The Physics Of Cooling Porridge

I think that Jonah is overanalyzing the situation:

Also, I have another peeve. Aside from the talking bears living in a nice middle class house, doesn’t the story defy the laws of nature? If the Papa bear’s porridge is too hot, that logically should be because it’s the biggest bowl and therefore would take the longest time to cool. The mama bear’s porridge should be “just right” because it’s the medium-sized one and the baby-bear’s should be too cool. Or, as is so often the case, do I have my physics wrong?

One overanalysis deserves another. He’s basically got it right, but it depends on the shape of the bowls. For any given shape, the larger the blob of porridge, the longer it will take to cool, because of the square-cube law. The volume of the porridge (which represents its heat capacity) goes up as the cube of the critical dimension (e.g., a diameter for a sphere) whereas the surface area (which is directly proportional to how fast it loses heat) goes up as the square.

For example, a cube of porridge an inch on a side will be one cubic inch of hot porridge that is cooled by six square inches of sides (assuming it’s floating in, say, a space station, and can have all six sides exposed to air). A two-inch cube has eight cubic inches (eight times as much) of hot porridge, but only twenty-four square inches of cooling surface (six sides of four square inches, that is, only four times as much). So if you double the size of the critical dimension, you double the cooling time as well.

Of course, if you have a spherical blob of porridge, and a large thin pancake of it, you could have a larger amount of porridge that cooled faster in the latter case. If, for example, we took the eight cubic inches from the previous example, and spread it out to an eighth of an inch thin in a pancake shape, then you’d have something with sixty-four square inches on each side (a hundred twenty eight) plus the side area (an eighth times the circumference, which would be the square root of 64 divided by pi times 2pi, or 2 times the root of 64, or about two square inches). So now we have eight times the volume of the one-inch cube, but over twenty times the surface area, so it would cool much faster.

So if Momma Bear’s porridge was in a wide flat bowl, and Baby Bear’s in a higher, narrower one (perhaps with a picture of a Teddy Human on it), it’s certainly conceivable hers could be colder than the baby’s.

Porridge and bears aside, this is the principle employed when one pours hot tea into a saucer to cool it (the metaphorical function of the Senate, in the Founders’ estimation, which would temper the urges of the House).

Why yes, I am in fact avoiding writing a proposal that’s due next week. Why do you ask?

[Update mid afternoon]

Welcome, Corner readers. Just curious, though, why no comments from any of you? No one in the comments section except the regulars, so far. Does this say anything about Corner readers?