Microwave ovens vs. cordless phones

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Discussion Overview

The discussion centers on the differences between microwave ovens and cordless phones operating at the same frequency of 2.4 GHz, specifically exploring why cordless phones do not heat the body like microwave ovens do. The scope includes technical explanations and conceptual clarifications regarding wave intensity and regulatory standards.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions why a cordless phone does not heat the body despite operating at the same frequency as a microwave oven, suggesting it may relate to the concentration or focus of the waves.
  • Another participant attributes the difference in heating effects to the power levels used, noting that microwave ovens operate at up to 1000 W while cordless phones use around 100 mW.
  • A participant inquires whether a cordless phone or similar device can cause a measurable increase in nearby water temperature.
  • In response, another participant provides a calculation indicating that 1 mW into a glass of water could raise the temperature by 8.5 mK in one hour, depending on measurement accuracy and insulation.
  • One participant acknowledges the importance of wave intensity in the heating effect.
  • A later reply discusses regulatory limits on RF energy exposure, mentioning the specific absorption rate (SAR) and how manufacturers mitigate exposure by using low-power transmitters and positioning antennas away from the head.

Areas of Agreement / Disagreement

Participants generally agree that power levels are a significant factor in the heating effects of RF energy, but there is ongoing inquiry into the measurable effects of cordless phones on water temperature and the implications of regulatory standards.

Contextual Notes

Participants express uncertainty regarding the specific absorption rate limits for cordless phones and the conditions under which measurable temperature changes might occur.

overlook1977
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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? I am 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?
 
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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.
 
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)?
 
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.
 
Makes sense...I was forgetting about wave intensity. Thanks!
 
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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