A way to make the standing-wave patterns in a microwave oven visible

[jeffinbath]

Following on from the recent discussion on the gap size in the metal grill covering the door of a microwave oven, I thought up a way to actually enable one to see some of the standing microwave patterns going on in a typical oven .

I bought a 20” x 20”cobalt chloride soaked cloth from Amazon [these are sold as water-leak detectors] and cut out about a 30 cm square piece which would just fit the floor of my oven. I then needed to convert this bit of blue cloth to a slightly off-white one by leaving the cloth over-night in a 100% (approx.) relative humidity chamber without actually letting it contact liquid water. I used a large transparent polypropylene box with a lid and held the cloth on a wall of the box using pairs of small ferrite magnets and covered the floor of the box with a soaking-wet newspaper.

I then removed the glass plate and its support from the oven and wiped off any water condensation from the 6 surfaces of the oven before placing my bit of white cloth on the oven floor. I then gave it 30 seconds of full power. I then put the cloth on a fridge door and held it in place with ferrite magnets and took photo 1. I then repeated the whole operation but this time hung the cloth from the roof of the oven by holding it in place with 2 ferrite magnets.

The conversion of the hydrated CoCl2 to its bright blue anhydrous form seemed to occur more rapidly in the vertical set-up . This is shown in photo 2. The need for food to be rotated is certainly demonstrated here.

One feature that did really surprise me was that ferrite magnets , famous for being electrical insulators, could actually get quite hot on the walls of the oven and even before the 30 seconds irradiation time was up , one magnet cracked in half. (shown in bottom rhs of photo 2 ). Could someone explain this please ?

 

[hutchphd]

Nice experiment. I've seen it done with slices of American Cheese (find the melty spots)but this certainly looks less messy

Could someone explain this please

Ferrites are ferromagnetic impregnated ceramic. Microwaves are electroMAGNETIC waves and at nontrivial energy density. It does not surprise me that they couple.

 

[A.T.]

Here they used a chocolate bar, but the aim wasn't to be accurate, but rather to explain error bars:

 

[jeffinbath]

Summary:: Why we need to rotate food while heating with microwaves is demonstrated in the experiment described here.

Following on from the recent discussion on the gap size in the metal grill covering the door of a microwave oven, I thought up a way to actually enable one to see some of the standing microwave patterns going on in a typical oven . I bought a 20” x 20”cobalt chloride soaked cloth from Amazon [these are sold as water-leak detectors] and cut out about a 30 cm square piece which would just fit the floor of my oven. I then needed to convert this bit of blue cloth to a slightly off-white one by leaving the cloth over-night in a 100% (approx.) relative humidity chamber without actually letting it contact liquid water. I used a large transparent polypropylene box with a lid and held the cloth on a wall of the box using pairs of small ferrite magnets and covered the floor of the box with a soaking-wet newspaper. I then removed the glass plate and its support from the oven and wiped off any water condensation from the 6 surfaces of the oven before placing my bit of white cloth on the oven floor. I then gave it 30 seconds of full power. I then put the cloth on a fridge door and held it in place with ferrite magnets and took photo 1. I then repeated the whole operation but this time hung the cloth from the roof of the oven by holding it in place with 2 ferrite magnets. The conversion of the hydrated CoCl2 to its bright blue anhydrous form seemed to occur more rapidly in the vertical set-up . This is shown in photo 2. The need for food to be rotated is certainly demonstrated here.

One feature that did really surprise me was that ferrite magnets , famous for being electrical insulators, could actually get quite hot on the walls of the oven and even before the 30 seconds irradiation time was up , one magnet cracked in half. (shown in bottom rhs of photo 2 ). Could someone explain this please ?

I suppose one could say that the standing wave pattern made by the microwaves with its heated blue “antinodes” and cool white “nodes”, is not fundamentally different in principle from the light generated pattern of standing waves I used to make the little hologram in my avatar here.

 

[tech99]

Another method is to use thermochroic film, which changes colour with temperature. Take out the rotary plate and put it on the oven floor, then give it just 2 seconds.