Re: Supercard

["Michael J. Mahon" <mjmahon@aol.com>]

Mark McDougall wrote:
> Henry wrote:
>
> > I just can't see how 5v will keep your cup warm though?  Traces on the 
> > Motherboard of the system are probably a little warm too.
>
>
> Yeah, interesting. They all claim to maintain a cup of coffee at 40-50 
> degrees Celcius, but I can't get my 'paper napkin' calcs to work...
>
> Given that USB is limited to 500mA @5V = 2.5W.
>
> I based it on a 420ml coffee cup (already) at 40 deg C, and an ambient 
> temperature of 20 deg C, assuming such a cup would take 30 mins to cool 
> to ambient temperature. I also assumed a linear temperature gradient 
> over the 30 mins (I believe the temperature gradient would actually 
> *decrease* as it cooled).
>
> By my calcs, it would require 19J to be transferred to the liquid every 
> second... (19W)... to *keep* it at 40 degs..
>
> .... out by a factor of 10!

That sounds about right, if you are assuming (as pictured) that the
cup is exposed to air on all surfaces except the bottom, and that the
top of the liquid is also exposed to air.

I wonder how the result would change if we assumed an insulated cup,
with heat input at the bottom and heat loss only from the liquid
surface on top?  (I'm sure that 2.5W is still insufficient to maintain
a 20-degree C temperature differential.)

Perhaps if the 2.5W were input to a thermoelectric heater, so that some
heat was being pumped as well as dissipated resistively...  (I've
actually seen one of these, that could be "reversed" to keep a drink
cool[er] as well.)

-michael

Parallel computing for 8-bit Apple II's!
Home page:  http://members.aol.com/MJMahon/

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