[jackyjoy123 (OP)]

Hello,

My boy Ben asked me why the plate in the microwave spins and
I was just wondering, is it vital for the plate in my microwave to spin ?...I assume it has to do with the microwaves penetrating equally eh ? but could a microwave oven expert explain to me the principles behind the reason why the plate must spin ? I told my son that I wasn't too sure but that I know a place where answers come freely and with a smile.

thanks
jackyjoy

[vhfpmr]

No, it's not vital, my first microwave had no turntable, and yes, it is to try to make the food cook more evenly.

The microwaves come from one side (usually the plastic covered aperture on the right), but then get reflected around off the inside of the oven, so at any point there will be multiple waves reflected from different parts of the oven walls. When two waves are incident on the same point in space the effect depends on their relative phase, if they are in phase they add up to one bigger wave, and if they are out of phase they will tend to cancel each other out. There are an infinite number of possibilities in between which will give rise to all sorts of other levels too, two waves equal in amplitude and out of phase will cancel each other out completely. This means that there will be different levels of microwave power all over the oven: hot spots and 'cold' spots. The idea is that if you keep the food moving each part will get a share of the cooking.

It doesn't work very well in my experience, anything that isn't a liquid, and can't be stirred frequently, doesn't cook evenly. I gave up thawing/cooking chicken breasts a long while ago, but it's great for reheating stews/casseroles etc.

https://en.wikipedia.org/wiki/Wave_interference#/media/File:Interference_of_two_waves.svg

[evan_au]

I've not tried it myself, but I understand that you can demostrate this experimentally:
- Remove the foil wrapping from a large chocolate block
-  place the chocolate block in the microwave, supported so it doesn't rotate.
- Give it a burst of (say) 40 seconds on full power
- you will find some parts of the chocolate block melt, while other parts remain quite solid

As a bonus, you can also measure the speed of light!
See: https://www.thenakedscientists.com/forum/index.php?topic=11200.0

Hot and cold spots is not a good arrangement if you are trying to kill all the bacteria in a chicken!
- The rotating plate makes the heating more uniform

[alancalverd]

Ants are particularly sensitive the the E field. If you have ants on a rotating plate microwave, they will form circles where the field strength is least.

Industrial microwave cookers, an increasing numbers of domestic ones, use a "stirrer" to even out the power distribution over a fixed flat plate - much quieter and easier to clean, though it still has a few hotspots..

[Colin2B]

Ants are particularly sensitive the the E field. If you have ants on a rotating plate microwave, they will form circles where the field strength is least.

I didn’t know that. Fascinating.
Before they cook of course!

Industrial microwave cookers, an increasing numbers of domestic ones, use a "stirrer" to even out the power distribution over a fixed flat plate - much quieter and easier to clean, though it still has a few hotspots..

Ours works this way and it seems quite even.  You still have to stir thick liquids and move food in contact with surfaces due to conduction.

[alancalverd]

Poppadoms are also good for measuring c with a rotating microwave oven.

[vhfpmr]

-  place the chocolate block in the microwave, supported so it doesn't rotate.
- you will find some parts of the chocolate block melt, while other parts remain quite solid

You can see this even with a turntable, the reason I've given up trying to thaw chicken breasts with the microwave is because I was ending up with small spots of chicken cooked whilst the rest still remained frozen. It seems to me that any component of the standing wave pattern that's determined by the shape of the food itself will tend to turn with the food, and not get averaged out by the rotation.