Author Topic: A question for science-guy!  (Read 92 times)

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Offline kimmy

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A question for science-guy!
« on: April 07, 2018, 02:47:08 pm »
Hey, science guy!   I have a question!

Why does metal spark in the microwave?  Why does this make the microwave die?


I had always imagined it was because of the photo-electric effect.  But apparently the photons in your microwave don't have nearly the energy required to punch electrons out of the orbit of any metal.

So then I thought, well, maybe the microwave energy induces electric eddy currents in the microwave, like in an induction welder.  But because of the chaotic reflections going on in a microwave the magnetic fields should be in all different directions and the net magnetic field should be about zero. So that doesn't make sense either.

I looked for scientific explanations of what is actually happening, and the closest I could find is something to the effect that "electric charge will accumulate on corners and sharp edges, and may arc to nearby metal."   Ok, but why?  What's driving the electric charge to accumulate at corners and sharp edges?  Electric charge doesn't like to do that. What's making it do that?

 -k
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Offline TimG

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Re: A question for science-guy!
« Reply #1 on: April 07, 2018, 03:30:36 pm »
https://en.wikipedia.org/wiki/Skin_effect

Microwaves are very high frequency electrical signals.

Offline kimmy

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Re: A question for science-guy!
« Reply #2 on: April 08, 2018, 11:36:09 am »
https://en.wikipedia.org/wiki/Skin_effect

Microwaves are very high frequency electrical signals.

Yeah, but doesn't this leave us in the same situation as with the eddy currents theory?  It seems to me that with all the bouncing around and reflections of the microwaves inside the metal box, the electric fields should largely cancel out and the net electric field at any point should be pretty much zero.

 -k
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Offline Omni

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Re: A question for science-guy!
« Reply #3 on: April 08, 2018, 12:09:10 pm »
Yeah, but doesn't this leave us in the same situation as with the eddy currents theory?  It seems to me that with all the bouncing around and reflections of the microwaves inside the metal box, the electric fields should largely cancel out and the net electric field at any point should be pretty much zero.

 -k

If you put high amounts of electrical fields into something metal, especially something as flimsy as tin foil, it they soon buildup to exceed the resistance of the air around and you get an arc.

Offline TimG

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Re: A question for science-guy!
« Reply #4 on: April 08, 2018, 12:33:16 pm »
Yeah, but doesn't this leave us in the same situation as with the eddy currents theory?  It seems to me that with all the bouncing around and reflections of the microwaves inside the metal box, the electric fields should largely cancel out and the net electric field at any point should be pretty much zero.
AC current always has an "average electrical current of zero". What we are talking about is setting up a standing wave that creates magnetic field patterns which, in turn, affect the distribution of electrons within the metal. With a simple cylinder you can use maxwell's equations to calculate the fields to explain the skin effect. With utensils in a microwave, the pattern would be much more complex but is it reasonable to assume that it would result in electrons being concentrated near the edges and causing sparks.

Take a bucket of water an tap the side at regular intervals. If you do it at the right frequency you can create a similar standing wave where the surface of the water at certain locations is consistently higher than others. Not a perfect analogy but should help understand what is going on.
« Last Edit: April 08, 2018, 12:35:40 pm by TimG »

Offline kimmy

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Re: A question for science-guy!
« Reply #5 on: April 08, 2018, 01:23:45 pm »
Ok, I get what you're saying.   

I'm still not clear on why this might cause damage to the microwave itself, though.

 -k

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Offline Omni

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Re: A question for science-guy!
« Reply #6 on: April 08, 2018, 01:42:16 pm »
Ok, I get what you're saying.   

I'm still not clear on why this might cause damage to the microwave itself, though.

 -k

That aluminum foil is full of electrons that get excited when they are exposed to the force of the magnetron that creates the wavelengths that stimulate the moisture in your food you want to heat. The arcs that are created flash back and can burn holes in the walls of your oven and burn the magnetron so it quits working not unlike when a light bulb filament finally overheats and ceases to be conductive and the lights go out.

