word from the others, yet an article has developed underneath the Physics section of Power (disambiguation). Once the section is merged, a link to this article should remain on the disambiguation page in context that makes it clear that more information about power is available here. --TantalumTelluride 00:51, 2 November 2005 (UTC)

I think I wrote most of the material that seems to be developing into an article. I thought that there was a need to distinguish between power and energy, but perhaps there isn't so much need to do that on the Power disambiguation page. I will work on cleaning it up. --C J Cowie 02:37, 2 November 2005 (UTC)

I cut back my previous edits: --C J Cowie 14:37, 2 November 2005 (UTC)

Thanks. I would have replied sooner, but a bot mistakenly blocked my IP address. Anyway, you're right; there does need to be an explanation of the difference between power and energy because there is a common misconception that they are the same thing. I just thought that there was way too much information on the disambiguation page, especially since the title of the page doesn't explicitly indicate that it is a disambiguation. It looks a lot better since your revision. Thanks again. --TantalumTelluride 16:37, 2 November 2005 (UTC)

This merge is now complete. --TantalumTelluride 16:40, 2 November 2005 (UTC)

Power expressed in elementary units

One never sees power expressed in elementary units, and this article is no exception. It's J/s, so that would be kg*(m²/s³). I'm surprised to see a power 3 in something so elementary. Is that the reason it is never given? I'd say every article on an SI unit should have an expression in the elementary units right in the intro.

Also, I thought that the more elementary the unit, the simpler it would be. Might this be an indication we're using the wrong basic units? DirkvdM 11:12, 31 January 2006 (UTC)

I agree that every SI unit article should give the unit in terms of SI base units, so I added it to the article on the SI unit Watt. Power is not such an elementary thing. It's actually a rather abstract concept.--Srleffler 13:39, 31 January 2006 (UTC)

Mechanical power

Srleffler, I agree with your caveat about constant force, since the article uses the word "power" in an indefinite sense that doesn't assume an instantaneous measurement. However, if the (total) force on an object is constant, then by Newton's law its velocity cannot be constant, so the formula still addresses only the instantaneous power. I'll try to make that more clear in the article... Melchoir 19:45, 9 February 2006 (UTC)

Looks good. I wasn't trying to address average power. I'm just a bit rusty on my calculus. I thought that the derivation of the formula for P from that for W wouldn't work if F were a function of t, but I see now that it does work.--Srleffler 22:54, 9 February 2006 (UTC)

Scalar or Vector

Is power a scalar or vector. What about energy? I thought that energy flowed and could therefore be considered a vector. Since power is merely the time derivative of energy, doesnt that make it a vector too? Im confused--Light current 19:36, 27 October 2006 (UTC)

Power and work (and energy) are both scalars. Work is the scalar product of the force vector and the displacement vector, and power is just the derivative of that, so they are both scalars. You cannot quantify energy in a specific direction; for example, if a book has gravitational potential energy of 10 J more than if it were on the floor, in which direction is that potential? It's just a scalar quanity. The reason it can seem like a vector is because in the case of kinetic energy, an object is always moving in a direction; however, an object moving in the opposite direction has exactly the same kinetic energy. Eebster the Great (talk) 01:47, 10 November 2008 (UTC)

Formula for mean power of casual (random) signal is missing

Those formulas are used e.g. in telecommunications.
--User:Vanished user 8ij3r8jwefi 18:46, 13 May 2008 (UTC)

Probably you mean causal, but still I can't interpret what you have in mind; how is causal related to random? And isn't mean power just a trivial application of the mean square? What kind of formula are you envisioning? Dicklyon (talk) 18:50, 13 May 2008 (UTC)

Probably I don't know right word for it in English. I mean no determinated.
I have no experience in math language here used. Integral [(s^2)*p(s)ds] in some bondaries, where s is signal and p(s) is probability. It think it is on R=1 [Ohm].
--User:Vanished user 8ij3r8jwefi 19:47, 2 June 2008 (UTC)

Confusion with strength

There used to be a section "Power and strength", which was deleted with this edit. I agree with the deletion and the rationale "there is no evidence that power is confused with strength" in an encyclopedic sense.

