1. 2010

    Death rays and thermal radiation

    It’s been far too long since I did a Mythbusters writeup, but I think it’s time to stop stalling and bring this series back. On this week’s episode, Adam and Jamie tested the myth of Archimedes’ heat ray for a third time — that has to be some kind of record — at the request of President Obama.

    The gist of the myth is this: by focusing enough of the sun’s rays, using a large number of mirrors, on an enemy ship, the Greeks hoped to heat it up enough to make it catch on fire. So far (spoiler alert), there’s no evidence that this thing ever could have worked. All three of the Mythbusters’ tests have failed.

    But I think I can shed some light (no pun intended) on why. As it happens, I taught a lab on thermal radiation transfer this week, and that (along with an interesting perspective on gravitationally baking a turkey) reminded me that it’s fairly straightforward to calculate, at least in a simple model, the amount of radiation it takes to heat something up to a particular temperature. It all stems from the Stefan-Boltzmann equation,

    $$P = \sigma\epsilon AT^4 …
  2. 2009

    Exploding a microwave oven with C-4

    Exploding grease, exploding microwave ovens, exploding cheese — it’s a Mythbusters fan’s dream episode :-) Of course, where there are explosions, there’s physics, and the latest episode of Mythbusters is no exception.

    Here’s one: you can’t blow up C-4 by microwaving it. Kari explained in the show that this is because C-4 is a plastic explosive, and microwaves are designed to pass through plastics (as well as metal and glass). So how exactly does that work?

    Microwaves heat food by a process called dielectric heating, which generally refers to the ability of many materials to absorb energy from electromagnetic radiation passing through them. Physically, an electromagnetic wave consists of rapidly oscillating electric and magnetic fields. These fields (well, primarily the electric field) exert forces on the charged particles that make up all matter — since the fields are oscillating, so do the forces. Essentially, an electromagnetic wave makes atoms and molecules rapidly jiggle back and forth, and as they do so, they bump into other nearby atoms and molecules, transferring kinetic energy to them and raising their temperature. Of course, if the atoms and molecules are gaining energy, that energy must be coming from somewhere, and the electromagnetic …