Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 13/04/2016 01:50:01
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Jason Deutsch asked the Naked Scientists:
How does a microwave burn/char things without infrared radiation.
What do you think?
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Burning and charring of food is actually a chemical reaction caused by heat in which the material breaks down to form other compounds. There are chemists here who will be able to describe the actual process but I suspect that the browning, charring of food is a combination charcoal and caramelisation plus a whole load of other chemicals.
Wood for example will begin to carbonise at 280C using up oxygen, above this temp the process becomes exothermic- generating it's own heat.
Think about food burning in the bottom of a pan, you don't need direct flame or infrared.
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Burning and charring of food is actually a chemical reaction caused by heat in which the material breaks down to form other compounds. There are chemists here who will be able to describe the actual process but I suspect that the browning, charring of food is a combination charcoal and caramelisation plus a whole load of other chemicals.
Wood for example will begin to carbonise at 280C using up oxygen, above this temp the process becomes exothermic- generating it's own heat.
Think about food burning in the bottom of a pan, you don't need direct flame or infrared.
I thought that is a Physics question Colin rather than chemistry?
I am uncertain of the answer, my observation of the process tell's me that firstly the food becomes dehydrated, the matter is then not losing heat by losing water and gases, so the matter gains more heat by the entropy gaining more energy which is a much greater rate than the rate of emittance of energy, creating ''burnt'', what ever burnt actually is I have no idea,
What is burnt?
Is it like ''dead'' atoms?
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Most food contains water, which is a polar molecule - the oxygen side is slightly more negative, and the hydrogen side is slightly more positive.
Microwaves have a local electric field which spins the water molecules around, absorbing microwave energy and turning it into molecular vibrations, which we feel as heat. It is this heat which causes the delicious chemical reactions.
The absorption spectrum of water has a broad peak around 1-3cm wavelength.
See the blue curve at: https://en.wikipedia.org/wiki/Electromagnetic_absorption_by_water#Microwaves_and_radio_waves
A wavelength of 12cm (2.45GHz) is usually used in microwave ovens. This is longer than the peak absorption, but is used because it penetrates the food more deeply, heating it more uniformly, and killing any bacteria more thoroughly.