Jasen Betts wrote in
:
On 2007-07-08, Spurious Response
wrote:
On Sun, 08 Jul 2007 00:25:22 -0700, wrote:
no, they radiate less total energy because they take 1/3 the time and
radiated power is no different to a 1kW unit, since that depends
entirely on water temp and kettle shape & finish.
Sorry, but the difference in time heating 4 quarts of water in a 1kW
vat is NOT 3 times that of the 3kW unit.
There are constraints. The water has to still be there when you are
done.
If it takes a 1kW unit to heat it to boil in ten minutes, there is no
way in hell a 3kW unit will do it in three.
Of course not! it'd take three and twentyy seconds.
It is NOT linear, chucko.
for a heating element immersed in the water it is almost exactly linear,
do the math.
Bye.
Jasen
Some useful information:
1 cup is about 236.6 ml.
It take 1 cal to raise 1 ml of water by 1 degree.
A watt is about 0.239 cal/second.
Let us assume the cup of water starts at 25 C and you want to raise it to
100 C, that is 75 degrees that we want to heat the water.
So, it will require (236.6 ml x 75 deg x 1 cal/(deg x ml)) = 17744.25 cal
to raise a cup of water 75 deg C.
You can deliver those cals at any rate you like[see notes below]. A 1 kw
heater will deliver 239 cal per second. A 2 kw heater will deliver at
twice that rate.
74.3 seconds to heat a cup with a 1 kw heater.
37.1 seconds to heat a cup with a 2 kw heater.
[notes: ignoring losses in wiring, assuming efficient heat transfer from
heater to water, assuming no loss of heat from cup+heater combination]
If you want to factor in any of those, please state your assumptions [such
as thermal transfer resistance between heater and water, between container
and air. Those act much like series resistors as far as the heat is
concerned. You can model temperature difference as voltage drop and heat
transfer as current if you like. Any good engineering heat transfer text
will show you exactly how to do this{but in this case, it won't really
matter all that much}].
--
bz 73 de N5BZ k
please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.
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