However, the fact that the section was there is evidence that at least some editors confuse it. "Power (physics)" is such a fundamental article that it should also be written with such editors or readers in mind. I wrote a comment to that effect in a hidden note, and I realize now it would have been better if I had written it here right away. So if someone feels up to clarifying that confusion, go for it. Maybe I'll do it at some later time. — Sebastian 20:40, 7 December 2008 (UTC)

I have no idea why the entire article on power made absolutely no mention of the concept of strength. But now with this edit, that deficiency has been fixed. That simple equation: $P=Fv$, along with the explanation that strength is a force should make it perfectly clear what the relationship between power and strength is. I'm coming to this more than seven years later, but at least it's been handled now.--Tdadamemd sioz (talk) 04:15, 22 February 2016 (UTC)

I don't think it's clear at all and have removed that sentence. See "Strength" section below. Kendall-K1 (talk) 02:10, 5 October 2016 (UTC)

Power vs. Energy

I believe that there should be a page created which details the differences between power and energy, primarily by explicitly defining them, including the use of dimensional analysis. The reason that i believe that there should be this specific page is that the terms "power" and "energy" are often used interchangeably in everyday conversation, so someone new to the topic (in its scientific context) could understandably be confused. I would write a page/section about this very issue but i am *very* new to Wikipedia and I don't even know if this idea has been brought up before. Mathiusdragoon (talk) 22:25, 6 April 2009 (UTC)

Faraday disk

The image was very pretty, but there was absolutely no mention of it anywhere in the article. If the image, and more importantly the concept, of the Faraday disk are relevant enough to belong here, then somebody ought to explain the relevance.—PaulTanenbaum (talk) 14:00, 20 August 2009 (UTC)

Example contrasting power, energy

Some may find the example I plugged in at the start of the intro a bit out of place. OK. I did feel that a bit of less-technical explanation is in order before the article dives into (eegad!) mathematical formulæ! And my mod scratches user:Mathiusdragoon's itch (see above).—PaulTanenbaum (talk) 20:24, 29 March 2011 (UTC)

Power as the product of force and velocity

I would like to make some minor changes to the introduction to identify the product of force and velocity, or torque and angular velocity as power. Prof McCarthy (talk) 16:10, 4 October 2011 (UTC)

It is, isn't it (contrary to what some contributors think). Be bold. --Wtshymanski (talk) 18:22, 4 October 2011 (UTC)

I moved the discussion of average power to is own section, and tried to simplify the lead. Prof McCarthy (talk) 01:52, 10 November 2011 (UTC)

A question arises in this context. If the power is used to accelerate a body from velocity v1 to velocity v2 considering constant acceleration a which velocity should appear in the expression P= F*v= m*a*v, the initial or the final velocity?--188.26.22.131 (talk) 12:21, 1 August 2013 (UTC)

The formula applies at every point in time, so the power is changing in proportion to the velocity in that situation. Dicklyon (talk) 15:18, 1 August 2013 (UTC)

That new section was not explaining how power is force times velocity. So I inserted a more basic section right before that to show simple equations that show this. It's 2016, and the article now clearly communicates that power is the product of force and velocity.--Tdadamemd sioz (talk) 04:18, 22 February 2016 (UTC)
Oh, I now see that this relationship was being stated in the section following the one that was being mentioned here.--Tdadamemd sioz (talk) 04:22, 22 February 2016 (UTC)

Revisions to the lead

Jc3s5h, thank you for your help with the revisions to the lead. I think it reads very well. Prof McCarthy (talk) 03:29, 17 November 2011 (UTC)

