nessus@nsc.UUCP (Kchula-Rrit) (07/13/85)
[You didn't expect me to put this in net.ham-radio, did you?]
Today, I had some macaroni and cheese for lunch(actually leftovers) which
I brought to work with me. Before eating it, I microwaved it to warm it
up. I noticed that some of the macaroni was really hot and a spot a couple
of inches away was still cold.
This seems like really odd behavior. Any ideas why it does this?
From the alter ego of--
Kchula-Rrit
!menlo70!nsc!nessushkr4627@acf4.UUCP (Hedley K. J. Rainnie) (07/13/85)
Consider the ancient microwave you must be using at work. Perhaps your
company could do more to provide their employees with a work environment
conducive to goofing off (i.e. eating when they whould be working). One
of the problems with older microwaves or cheaper models was the formation
of standing waves with their attendant nodes (areas of little or no energy).
In the early days, this problem was treated with the Deluxe Carousel models
of microwave (shades of fifties technology!). I'd like to say that I know
how they handle the problem today, but, alas, I don't. Also, consider: the
microwave energy is most efficiently absorbed by water molecules in the
food. In a given edible sample, the high water content of one region may
result in it heating to incandescence before a low-water region even reaches
tepidity. Try stirring periodically and moving the mass-quantity around in
the chamber -- the microwave cavity.
Rjp@lanl.ARPA (07/13/85)
> > Today, I had some macaroni and cheese for lunch(actually leftovers) which > I brought to work with me. Before eating it, I microwaved it to warm it > up. I noticed that some of the macaroni was really hot and a spot a couple > of inches away was still cold. > > This seems like really odd behavior. Any ideas why it does this? Yes. The energy in your microwave oven is electromagnetic waves bouncing around inside a closed metal box. The reflections at the boundaries result in a standing wave pattern with regions of high electric field (and, consequently, low magnetic field) and vice versa. Microwave cooking occurs from heating by dielectric loss (mostly in the water). The parts of the food that are near electric field maxima heat more than those near electric field minima. Most microwaves have a rotating metal blade that "stirs" the microwaves. That is, it disturbs the standing wave pattern in an attempt (usually not very successful) to move the electric field maxima and minima around. The rotating table in some microwaves is an attempt to even out the cooking by moving the food around in the standing wave pattern. Unforunately, the middle of the rotating tray doesn't move very far. Once, someone asked me how to measure the field distribution in a microwave oven. (He was curing some kind of ceramic glue or something like that.) Since my main job is measuring microwave field distributions in cavities he thought I was a good person to ask. He didn't seem fully satisfied with my answer, however. I suggested that he cover a pizza crust more or less uniformly with cheese and place it in the microwave oven. The electric field maxima would be at the points where the cheese first began to melt. Actually this is a tough problem because the frequency of the oven cavity and the standing wave pattern depend on the distribution of food (dielectric loading) in the cavity. If all you want to do is cook hotdogs, just apply 120VAC from end to end for 60 secs. It works, and it's cheap. Yours for scientific cooking, Jim Potter jp@lanl.arpa
ethan@utastro.UUCP (Ethan Vishniac) (07/15/85)
> ... One > of the problems with older microwaves or cheaper models was the formation > of standing waves with their attendant nodes (areas of little or no energy). > In the early days, this problem was treated with the Deluxe Carousel models > of microwave (shades of fifties technology!). I'd like to say that I know > how they handle the problem today, but, alas, I don't. *** REPLACE THIS LAMP WITH YOUR MICROWAVE *** I believe the problem is solved by installing a small metal fan in the top to scatter the microwaves. -- "Don't argue with a fool. Ethan Vishniac Borrow his money." {charm,ut-sally,ut-ngp,noao}!utastro!ethan Department of Astronomy University of Texas
nessus@nsc.UUCP (Kchula-Rrit) (07/15/85)
> [You didn't expect me to put this in net.ham-radio, did you?] > > Today, I had some macaroni and cheese for lunch(actually leftovers) which > I brought to work with me. Before eating it, I microwaved it to warm it > up. I noticed that some of the macaroni was really hot and a spot a couple > of inches away was still cold. > > From the alter ego of-- > > Kchula-Rrit > !menlo70!nsc!nessus My question has been answered[varying microwave density throughout the oven]. Thank you. Kchula-Rrit
JGA@MIT-MC.ARPA (07/19/85)
From: John G. Aspinall <JGA@MIT-MC.ARPA>
Article-I.D.: <356@utastro.UUCP>
... One of the problems with older microwaves or cheaper
models was the formation of standing waves with their
attendant nodes (areas of little or no energy). In the
early days, this problem was treated with the Deluxe
Carousel models of microwave (shades of fifties
technology!). I'd like to say that I know how they handle
the problem today, but, alas, I don't.
I believe the problem is solved by installing a small metal fan in the top
to scatter the microwaves.
Ethan Vishniac
Ethan is, as usual, right. To elaborate just a little; any cavity, no
matter what the shape, will have standing waves as long as reflection at
the boundaries is large compared to absorbtion. (Just think about
conservation of energy to convince yourself of this in an approximate
way.) So the solution is to continually change the shape of the cavity.tan@ihlpg.UUCP (Bill Tanenbaum - AT&T Bell Labs - Naperville IL) (07/22/85)
> ... One of the problems with older microwaves or cheaper > models was the formation of standing waves with their > attendant nodes (areas of little or no energy). In the > early days, this problem was treated with the Deluxe > Carousel models of microwave (shades of fifties > technology!). I'd like to say that I know how they handle > the problem today, but, alas, I don't. ---------------------------------------------------------------------- Today's microwave ovens attempt to solve the problem in three different ways, depending on the manufacturer and model. 1) A turntable to rotate the food. Presumably the axis of symmetry of the turntable does not pass near a node. 2) A metal fan, to continually change the cavity shape. 3) Dual beams. I presume they have different standing wave patterns. Having just been shopping for a microwave, I know that all these methods are used in today's top of the line models. I have not seen a single model that uses more than one of these methods simultaneously. -- Bill Tanenbaum - AT&T Bell Labs - Naperville IL ihnp4!ihlpg!tan