Science Buddies: "Ask an Expert"

I am researching the possibility of using parabolic mirrors to focus microwaves within a microwave oven to increase the efficiency of energy. What do you think? Is this project safe? Has anyone else already researched this, specifically the relation between energy consumption and heat generated? And is there some reason why this will not work that I overlooked? Thanks!

The problem with putting anything inside a microwave oven that reflects microwaves is that it alters the designed in cavity reflection and absorption properties of the oven that the manufacture used to create a relatively large volume of even cooking space and to prevent the reflection of the microwaves back into the generator out of phase.

If you manage to reflect microwaves out of phase back into the generator you will significantly decrease the microwave energy available to cook with and you can easily overload the generator and cause it to fail or blow whatever safety fuse element exists.

Everything which strongly reflects microwaves is probably on the list of the things the manufacturer tells you not to put into their oven.

I don't quite understand what you mean by microwaves entering the generator out of phase. Could you please clarify? Because from my understanding, I thought the generator is designed to withstand stray waves. Also, the parabolic mirrors should be focusing the waves in the center, and once the waves passes the focus point in the middle, they should be spread out in an uniform manner, not focused into the generator.

You need to do some reading on basic wave priciples, particularly reflection and phase and amplitude to understand how the microwave generator in a microwave oven is coupled into the oven cavity. You also need to do some reading on anteneas and transmission lines and reflections. The microwave oven has components that act like an oscillator and amplifier and a transmitting antena to couple the amplified electrical oscillations into electromagnetic waves in the oven cavity.

Lets take a simpler case of a transmission line and a pulse generator setup to produce a sequence of pulses. If you send a pulse down a transmission line into an open circuit, the wave will be reflected back. If the length of the transmission line is such that the wave comes back in phase and alligns with the next pulse in time, the amplitude will keep adding and will quickly exceed the voltage breakdown of some component. If the length of the transmission line is such that the pulse comes back 180 degrees out of phase, then it will cancel the next pulse If the generator is designed with a feedback mechanism then the generator is subject to over current failures.

In the case of the microwave oven, the "transmission line" is a wave guide that is used to couple the generator to the oven; however, the same kinds of problems happen when the impedance is disrupted from what the design anticipated. It is just a lot more complex mathmatically to describe the effects.

O.K, I think I see what you're getting at. Correct me if I'm wrong (I haven't had much experience with EM waves), but I think you're saying the wave entering the generator might not be that strong, but once it reflects and produces a standing wave, it will amplify itself. Those that don't cancel themselves out. And if I disrupt the normal activity of the waves , I may be amplifying waves into the generator, which probably won't have beneficial effects. But, if there was some way to not blow out the safety fuse, would the idea theoretically work? (would it focus the waves?)

I can't really recommend this kind of experiment, because there are factors that are hard to control for that might damage your microwave oven in various ways. Unless you manage to damage the safety interlocks or the metal screen/casing, it's not likely to let microwaves out, but it is possible to set the oven on fire. So have a fire extinguisher on hand.

If you were going to do this kind of experiment, you'd want to think about how you would measure the results. Perhaps the amount of heat absorbed by a known size container of water placed in various spots. Make it large enough, and microwave for a short enough period that it only raises the temperature by a small number of 10s of degrees, though. Water can become superheated in microwave ovens if heated too long, resulting in dangerous (explosive-like) conditions. A small stone or other rough object in the container would prevent that without changing the results much.

[quote="MrRice5555"]But, if there was some way to not blow out the safety fuse, would the idea theoretically work? (would it focus the waves?)
[quote]That would depend on how you define "work". When you reflect waves, you change their propagation direction and their phase. When waves pass each other (except orthoganally), they add and subtract (using complex vector rules - lookup complex numbers) so you get "nulls" and "peaks" in the "field strength" of the microwaves within the containment volume. Even without the addition of "reflectors" inside a microwave oven, you will get uneven cooking because of these variations in field strength. This is why a lot of microwave ovens have a turntable in them to rotate what you are cooking to try and balance out the effects of these variations. Putting in things that are microwave reflective will certainly change the field strength pattern and it would be extremely difficult to predict the resulting pattern for any given setup. The test equipment and methods to determine where the nulls and peaks are within a microwave oven is a lot more "art" than "science" because the equipment must either fit inside or alter the containment to bring a signal outside which alters the cavity. The absorption and reflective properties of any measurement equipment inside the oven is again an alteration to the behavior. In other words this is yet another kind of behavior that involves an uncertainty principle.