Revisions to Mechanical Power

I tried to smooth out the language for this section on Mechanical power, and found that things got worse and worse. Sorry for the long line of revisions. I hope the result is considered to be an improvement. Prof McCarthy (talk) 05:48, 21 November 2011 (UTC)

Wrong formula in Mechanical advantage

Evidently, one of the two formulae for mechanical advantage (that relating forces or that relating torques) is inverted. As I do not know the definition of mechanical advantage, I do not know which is wrong and can not correct it. Kurt Artindagi (talk) 07:25, 14 December 2013 (UTC)

I fixed it to agree with Mechanical_advantage#The_law_of_the_lever. Dicklyon (talk) 07:59, 14 December 2013 (UTC)

Strength

I don't think we should be talking about strength. When you say that word to me, I think of units N/m², not N. It is not a precise term used in physics. The article right now links to Physical strength which does define it as a force, but then goes on to talk about moments about a joint. I think talking about strength when we mean force just muddies the waters. Kendall-K1 (talk) 12:26, 22 February 2016 (UTC)

I removed this:

A term that is typically associated with the concept of power is strength. Strength is a force, so the above equation gives the relationship between power and strength.

This had been linked to Physical strength. That article doesn't define strength at all as far as I can tell, as a force or otherwise. The opening sentence is nonsense: "Physical strength is the measure of an animal's exert of ability on physical objects." Someone changed the link to point to Strength#Physics. There is no information there, it is just a list of possible things "strength" could mean, and none of them are relevant; the various material strengths are measured in force per unit area and don't have anything to do with power. Unless we can find some RS that equates strength to force and says why that's important in understanding power, I don't think this belongs here. Kendall-K1 (talk) 02:07, 5 October 2016 (UTC)

TNT

I find this confusing: "Burning coal produces around 15-30 megajoules per kilogram, while detonating TNT produces about 4.7 megajoules per kilogram... The coal value does not include the weight of oxygen used during combustion, while the TNT number is TNT only." Exploding TNT in a vacuum produces around 4.2 Mj/kg. In air, around 4.7 (from TNT equivalent). In oxygen at 2.5 bar, 15.[1] I think we should change this to say it doesn't include the weight of oxygen from the air for either fuel. Kendall-K1 (talk) 21:29, 17 October 2016 (UTC)

Electrical Power

At the moment electrical power has the following set of equations defining power using Ohm's law:

$P=I\cdot V=I^{2}\cdot R={\frac {V^{2}}{R}}\,$

where

$R={\frac {V}{I}}\,$

is the resistance, measured in ohms.

But doesn't it make more sense to state Ohm's law in terms of $I$? Since $I^{2}$ is the term that is substituted to get the final equation $P={\frac {V^{2}}{R}}\$ Wrightgw (talk) 15:05, 18 November 2016 (UTC)

I don't think the statement should be a derivation that leads up to the final expression, $P={\frac {V^{2}}{R}}\$. Rather, I think the statement is giving several equivalent expressions, any of which may be the most useful, depending on which quantities are known. Jc3s5h (talk) 17:05, 18 November 2016 (UTC)

Per unit time

I don't think we need to have "per unit time" at the end of both of the first two sentences. I would go for removing it from the first sentence. Kendall-K1 (talk) 00:25, 18 September 2017 (UTC)

Perhaps combining the first two sentences would be more appropriate, perhaps something along the lines of: "In physics, power is the rate of doing work (amount of energy consumed) per unit time."--☾Loriendrew☽ ☏(ring-ring) 01:51, 18 September 2017 (UTC)

Shouldn't have been at the end of the first sentence, ever. I have merged the two sentences as described here. Power is a rate at which work is done. --Wtshymanski (talk) 02:18, 18 September 2017 (UTC)

Incorrect dimensions in the infobox

The dimensions should be L^2*M*T^-3, not L*M^2*T^-3. I'm not sure how to make the edit, though. — Preceding unsigned comment added by 71.238.21.161 (talk) 06:00, 12 January 2018 (UTC)

