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High and Low Frequency:Energy

  1. May 26, 2009 #1
    Consider following:
    The RMS value of any sine wave is Ampiltude/sqrt 2. It is independent of frequency.
    So consider two sine wave generators, one generates at 50 Hz and the other one generates at 500Hz. Both have same amplitude A for volt wave and B for current wave. Consider same pure resistive load connected to both.
    The power equation for both would be Vrms.Irms = AB/2.

    Que1. Is this per wave cycle or per second ?

    Que2. Is it correct that the 500Hz wave will deliver same energy as 50 Hz in 10 times
    LESSER time per cycle??

    Thanks in advance.
  2. jcsd
  3. May 26, 2009 #2


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    Staff: Mentor

    It's per second, averaged over an integer number of cycles.

    Both waves deliver the same energy in the same amount of time (e.g. one second). The 500Hz wave delivers 1/10 the energy as the 50Hz wave per cycle, but the 500Hz wave makes 10 times the number of cycles as the 50Hz wave during the same amount of time, so the total energy is the same for both.
  4. May 26, 2009 #3
    Thank you very much jtbell,
    Just one more thing to confirm.
    According to Planck's law, we need higher min energy to create one photon of high frequency than to create one for low frequencies, right?

  5. May 27, 2009 #4


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    Science Advisor
    Gold Member

    Planck's Law is the distribution of emitted radiation of a black-body radiator at a given temperature. It is not the energy of a photon, I believe that was first given by Einstein in his photoelectric paper of 1905. But yes, the energy of a photon is directly proportional to its frequency, the higher the frequency, the higher the energy of the photon. This does not mean that if I have an electromagnetic wave that is of frequency 500 MHz and one of 50 Hz that the 500 MHz wave is of higher energy. Classical EM relates energy to be proportional to the amplitude of the waves. So, given two waves of differing frequencies but the same energy (classical), then the difference is that the higher frequency waves have fewer photons being emitted per unit time.
  6. May 27, 2009 #5
    Ok, I got that. Thanks.

    This is exaclty what I wanted to confirm.

    Thanks again.
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