Talk:Power (physics)

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[edit] Discussion

Can the power consumed by a DC circuit with changing voltage (as the battery is depleted) and changing current from times ti to tf 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)


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

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)

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)

[edit] 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)


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)

[edit] 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)

[edit] 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)

[edit] 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)

[edit] 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)

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

Those formulas are used e.g. in telecommunications.
--Čikić Dragan (talk) 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].
--Čikić Dragan (talk) 19:47, 2 June 2008 (UTC)