On Thu, Nov 12, 2009 at 6:52 AM, Cordwell, William R <wrcordw@sandia.gov> wrote:
So, does it pump up to 1000 J/s into the box? If the microwave is empty, how much energy is actually emitted by the electronics, and where does it go?
Yep. It's like ten 100 watt lightbulbs, except emitting microwave light instead of visible light. If it's not absorbed by food, it's got to go out the hole it came in, since all the walls are reflective and air is transparent. Thus the earlier comment about damaging the magnetron. Plasma is not transparent, on the other hand; see youtube for lots of fun videos involving microwaves and plasma.
-----Original Message----- From: math-fun-bounces@mailman.xmission.com [mailto:math-fun-bounces@mailman.xmission.com] On Behalf Of Mike Stay Sent: Wednesday, November 11, 2009 5:14 PM To: Dan Asimov; math-fun Subject: Re: [math-fun] Physics/chemistry question
On Wed, Nov 11, 2009 at 3:48 PM, Dan Asimov <dasimov@earthlink.net> wrote:
I believe I've seen paper catch fire in a microwave oven when it's been on too long and the water has evaporated away.
But how is this possible, considering that water boils at well below the kindling point of paper?
The microwave oven is pumping 1000 joules of energy per second into that box. It's chosen to be a wavelength of light where water's opaque, so water absorbs the light until it boils; steam is transparent to microwaves. If the paper absorbs any of the light, it will begin to heat up, too, and once the water is gone, it's the only thing in the microwave that *can* absorb the light: the walls are all reflective. -- Mike Stay - metaweta@gmail.com http://math.ucr.edu/~mike http://reperiendi.wordpress.com
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-- Mike Stay - metaweta@gmail.com http://math.ucr.edu/~mike http://reperiendi.wordpress.com