Tag Archives: science

Thermodynamics

I asserted to my daughter last week that a paper cup with water in it will not catch fire if placed directly in a fire. So, of course, we had to try it.

I was a little nervous, but it turns out this is completely true. The cup burned down to the water line, and then didn’t burn until the water had completely boiled off. The *instant* the last of the water boiled off, the cup burst into flames and was gone almost immediately. (Animated version of image is here.)

Why? Well, it’s because water boils at 212°F (100°C) and paper combusts at 451°F (843.8°C) so as long as there is water in the cup, the heat of the cup is being convected away into the water to heat it towards boiling and the cup remains too cold to ignite. Once the water starts boiling, the cup is full of steam, which is quickly carrying away the heat. The moment the water has all evaporated, though, the cup is abruptly at combustion temperature and goes up in a flash.

You should try it. It’s a great way to impress your kids. Or win a bet.

Obligatory eclipse post

Three videos from the eclipse. (As usual, if you’re reading this on Facebook, you’ll miss all the good stuff. Follow the link above for the actual post.)

First, the making of the tinfoil hats:

Then, there’s the eclipse itself:

 

It is incredibly hard to hold the camera still, and also follow the sun’s motion, with a tiny hand-held camera, while also holding eclipse glasses over the lens.

Here’s the same video sped up x 6, for the timelapse effect, but it ended up being far too jumpy.

 

I’m going to see if I can figure out how to stabilize it.