ricecake comments on Oh lord yes

[–] ricecake@sh.itjust.works 3 points 2 months ago (2 children)

Nope, there's just even more energy in a lump of coal.
I believe their point is that it's a bit silly to sit around focusing too much in thermodynamics beyond the raw limits, when there's a lot of factors that weigh in to how much of the theoretical maximum is available or used. Beyond just the basic chemistry involved, there's hormones that influence how it's used, and peoples urges to consume.

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[–] platypode@sh.itjust.works 3 points 2 months ago (1 child)

That’s just not true.

The average home in the US uses 10,000 kWH per year; let’s be generous and call that 500 kWH per month, or 2e9J.

A sugar cube is 5g. To power a home for a month, it would therefore need a specific energy of 2e9J/5g = 0.4e9 J/g. Normally specific energy is given in MJ/kg, so we get a minimum needed of 400,000 MJ/kg.

Coal, per Wikipedia, comes in at 26-33 MJ/kg. Liquid hydrogen gets over 140, and that’s the most efficient fuel on the list.

I didn’t bother to look up the specific energy of sugar, but somehow I doubt it laps coal by orders of magnitude. The only possible way it could have that much energy is if we could convert its mass directly to energy, but neither technology nor biology can do that right now so it’s a bit silly to even mention.

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[–] ricecake@sh.itjust.works 3 points 2 months ago

neither technology nor biology can do that right now

Yes, that was the point. If we focus purely on the math to the exclusion of reality you get results that don't apply to reality.

If you only look at thermodynamics when discussing weight loss you get the best possible weight loss advice: "eat less".
It happens to ignore the reality of biology, how the body reacts to changing nutrition, or how it reacts towards changing your desires when nutrition changes, but it is technically correct.

Just like it's technically correct to say that a sugar cube has enough energy to power your home for a long time. It may be a useless observation, but it's technically true.

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