Electromagnetic Fields: Microwave Oven

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Offline alphadelta

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Electromagnetic Fields: Microwave Oven
« on: 19/04/2014 05:13:42 »
Hi Guys!
I have an assignment, which is grilling me for over a week.
Please share some ideas on it.

Estimate the time it takes to cook a potato in a microwave oven. Assume that the oven is irradiated by a plane wave in top half of a window on one side of the oven. Find all necessary values (size and power of microwave oven, size and heat capacity of potato, etc) and then

1 )Estimate the power in each mode of the oven.
2) Integrate the energy density over the volume of the potato, for each mode
3) Estimate the change in temperature of the potato per unit time and subsequently the time to cook it


Offline evan_au

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Re: Electromagnetic Fields: Microwave Oven
« Reply #1 on: 19/04/2014 13:49:07 »
We try not to do people's homework assignments, but here are some ideas:
  • It sounds like they want you to do some experiments.
  • Pick a microwave oven - one at home sounds fine
  • Measure it's power.
  • Pick a potato - one at home sounds fine
  • Measure how long it takes to cook a potato.
  • Microwave ovens use a standard wavelength, absorbed by water.
  • The microwave has a slightly irregular shape, sometimes with a metallic "stirrer" so that energy is reflected in every direction.
  • You can work out which wavelengths will resonate well in a box of this size. (You might need to simplify it to a rectangular box).


Offline alancalverd

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Re: Electromagnetic Fields: Microwave Oven
« Reply #2 on: 19/04/2014 21:37:52 »
It's nothing to do with microwave ovens per se. If you have to "estimate the power in each mode" then just assume that the potato absorbs x % of the input power P watts from whatever the source (microwaves, saucepan...) and has a specific heat capacity of y joules/gram.degree. Then the rate of heating is Px/(100ym) degrees/second where the mass of the potato is m grams.

The real unknown is the amount of heat required to cook a potato. It's a lot easier to do the temperature rise calculation for a small potato in a microwave oven because you can assume it's heated evenly, unlike boiling, baking or frying, but what is the required temperature for hydrolysis of the cell walls and starch? It certainly isn't 100 deg C.

Best solution is to look at the instruction book for a microwave oven. Believe it or not, technicians and chefs in the factory have spent many hours calibrating their product and writing the book, and they really know the answer.  And the power of the oven is written on the back (by law) and usually on the front (so you know how to use it) - no need to estimate. Which is just as well because they vary from 500W (small domestic use) to 5000W or more (industrial kitchens).

What a stupid assignment.
« Last Edit: 19/04/2014 21:42:27 by alancalverd »
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