Microwave ovens vs. cordless phones

[overlook1977]

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?

 

[Manchot]

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.

 

[overlook1977]

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)?

 

[mgb_phys]

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.

 

[overlook1977]

Makes sense...I was forgetting about wave intensity. Thanks!

 

[Dale]

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.