Equations for power

I'm not sure it's useful to express power as dW/dt in this section. The typical reader won't know what this means, and the distinction between instantaneous and average power is already discussed in the following section. On the other hand, we already say work/time in the lead, so maybe it's ok. Kendall-K1 (talk) 15:34, 17 January 2018 (UTC)

I think the dW/dt thing is fine in general, but it's the whole section that probably needs to go, and the article is in need of a thourough rewrite. P = Fv is true for the mechanics of moving bodies. But this has little bearing on say rotational power, or electrical power. Headbomb {t · c · p · b} 19:10, 17 January 2018 (UTC)

Lead equation

I think the very first equation, in the first paragraph of the lead, was better written as "power = work / time". We can add more rigor later in the article, but it was more clear as an introduction when it was written this way. Kendall-K1 (talk) 00:49, 25 June 2018 (UTC)

Peak power and duty cycle

The present section "Peak power and duty cycle" needs to be removed/replaced/rethought and retitled. "Duty Cycle" is only meaningful, if at all, in ON/OFF situations, in which the average power is the ON power times the Duty Cycle. 94.30.84.71 (talk) 14:39, 20 November 2018 (UTC)

Isn't that exactly what the last equation in that section says? Kendall-K1 (talk) 15:04, 20 November 2018 (UTC)

Motive propulsion made ridiculous

Was:

The power of a jet-propelled vehicle is the product of the engine thrust and the velocity of the vehicle.

Changed to:

In classical mechanics, as quantified from a stationary frame of reference, the motive power of a jet-propelled vehicle is the product of the engine thrust and the velocity of the vehicle (note that by this definition, a propelled vehicle hovering at stationary elevation over a gravitational body, where the upward thrust exactly cancels the downward acceleration of gravity, the motive power is zero).

I wasn't quite sure how to get this right, so I settled for making it sound slightly ridiculous.

In the relativistic setting, the observable is acceleration. Acceleration times inertial mass (if known) yields a force (newtons). But then you need to multiply by distance (meters) to obtain energy (joules). I suppose that the occupant of the enclosed elevator could work out a presumptive distance (for the component of acceleration not attributed to gravitational force) but that gets weird in a big hurry (for constant acceleration, your presumed energy output under presumed displacement increases in time, in a manner seemingly dependent on where you assign t=0). Is power actually relative in a way that forces the elevator occupant to look out a window in order to come up with a sane denomination? What am I getting hopelessly wrong here?

In any case, the real issue is what was wrong with the original definition of the motive power of jet propulsion, because I'm personally not entirely satisfied by a definition of power where a stationary hovercraft thruster drowning out conversation for miles around is entirely impotent. — MaxEnt 20:47, 27 January 2021 (UTC)

I puzzled this out for a few minutes longer, after communing with Physics 12 from decades ago (which came easily to me at the time). The elevator occupant can observe his rocket engine, and quantify the relative momentum exchange by the velocity of the expended exhaust times the exhaust flux (mass integral). Assigning an arbitrary t=0 to obtain absolute momentum doesn't strike me a problematic. As for expended power, we can certainly take the square of the velocity of the exhaust jet times the exhaust flux. But then attributing this to "motive" power (or not) does seem to depend on a predetermined external coordinate frame in which present velocity is a known quantity. I'm still left feeling that the original definition is silly unless supplied with additional explanation. — MaxEnt 21:11, 27 January 2021 (UTC)

Improving the Radiant power section

I have a couple of suggestions for improving the [[2]] section:

• First, the reference for the formula is already found at [[3]].
• Second, given some other signal processing and information theory notation, I think it is better (less confusing upon fist read) to remove the parentheses (as though $I$ could be a function), and instead write this formula as $P=|I|\cdot 4\pi r^{2}$, as is done in the first reference on intensity.

Maxie (talk) 23:23, 6 March 2021 (UTC)