Dive Into The Microwaves, The Water’s Dipolar

a microwave-oven with animated wave diagram

When the microwave oven started to gain popularity in the 60s and 70s, supporters and critics alike predicted that it would usher in the end of cooking as we knew it. Obviously that never quite happened, but not because the technology didn’t work as intended. Even today, this versatile kitchen appliance seems to employ some magic to caffeinate or feed a growing hacker in no time flat. So, how exactly does this modern marvel work?

interior of a microwave-oven with a wave length overlay

That’s exactly what [Electronoob] set out to explain in his latest video. After previously taking apart a microwave and showing off the magnetron within he’s back with a clear explanation of how these devices work.

Maybe you have no idea, or have heard something vague about the water in the food wiggling in response to the microwaves. Do you know why microwaves and not some other part of the electromagnetic spectrum? Why the food spins on a platter? How the size of the oven relative to the wavelength affects the efficiency of its cooking? We didn’t, and think the video is a great primer on all of this and more.

Here at Hackaday, we sure love using and abusing microwave ovens. From upgrading them with voice control back in 2013, to turning them into UV curing chambers and mini foundries, to the limitless possibilities for the transformers and magnetrons that await us inside, we just can’t get enough. (this is our 82nd article tagged with microwave!)

22 thoughts on “Dive Into The Microwaves, The Water’s Dipolar

  1. A Tin Hat person asks… Does a microwave oven leak energy? It must leak, cause if I place my cell phone inside, the phone can still be called and will ring, so cellular signals get in and out. Well… yes, using WORDS, you are right, it does leak! But the answers are in the numbers and how much. Closing the door drops the signal by 30dB or more. That is 3 orders of magnitude (3 zero’s). A 1000W of cooking energy is reduced to 1W of leakage putting it on par with a WiFi access point. And yes, you must have really strong cellular coverage if your phone can still work inside the oven with 30dB of attenuation.

    1. Defeat the interlocks, open the door and stand a meter away from the oven when it’s running, and you’ll get about 300 watts per square meter of power incident upon you. That’s about a third of what you get walking outside on a sunny day.

      And it’s even much safer than that: sunlight gets absorbed in less than a millimeter of skin, but microwaves are absorbed at a rate 20x lower, so their power deposited is diluted by that much.

      When next to your ear, your cell phone deposits more power per gram of brain than you would get standing in front of an open microwave.

    2. The microwave RF door seal is made to provide maximum attenuation at the oven operating frequency. The RF seal will not be as effective at other frequencies such as mobile phone frequencies. The consequence is that mobile phone signals can get in and out of the oven better than microwaves at the oven operating frequency.
      A lot of engineering went into the microwave oven door and door frame.

    3. … or a mechanical problem with the oven, for example a bent door or food jammed around the door edge. For a mobile phone to work correctly, it requires bidirectional communication. The mobile phone energy has to be getting out of the oven to reach the mobile site receiver as well as energy coming in from the system.

      A good rough test is if Wi-Fi in the 2.4 GHz band becomes unusable while the oven is operating. The oven operates on a frequency slightly higher than the Wi-Fi band, but if enough energy leaks from the oven, it may desensitize the preamplifiers in the Wi-Fi equipment to the extent they won’t respond to their intended signals. I you are seeing Wi-Fi interference, it may warrant further investigation. As noted in the comment above, it takes a lot of leakage to present a biological hazard. Keeping your body at least a few feet (more than one meter) from the oven while it is operating is a smart idea.

      Remember that Percy Spencer survived having the chocolate bar in his pocket melted without apparent biological damage. There are also apocryphal stories of employees using search RADARs in the arctic circle to warm themselves. Literature suggests that eyes are more sensitive to damage, so it probably isn’t smart to plant your face right on a microwave oven while it is operating.

  2. Rotating platforms are an asinine solution to the problem. You can’t put a square dish in the oven unless it’s tiny. A quarter of the space in the oven is unusable. The glass platter keeps getting dislodged. It’s hard to clean around the tiny parts underneath. You can’t put an unbalanced load in there. The platter continually comes off its drive pedestal. Worst of all, the coffee cup you put in to reheat appears not where you left it, with the handle invariably oriented in the most awkward position.

    Rotating platforms are also completely unnecessary. They exist to pander to the notion that if it’s present then the oven is ‘better’, and so sells better to the unwashed masses. Commercial-grade ovens don’t have them, and don’t need them. Instead, they have a ‘mode mixer’ paddle that rotates behind a radiolucent panel, eliminating the need to rotate the dish of food.

