Talk:Power (physics)

From Academic Kids

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

<math>P=\int_{t_{i}}^{t_{f}} \! \left[ v(t) \cdot i(t) \right] \, \mathrm{d}t<math>

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

<math>E=\int_{t_{i}}^{t_{f}} \! \left[ v(t) \cdot i(t) \right] \, \mathrm{d}t<math>

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

<math>P_{avg}=\frac{E}{t_f-t_i}<math>

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

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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 (http://www.wikipedia.org/w/wiki.phtml?title=Special:Whatlinkshere&target=Power), 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)