1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

EM waves: energy and power related to frequency

  1. Aug 27, 2013 #1
    I was reading articles about EMR and health and I realized I never got how "output power" is related to frequency.

    Let's talk about energy, the rule is: the higher the frequency (shorter wavelength) the higher is the energy transported by the wave.
    Now power, power is the energy consumed in a certain period of time. But that contradicts (at least in my mind :D) the definition of energy/frequency.

    Take a Wi-fi signal vs a microwave oven for example, they both operate on similar frequencies (2.4Ghz) but one has an output of about 200mW and the other up to 1KW :| So if the microwave oven has this amount of power it means that it also carries a lot more energy than Wi-fi, but how does WiFi carry such a lower energy at that high frequency, so the question is how do you control the output power ? (1)

    Also, it's been demonstrated that gamma and X-rays do damage human tissues, what I don't understand is: is the wavelength related to it being dangerous or is the power also part of it? (2) Considering that I can send waves with different power what is it more dangerous a AM radio wave of 1MW of a x-ray of 1mW? :D (3)

    Lastly, the spacing related to energy. 3G (UMTS Cellular technology ) usually operates on 900 Mhz and 2100Mhz with a spacing (or width) of 5Mhz for each block. I know that 5Mhz is always 5Mhz but why do they need the same amount of width to carry the same datarate if those "5mhz" carry more information in higher frequency of the spectrum? (4)
  2. jcsd
  3. Aug 27, 2013 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    The total power of the wave is mostly independent of the frequency. It is quite possible to be transmitting 100 watts of power with a 100 MHz antenna and a 100 GHz antenna.

    Higher frequency waves have more energy per photon than lower frequency waves. A 200 GHz EM wave has photons with an energy of about 8.2713x10-4 eV.

    On the other hand, an X-Ray with a frequency of 3.0x1016 Hz has an energy per photon of about 124 eV. It only takes somewhere around 10 eV to start knocking electrons out of their orbitals, which results in ionization of the atom/molecule. This ionization is how UV Rays, X-Rays, and Gamma Rays damage the human body. Each individual photon has enough energy that it ionizes DNA and other parts of cells, damaging or destroying them.

    To my knowledge they do not carry more information at the higher frequency unless the spacing is larger than 5 MHz. When modulating a signal, the higher the frequency of the information signal, the larger the bandwidth required. So transmitting a voice call at 20 Kbps requires FAR less bandwidth than broadband internet at 10 Mbps.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook