Microwave tube mechanism of heating

[Vrbic]

Hello,
first of all, I understand, how microwave tube works but I'm interesting in how exactly the microwave heat up a things? What's the matter inside. I have heard about increasing energy of molecules of water by polarization them by micro-wave. But I have got a plate which is not from glass and it is heated up more than things on it. I suppose it is not from water :)
My idea is that lattice parameter "a" of that material has resonance length for micro-waves. Or frequency of these waves is resonance frequency for such lattice. Or where is true?

 

[Alfred Cann]

All that is necessary for a material to be heated is that it be a conductor which is not ideal, but lossy, i.e. has some resistance. It could also be a lossy dielectric or a lossy magnetic material; both of these have characteristic curves that show some hysteresis. All such materials absorb some energy from the microwaves. For example, most foods are lossy conductors. The microwave field induces currents in those materials and the I squared R loss produces heat.

 

[davenn]

All that is necessary for a material to be heated is that it be a conductor which is not ideal, but lossy, i.e. has some resistance. It could also be a lossy dielectric or a lossy magnetic material; both of these have characteristic curves that show some hysteresis. All such materials absorb some energy from the microwaves. For example, most foods are lossy conductors. The microwave field induces currents in those materials and the I squared R loss produces heat.

no, this isn't correct

But I have got a plate which is not from glass and it is heated up more than things on it. I suppose it is not from water :)

ceramics in general are very good at absorbing microwave energy. and if there is still any moisture within the ceramic.
Then those water molecules will get warm in the same way as water molecules in the food via the absorbed energy
There are various ceramics that manufactured for their energy absorption without getting too hot to touch
these are available for use in microwave ovens. Also consider the ceramics that were used in the space shuttle heat protection tiles,
these tiles could be glowing red but you could still pick them up by handDave

 

[Blibbler]

The dielectric heating process caused by microwave absorption is basically frictional - the electric and magnetic field vectors of the microwaves cause separated charge to move, so any molecules with a degree of charge separation (including any polar molecules such as water) will move in response to the incident microwaves and this will cause bulk heating of the material.

 

[Vrbic]

The dielectric heating process caused by microwave absorption is basically frictional - the electric and magnetic field vectors of the microwaves cause separated charge to move, so any molecules with a degree of charge separation (including any polar molecules such as water) will move in response to the incident microwaves and this will cause bulk heating of the material.

So if I understand "classical" heating up of water: It is caused by ROTATING or MOVING of charged particles? My idea is that dipole molecules of water just turn round because of changing of electromangnetic field by microwave. Is it true?

 

[Vrbic]

Then those water molecules will get warm in the same way as water molecules in the food via the absorbed energy

Absorbed energy seems very general :) How exactly? Is it by rotating of dipol moleculs of water because of microwave (electromagnetic field)?

ceramics in general are very good at absorbing microwave energy. and if there is still any moisture within the ceramic.

Do you want to say that in ceramics are moleculs of water or that moleculs of ceramics behave similary as water? My plate was definitely dry (on surface).

 

[davenn]

My plate was definitely dry (on surface).

yes, but you don't know how dry the ceramic is inside. Standard kiln fired ceramics can still have small quantities of water in it

ceramics produced specifically for microwave oven use are more carefully produced