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Old 02-18-2008, 01:27 PM   #1
JoeyAdams
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Distribution of power in a microwave oven


The microwave directions for a calzone I ate today say to cook only one at 50% power. I decided to cook two at a time with 100% power, and they tasted great.

My question is: What is the mathematical relationship between the number of calzones in a microwave and the amount of cooking power one of the calzones receives? Is it an inverse relationship like this:

P = k/n where P is cooking power, k is a constant based on time of cooking and microwave's wattage, and n is the number of calzones.

Or is the P to n relationship a more interesting curve? For instance, if one were to put a very small drop of water into the microwave and cook it, would all 1200 watts focus onto that drop of water and make it instantly vaporize, or would most of the energy reflect back into the heating element? The answer to that question would be key to understanding if microwave heating is perfectly inverse or not.
 
Old 02-18-2008, 01:35 PM   #2
Jeebizz
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Shouldn't also the mass of the food in question be taken into consideration, and not just the number of calzones?? That also usually affects how it is cooked in the microwave.
 
Old 02-18-2008, 02:36 PM   #3
michaelk
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Typically the more mass the more cooking time required. Lowering the power setting when just heating a single calzone will allow it to heat more evenly and not burn the outsides first. Since no oven heats identically then there should be some additional factor. If all we are talking about is calzones then the total mass is just a function of the number.

There will be microwave reflections based on the what type of food you are cooking and there will be reflections so not all power will be focused at a single spot.
 
Old 02-18-2008, 02:44 PM   #4
pixellany
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I think the fundamental relationship is energy vs. water content (specifically the mass of water). A microwave works by agitating the water molecules ( at about 2.5GHz ).

If you are just heating water, there is probably a pretty linear relationship between temperature rise and total energy (power X time). the "power" setting is really duty cycle, So the energy delivered is:
E = W x P x t
where:
W = the total power output of the oven (eg ~ 1000watts)
P = the "power" setting as a decimal (eg a setting of "5" is 50% or 0.5)
t = cooking time

To raise a cup of water by 100 deg F should take the same total energy, regardless of where P is set

For food that is a mixture of ingredients, I suspect things are more complicated. For example, you would need to take into account the heat transport in the mixture. (The reason reduced power is used for thawing is to allow the energy to uniformly distribute in the material being heated)

Another factor is the coupling efficiency. The microwave source sends the radio waves into the cavity, where they bounce around and set up patterns known as standing waves. These patterns will be influenced by how much material is in the cavity absorbing the waves.
(All this is why the better ovens have a turntable)

Why does bread get tough when microwaved? My guess is that the heat conductivity is poor and so there is local cooking where the water is.
 
Old 02-19-2008, 04:59 PM   #5
JoeyAdams
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Quote:
Originally Posted by Jeebizz View Post
Shouldn't also the mass of the food in question be taken into consideration, and not just the number of calzones?? That also usually affects how it is cooked in the microwave.
Calzone is my unit of mass I'm asserting all calzones are equal in this analysis. Indeed, both mass of food and cookability (resonance of molecules to microwave radiation) must be taken into account.

Quote:
Another factor is the coupling efficiency. The microwave source sends the radio waves into the cavity, where they bounce around and set up patterns known as standing waves. These patterns will be influenced by how much material is in the cavity absorbing the waves.
(All this is why the better ovens have a turntable)
Hmm, I never thought about that benefit of a turn table
 
  


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