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Microwave ovens vs. cordless phones

  1. Dec 11, 2007 #1
    If a microwave oven is using the same frequency as a 2.4 GHz cordless phone, why is it that a cordless phone does not heat your body when you use it? Im assuming it has something to do with concentration or focus of the waves? I would assume a cordless phone or anything that emits waves in this frequency would have to have some measurable effect on water molecules in its proximity, am I correct?
  2. jcsd
  3. Dec 11, 2007 #2
    It's simply the amount of power being used. Microwave ovens use up to 1000 W, while cordless phones and cell phones use powers on the order of 100 mW.
  4. Dec 11, 2007 #3
    do you know if a cordless phone or other 2.4 GHz wireless device can make a measurable difference in nearby water temperature (however small)?
  5. Dec 11, 2007 #4


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    Science Advisor
    Homework Helper

    Depends how good your measurement is (and how well insulated the water is) !

    1mW into a glass (100cc) of water, would heat it by 8.5mK in 1 hour assuming no heat was lost.
  6. Dec 11, 2007 #5
    Makes sense...I was forgetting about wave intensity. Thanks!
  7. Dec 11, 2007 #6


    Staff: Mentor

    The various regulatory agencies limit the amount of RF energy that is allowed to be deposited by a device. The limit of interest is called SAR (specific absorption rate) and is in units of W/kg. I don't know what the limit is for cordless phones, but manufacturers generally do two things to reduce it. First, they use low-power transmitters. Second, they position the antenna as far away from the head as they reasonably can.
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