    My commercial-grade Toshiba lasted 33 years, and had the rotating paddle above the food. My more recent Panasonic (also commercial grade) has its paddle underneath, under a glass floor, and the microwaves enter the cavity from underneath the food. It’s perfect.

    The Panasonic also has dimensions 33×33 cm internally. Absolutely not an integer multiple of the magic 6.1 cm electronoob fancifully speculates is required. Spoiler: you actually *don’t* want to encourage standing waves in the cavity, because that just promotes hot spots…

    1. I believe the rotating plate is more a marketing ploy than a technical advantage. The food rotating makes it more obvious to the user that something is going on inside them oven. The turntable motors seem to almost always be 6 RPM synchronous and dials typically allow 10 second increments for the exposure duration. This assures the food to be presented to the operator in the same orientation after cooking as when it was placed in.

      The paddle wheel actually makes more sense because it eliminates the complexity of the turntable and allows for arbitrary cook times not to affect the position of the food at the end of the cook cycle,

      As an aside, the cheapest possible synchronous motors are used, thus there is a 50/50% chance of the rotation being clockwise/counter clockwise. There could be some advantage distributing wear, but I doubt it is significant.

  3. “Rotating platforms are an asinine solution to the problem. You can’t put a square dish in the oven unless it’s tiny.” – Uh…I have no problem with a 9×9 dish in mine.

    “The glass platter keeps getting dislodged…..The platter continually comes off its drive pedestal.” – Never had this happen even with the cheap Walmart microwaves.

    “It’s hard to clean around the tiny parts underneath.” – What tiny parts ? Mine has a platter and ring with support rollers that come out for easy cleaning.

    “You can’t put an unbalanced load in there.” – Never had a problem.

    “Worst of all, the coffee cup you put in to reheat appears not where you left it, with the handle invariably oriented in the most awkward position.” – HaHaHa ! Maybe you should do a study on the odds of the handle pointing in the correct direction or create a 4 handled coffee cup or how about waiting for the cup handle to face the right direction and… Oh I don’t know…. Hitting the Stop button ???

    1. The turntable is supposed to be designed to leave the contents in the same position at the end of the cycle as when they went in. Of course wear, etc, or stopping before the cycle is programmed to end could result in the turntable stopping in a random position.

      1. This would only work for even minutes and assumes perfect operation of what are on average very cheap components. Even if that is the intent it rarely comes out. Not that it matters.

  4. In the world where you try to extract every watt of usefulness, microwave are very bad at it. At best, their efficiency is close to 50% (but the low end microwave oven only reach 30%). That’s so poor it would be completely dismissed compared to any other cooking mean. Even a dumb kettle reach 90% of efficiency to heat water molecules.

    The only reason MW are so used is because they don’t require long pre-heat time and are easy to clean.

    There is a company using semiconductors to throw kW of MW energy via usual RF technology (instead of the dumb magnetron). They use beam forming to avoid hot spot, a PID to ensure that the energy isn’t wasted on unwanted area, etc… At least, they could raise the efficiency to 65% or so.

    The companies are hubersuhner, goji, midea…

    1. I would never waste my time making baked potatoes in a conventional oven if a microwave oven was available and never try to cook a pizza in a microwave oven if a pizza oven was available. Horses for courses and all that and, for the small amount of time a microwave oven is used, a magnetron is a good solution.

    2. Microwaves also penetrate the food so that cooking is faster. Infrared heat in a conventional oven is absorbed mostly at the surface and must be reradiated to food’s interior. True, a microwave oven isn’t a paragon of efficiency, but it cooks most foods faster than a conventional oven.

    3. Except, when you use a microwave most of the energy that turns into a microwave photon gets absorbed by the food instead of the walls. An oven, a toaster, etc all heat up a bunch of other mass that isn’t food. The lack of preheat time is a massive savings in the real world, and just because percentage efficiency looks low doesn’t really matter.

      An electric kettle can be insulated and pure water holds a lot more heat than thin metal, so it’s one of the best options – not a typical example. Even then if it runs too long, the steam carries a lot of energy away. An oven? Those throw off massive amounts of heat into the house, which means if you’re using air conditioning with a COP of 3 to 4, your efficiency might already be down to 75% with a completely perfect oven. That said, you’ve got to eat, so use what makes sense for what you’re cooking.

    4. I don’t think this is correct. If only 30% of the energy was going into the food, that microwave would be acting as a space heater. Your 1000W microwave would be putting out 700W of heat. Here’s a thought experiment. Many people have their microwave inside a cupboard. If the microwave was only putting 300W into the food, that cupboard would be getting very hot very fast. This doesn’t happen.

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