[math-fun] Physics/chemistry question
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? --Dan _____________________________________________________________________ "It don't mean a thing if it ain't got that certain je ne sais quoi." --Peter Schickele
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
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? -----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 _______________________________________________ math-fun mailing list math-fun@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/math-fun
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
From: Dan Asimov <dasimov@earthlink.net> To: math-fun <math-fun@mailman.xmission.com> Sent: Wed, November 11, 2009 3:48:44 PM Subject: [math-fun] Physics/chemistry question 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? --Dan _____________________________________________________________________ I use paper in the microwave to sop up spills. I've never experienced the conflagration of originally dry paper, but I have seen paper that has been wet catch fire. Here is my guess for the mechanism. Water molecules are attracted to the cellulose molecules in the paper fibers. Because of this attraction, the effective boiling point for bound water exceeds that of bulk water, the boiling point being the temperature at which the vapor pressure of the water equals one atmosphere. The bound water is conductive and heats, leading to char. The carbon char is also somewhat conductive leading to further heating. If in addition, as in my case, there are several layers of paper, the heat is confined, and finally the paper bursts into flame. -- Gene
From: Dan Asimov <dasimov@earthlink.net>
To: math-fun <math-fun@mailman.xmission.com> Sent: Wed, November 11, 2009 3:48:44 PM Subject: [math-fun] Physics/chemistry question
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?
--Dan _____________________________________________________________________
I use paper in the microwave to sop up spills. I've never experienced the conflagration of originally dry paper, but I have seen paper that has been wet catch fire. Here is my guess for the mechanism. Water molecules are attracted to the cellulose molecules in the paper fibers. Because of this attraction, the effective boiling point for bound water exceeds that of bulk water, the boiling point being the temperature at which the vapor pressure of the water equals one atmosphere. The bound water is conductive and heats, leading to char. The carbon char is also somewhat conductive leading to further heating. If in addition, as in my case, there are several layers of paper, the heat is confined, and finally the paper bursts into flame.
-- Gene
I think something like this even holds for plastic dishes, especially old ones. I have had empty Melamine dishes scorch, blister, and split, and give off a horrendously sickening stench, quite suddenly as the char phenomenon has positive feedback. I have even seen specs for rate of weight gain of submerged plexiglass. joerg>Even if the magnetron survives you might get
more than you ask for. For example, a tiny flame will ionize the air, rendering it conductive and absorbent to the microwaves. Spectacular blazing flames may well follow.
There are some web sites about these effects. The experiments are usually done outside the building, for very good reasons.
Finally, a use for used birthday candles. --rwg
All paper is not created equal. I've had waxy paper used to wrap tamales burst into flames while the tamale was still cold.
* Dan Asimov <dasimov@earthlink.net> [Nov 12. 2009 12:37]:
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?
--Dan
semi off topic: If you run the microwave 'dry' (i.e. with next to nothing that absorbs the energy), you risk breaking the magnetron. This used to be a standard warning in all manuals, don't know whether this still applies with modern ovens. Even if the magnetron survives you might get more than you ask for. For example, a tiny flame will ionize the air, rendering it conductive and absorbent to the microwaves. Spectacular blazing flames may well follow. There are some web sites about these effects. The experiments are usually done outside the building, for very good reasons. cheers, jj (who reads manuals)
participants (7)
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Cordwell, William R -
Dan Asimov -
Dave Dyer -
Eugene Salamin -
Joerg Arndt -
Mike Stay -
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