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<   No. 417   2004-03-17   >

Comic #417

1 Jar Jar: But iffa yousa planet bein' too hot, cannot yousa use dissa heat energy to runna som-a freezin' units, muy muy?
2 Anakin: Jar Jar, the efficiency of a Carnot heat engine is only 1-TC/TH and any possible cold reservoir isn't going to be very cold.
3 Anakin: And that's the best theoretical case. The additional energy needed to pump the heat around will just heat Coruscant even more.
4 Anakin: Sometimes I don't think you understand basic thermodynamics at all.
4 Jar Jar: Meesa learn-a from George Lucas!

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The saga continues: #386, #393, #396, #399...

Admittedly if you haven't studied thermodynamics it's not obvious that Coruscant can't use the waste heat its civilisation generates to run cooling systems. So if you were wondering why I didn't mention this as a possibility, it's because it's not. (Wow, how many negatives can I squeeze into two sentences?)

Basically, yes, you can use heat energy to do work. The trouble is the efficiency is always less than 100%, because of Carnot's theorem of thermodynamics. What this means is you can't use heat energy to pump the heat around without also adding some external energy to the system. And this extra energy shows up eventually as waste heat. So not only do you fail to cool things down, you actually make it hotter.

Think about your fridge. It cools down the inside. But to do it, it pumps that heat out through the coils on the back. And to move that amount of heat, it consumes electricity, which also comes off the coils as heat.

So a civilisation the size of Coruscant generates so much heat simply from all the energy they're using that the planet will reach a temperature of about 500 K (227°C or 440°F) before it reaches equilibrium by radiating a black body spectrum to space (that's another physics lesson for another comic strip) and, because of the laws of thermodynamics, can't do anything to cool itself down*. And no, that doesn't count incoming heat from its sun - which will only make the situation worse. Even if it was in interstellar space with negligible sunlight, they'd still have this problem.

So now you know.

For those who wish to learn more, try reading about the Carnot heat engine.

For those who wish to come up with new excuses for how Coruscant could possibly be realistic, try reading about the Time Cube.


*: Okay, smartypants, yes, it could fire high thermal energy particles into space. Eroding the planet fast enough to need those 80,000 Super Star Destroyers bringing in shipments of dirt and rock every day. Not to mention creating a radiation hazard to any shipping in the system that would make ion cannons look like popguns.

2013-02-27 Rerun commentary: I've said about as much as I want to say about the physics of this already. Instead let me point out that two of the ship control panel LEGO pieces I've used here are white, and one is grey. The reason for this was that at the time I only had two white control panel pieces, so had to sub in a grey one for the third.

IN hindsight, given you can't actually see the control panel printing on the pieces from this camera angle, I could have used any old white 2×2 slope piece, but I prefer to know the set is complete for if I ever change the camera position.

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Last Modified: Wednesday, 27 February 2013; 02:11:01 PST.
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