Offline TimG

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Re: A question for science-guy!
« Reply #7 on: April 08, 2018, 01:44:47 pm »
I'm still not clear on why this might cause damage to the microwave itself, though.
Could be nothing more than manufactures saying they can't predict what might happen so they say 'don't do it'. I guess it is theoretically possible that enough charge could build up that a spark could break through the plastic interior wall.

Offline Omni

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Re: A question for science-guy!
« Reply #8 on: April 08, 2018, 01:50:30 pm »
Could be nothing more than manufactures saying they can't predict what might happen so they say 'don't do it'. I guess it is theoretically possible that enough charge could build up that a spark could break through the plastic interior wall.

Actually the walls in a microwave are metal, some are covered by plastic.

Offline ?Impact

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Re: A question for science-guy!
« Reply #9 on: April 09, 2018, 12:00:58 pm »
I believe if you put a metal bowl or pot in a microwave oven, it will not spark. The problem of course is it will reflect the microwaves back and therefore the food inside the bowl will not heat up. Some microwaves will reach the food from an open top, but most will be reflected away. The problem with sparking seems to be associated with thin metals, like aluminum foil or the wrappers on some food items that contain a very thin metal usually bonded to a plastic. I am not sure if something like a fork would spark in microwave, perhaps some of the cheap flatware would.

Offline Omni

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Re: A question for science-guy!
« Reply #10 on: April 09, 2018, 12:28:30 pm »
I believe if you put a metal bowl or pot in a microwave oven, it will not spark. The problem of course is it will reflect the microwaves back and therefore the food inside the bowl will not heat up. Some microwaves will reach the food from an open top, but most will be reflected away. The problem with sparking seems to be associated with thin metals, like aluminum foil or the wrappers on some food items that contain a very thin metal usually bonded to a plastic. I am not sure if something like a fork would spark in microwave, perhaps some of the cheap flatware would.

That's correct. I have a micro that is made of steel and happily it doesn't blow up every time I turn it on. If you are going to put some cutlery in your oven put spoons or knives, much less likely to spark than forks. The tines of the fork are fine enough to create the same environment for arcing as does a sheet of aluminum. Better yet use a glass dish and be worry free. I haven't got around to any research on this but a buddy of mine once told me he read an article that suggested that plastic in a microwave is not so healthy as the radiation penetrating the plastic can drive certain elements into the food in the container which are carcinogenic and so I avoid heating in plastic. It might be an old wives tale but I used to find if you had say a bowl of soup on the go the bowl would get so hot you could barely touch it while the soup was still tepid.   

Offline ?Impact

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Re: A question for science-guy!
« Reply #11 on: April 09, 2018, 12:51:38 pm »
I haven't got around to any research on this but a buddy of mine once told me he read an article that suggested that plastic in a microwave is not so healthy as the radiation penetrating the plastic can drive certain elements into the food in the container which are carcinogenic and so I avoid heating in plastic.

Doesn't bode well for frozen microwave meals which are always packaged in plastic.

Offline Omni

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Re: A question for science-guy!
« Reply #12 on: April 09, 2018, 01:30:04 pm »
Doesn't bode well for frozen microwave meals which are always packaged in plastic.

I resort to those once in awhile ( I have a frozen lasagna in the freezer as we speak) but I pluck them out and throw them in a glass baking dish to heat. I reckon I'm just paranoid. But hey I recycle the plastic.

Offline Omni

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Re: A question for science-guy!
« Reply #13 on: April 09, 2018, 01:36:43 pm »
Doesn't bode well for frozen microwave meals which are always packaged in plastic.

So I did some research:

The FDA long ago recognized the potential for small amounts of plasticizers to migrate into food. So it closely regulates plastic containers and materials that come into contact with food. The FDA requires manufacturers to test these containers using tests that meet FDA standards and specifications. It then reviews test data before approving a container for microwave use.

https://www.health.harvard.edu/staying-healthy/microwaving-food-in-plastic-dangerous-or-not

Now I'll have another beer while I read about how to keep healthy.

Offline SirJohn

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Re: A question for science-guy!
« Reply #14 on: April 10, 2018, 10:57:30 am »
Now I'll have another beer while I read about how to keep healthy.

There is plastic in beer. And in all water.
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