Linux and RIM Blackberry

[Peter]

On Tue, 17 May 2005, Lennart Sorensen wrote:

> On Tue, May 17, 2005 at 09:02:39AM +0300, Peter wrote:
>> Eventually you will be able to warm your coffee by holding the cup out
>> of a window for 2 minutes.
>
> And just how much power would people have to be broadcasting at
> (assuming it was the right frequency microwaves) for that to work?

Roughly 700W/dm^2. there should be several times that amount in the near 
field of a transmitter (esp. TV UHF transmitter or near the main beam of 
a broadband dish). Why ? ;-)

Calculation is:

1J = 1W*s = 0.23cal,
1cal = 1K/cm^3 delta-t H2O at a certain temperature

1 cup = 250ml = 250cc => 250*4.18 ~= 1000 Joules per degree of heating. 
We need about 60 degrees (from 4 deg C to 64 deg C) so 60,000 Joules. 
The cup cross section is about 10x10cm^2. A 700W/dm^2 beam delivers 
about 84,000 Joules in 2 minutes. Which is about right imho (there is 
some mismatch, reflexion etc).

You only have such conditions inside a beam or a waveguide. 700W/dm^2 is 
70kW/m^2, about 40 times the power of the sun at noon. A normal 
microwave oven also uses about 700W and it heats a cup in about that 
amount of time.

It was meant as a joke. Normal exposure to mw is much lower. Something 
that rises your skin temperature by 1 degree in 1 second is easily 
noticeable by anyone and that occurs at much lower power levels. Afaik 
that level is around 5W/dm^2 or so (500W/m^2, about the power of a 
1-element electric space heater at 1 meter). Touching the antenna of a 
5W HF transmitter should have a similar effect (it does).

Broadcast transmitters must use large amounts of power to give good 
coverage. E.g. FM radio will run 50kW or more to cover an area of about 
60km^2 with marginal quality around the edges (60km^2 is a circular area 
with about 20km radius). TV needs even more power for the same area. Put 
10 mixed FM and TV transmitters like that on a radio tower, and you can 
probably heat your cup by sticking it out of the window when close 
enough.

Peter
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