[sdiy] help needed: thermal equations to RC networks analogy

[ChristianH]

On Tue, 11 Feb 2003 14:58:07 +0100 Czech Martin wrote:

> I believe that a heat source plus thermal medium can be described
> in electrical analogy:
> 
> temperature -> voltage
> heat stream -> current
> heat source -> voltage source
> thermal isolation -> resistance
> heat capacity -> capacitor
> 
> It's only a believe, but I'd like to know instead!
> 
> So ,e.g. a resistor firing a volume of air can be described
> as a voltage source (voltage ~ total heat power),
> and a RC network: a string of resistors, and from nodes
> in between capacitors to ground.
> If the whole is to take place in a package one needs some 
> resistor to model heat losses to the outside world.
> 
> Problem:the voltage source will finally give voltage
> to all nodes (i.e. heat will distribute).
> But if the voltage is switched of, the voltage source
> will draw current back from the network.
> This would mean that a resistor that is not burning
> any power input will take some heat current back
> from the air, actually cooling it. ouch!

I think this is where the analogy is limping a bit. An electrical
resistor generating heat has a heat capacitance as well. When it stops
running electrical current through itself, it doesn't stop emitting heat.

It would do so if it could be immediately put back to room temperature
(i.e. thermal ground), and in this case it would indeed drain heat from
the surrounding air. So much for the theory part...

Christian