What is the process called when metal is heated in a microwave oven?

[zach88]

Recently I came across with one video in youtube where the guy in the video did experiment on microwave oven. He heat up aluminium foil in the oven and after a few second, the foil started to produce spark. This is my first time watching something like that. I never knew that when metal is heat up in microwave oven, it will produce spark. I have read several website which explain about this, but I just could not understand them. So, I really appreciate if someone could explain to me in the simplest way..

 

[davenn]

hi Zach

remember google is your friend ! :) there were 1000's of hits on that subject

here's just one comment from wikipedia...

" Metal objects

Any metal or conductive object placed into the microwave will act as an antenna to some degree, resulting in an electric current. This causes the object to act as a heating element. This effect varies with the object's shape and composition, and is sometimes utilized for cooking.

Any object containing pointed metal can create an electric arc (sparks) when microwaved. This includes cutlery, crumpled aluminum foil (though not all foil use in microwaves is unsafe, see below), twist-ties containing metal wire, the metal wire carry-handles in paper Chinese take-out food containers, or almost any metal formed into a poorly conductive foil or thin wire; or into a pointed shape.[34] Forks are a good example: the tines of the fork respond to the electric field by producing high concentrations of electric charge at the tips. This has the effect of exceeding the dielectric breakdown of air, about 3 megavolts per meter (3×106 V/m). The air forms a conductive plasma, which is visible as a spark. The plasma and the tines may then form a conductive loop, which may be a more effective antenna, resulting in a longer lived spark. When dielectric breakdown occurs in air, some ozone and nitrogen oxides are formed, both of which are unhealthy in large quantities."

cheers
Dave

 

[zach88]

Thanks for the respond Dave..
Actually I have read about that. I just need more clarification about that.
From what I can understood from that, every metal will act as antenna and the effect of microwave depend on geometry of the metal. right?
I am not understand how the metal can become antenna which later produce electric current.
By the way, sometimes I am a slow learner. Sorry to trouble all of you..

 

[davenn]

Thanks for the respond Dave..
I am not understand how the metal can become antenna which later produce electric current.
By the way, sometimes I am a slow learner. Sorry to trouble all of you..

no probs. :)

Any piece of metal will act as an antenna, be it inside a microwave oven or out in the open air picking up radio / TV signals. There will be an induced voltage/current in that piece of metal from the passing EM field (waves). As that piece of metal becomes closer in size to a physical wavelength, 1/2 wavelength etc of the frequency being received the more efficient it becomes and the more voltage is induced into it.
Hence why you see antennas cut to specific dimensions to suit the frequency of operation.

Back to the microwave oven, with the crumpled Aluminium foil and all its sharp points etc
the comments in that quote I made above covers that.
You have to remember that inside the microwave oven the RF (EM) field is extremely high inside that small area. microwave ovens range from ~ 500 to ~ 1100 Watts, which allows for VERY large voltages to be induced into the metal objects and therefore causing VERY large currents to flow.

hope that helps

Dave

 

[zach88]

Forks are a good example: the tines of the fork respond to the electric field by producing high concentrations of electric charge at the tips. This has the effect of exceeding the dielectric breakdown of air, about 3 megavolts per meter (3×106 V/m). The air forms a conductive plasma, which is visible as a spark.

Thanks for the explanation. Now, for my next question. From the quote, it said that the tines of the fork respond to the electric field by producing high concentrations of electric charge at the tips which has the effect of exceeding the dielectric breakdown of air. Can you explain what is it means by dielectric breakdown?

 

[davenn]

Thanks for the explanation. Now, for my next question. From the quote, it said that the tines of the fork respond to the electric field by producing high concentrations of electric charge at the tips which has the effect of exceeding the dielectric breakdown of air. Can you explain what is it means by dielectric breakdown?

Hi Zach,
I will assume from that, that you don't know what a dielectric is. There are some pretty complex explanations online some googling will lead you through some of them :)

but for the ease of explanation, let's view a dielectric as a type of insulator. There are many variations ... eg. air, paper, plastic, mica, foam, teflon ... all used in day to day electronics.
Air, paper, plastic, mica all often used in capacitors as an insulator between the 2 plates of a capacitor. Air, foam and teflon are often used in coax cable as an insulator between the inner conductor and outer conductor of the coax cable.

All the different dielectrics mentioned above ( and others I havent) have an ability, for a given thickness to withstand an electric field across it without breaking down --
This is called the Dielectric Strength -- of an insulating material.
Different materials have different strengths. Dry air is used in very hi voltage capacitors, and coax cables as 2 examples. In the old days, valve radio etc era, paper was commonly used in capacitors and the paper was impregnated with wax to help improve its dielectric properties. These days you will normally only find paper in electrolytic capacitors ( the ones in metal cans)
In a coax cable the type of dielectric used has 2 major effects on the makeup of the cable.
1) it affects its dielectric strength abilities --- foam worse, air very good, teflon the best
2) it also affects the velocity factor of the cable -- teflon worse, foam very good, air the best
we won't go into coax velocity factors here ---- that's for another whole topic. ;)

Now hopefully you have a handle on that :)

a fork in the microwave with its individual prongs (tines). A large electric field builds up on each prong due to the intense RF EM field present. You have an air dielectric between each prong. That air gap has a dielectric strength that will finally be overcome in a strong enough electric field that you get a flashover (spark) between the prongs.

have a look at this wiki page for a list of materials and their relative dielectric strength
http://en.wikipedia.org/wiki/Dielectric_strength

Air is very low on the list compared to waxed paper, teflon and mica

cheers
Dave

 

[zach88]

Thanks a lot for the explanation. Last time I only know that dielectric term is used in capacitor only. :)
Is it possible to have spark when I replaced the fork with spoon?

 

[davenn]

Thanks a lot for the explanation. Last time I only know that dielectric term is used in capacitor only. :)
Is it possible to have spark when I replaced the fork with spoon?

less likely, but still depends on the individual shape of the spoon
That wiki quote above did comment on spoons

Dave

 

[zach88]

If I heat food using microwave, it is called dielectric heating right? What is it called if I heat metal? Is it induction heating?