The basics:
A microwave (not the appliance yet, the light wave) is a type of light - like a radio wave or visible light wave. Visible light is high-energy, and it's waves are really close together - nanometers. Radio light is low-energy and it's waves are far apart - meters. Microwaves are right in between those.
Microwaves (the appliance) produce ... microwaves (the light wave)! The waves are medium energy, and they're a few centimeters across. The microwave in your kitchen produces a "standing wave." All that means is that is doesn't really travel, it just goes up and down, up and down in the same place.
So how does this standing wave cook food? Water molecules in your food are like little magnets - they have a positive and a negative side. They also like to line up with the magnetic field around them. If a water molecule is caught up in the middle of the standing wave in your microwave, the magnetic field that is being produced is switching back and forth really quickly, and the molecule goes back and forth trying to line itself up with the shifting field.
Imagine being in the middle of a jump rope, and your job was to always look at the rope - you'd soon find yourself dizzy from looking up, then down, then up again. By forcing all the water molecules in your food to 'look' at the direction of this shifting field, they'll be moving around, turning back and forth, and that energy goes to heat up your food.
Last piece of the puzzle - warm spots and cold spots. You'll know from experience that there are spots in your microwave that don't heat food. These are the nodes - the spot where the standing wave doesn't go up or down, but rather stays put - the handles of the jump rope. The water molecules here don't feel much a pull in either direction, so they're quite content to not waste energy in all this 'heating up' nonsense.
That's the basics done - so here's how I put that info to the test to figure out a few things about my microwave.
Speed of Light:
Because I work in education, one of the first things I did was use my microwave to calculate the speed of light. If you google "Calculate the Speed of Light with a Microwave" you'll find the basic process - find the frequency of microwaves being produced on the back of your microwave (mine is 2450 MHz, or 2.45 billion waves per second). Figure out the wavelength of the microwaves by putting something that melts into the microwave for a few seconds, and measuring the distance between the points that melted, doubling that, then use the equation:
Speed of light (m/s) = frequency (Hz) x wavelength (m)
I wanted a little better information and visualization, so instead of the standard chocolate, cheese, or marshmallows, I used a papadums - an Indian treat generally not made in the microwave, to generate a 'heatmap' of my microwave in two dimensions. This told me where my hot spots and cold spots were in my particular microwave.
So not great. My microwave is pretty patchy. It's a really good thing that it rotates, otherwise it would just be this patchy mess of hot and cold. I tried putting the rotating tray back in and that yielded much better results:
In my experiment, I calculated the speed of light to be 318,500,000 m/s - a percent error of 6.2%. Not bad, but I've seen better.
<sidenote>
Here's the math:
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| My microwave's frequency is 2.45 billion waves/second (Hertz [Hz]) |
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| 13 centimeters seemed about average between hotspots. This is the shadiest part of the calculation. |
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| My microwave's frequency, times the measured wavelength, 13 cm, in my case |
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| Calculating percent error. 299,792,458 is the known value. |
</sidenote>
Oil, Ice, and Water
Ice is weird. Its solid form floats in the liquid form, it'll dissolve nearly anything, and there are at least 17 at least 17 different forms of it. It's basic form is a crystal structure that comes from the fact that the positives like to like up with the negatives and form bonds when you take energy away from it.
When heating things up from frozen, you have to deal with the issue of our water molecules being bonded to each other in ice. Any individual molecule has to break that bond to be free to rotate back and forth and heat up in a standing microwave.
Another issue is oily food. Oil molecules are non-polar - they don't have distinct positive and negative areas, they're sort of well-adjusted and boring all over. If there's no pull to 'look at the jump rope,' or follow the shifting magnetic field, oil molecules aren't going to want to move around and heat up.
To test this effect, I put an ice cube, the same volume of water, and the same volume of oil in a microwave; each for 30 seconds, each in the center.
- The water changed from 65°F to 191°F - a change of 126°F
- The ice changed from 19°F to 55°F - a change of 36°F (a little melted)
- The oil changed from 68°F to 154°F - a change of 86°F
We'll keep these differences in mind as we get to the point:
Microwaving Power Tips:
1) Use the power level
A lot of microwave instructions terminate in a step that says "Let cool for x minues before enjoying," or something similar. That step is there for an important reason. It doesn't really let the food cool to a safe eating temperature - that could be accomplished by simply reducing the cook time. It is there to let the food equalize in temperature. At this point, liquid water in the hot spots of the microwave have a lot of heat, ice is less icy, and oil is pretty much unchanged. If you bit into it now, you'll have hot, warm, and cold patches.That minute or two allows the heat to spread around and for the whole thing to equalize out to be the correct temperature - melting the ice and warming the oil, while cooling off the magma.
Now onto the tip - microwaves are like the carts at Gringott's bank - one speed only. The power level setting doesn't make lower-powered waves - it simply turns off the microwaves for that percentage of the total cook time. You can hear the microwave turn on and off while in a power setting other than 10.
Putting that together, if you double the cook time and half the power you get built-in waiting periods for the heat to move around and melt ice and warm oil. You end up with a much more even heating, with no waiting after you're done!
2) Get rid of the interior cold spot
It's hard for heat to travel within food - it's a slow process. If the outside of food gets hot it'll take a while for that heat to travel into the center of the mass.When possible, arrange your food into a donut shape. This effectively halves the distance from any outer edge to any point in the interior.
3) Give your food some variety
If your food is centered on the rotating tray, and there is a cold spot 4 cm from the center point on either side, any food that particular distance away from the center will never be warmed up. The simplest way to combat this is by placing your plate of food off-center in the microwave. This ensures that every point not in the exact center will experience a variety of locations in the microwave and have a higher chance of spending more time in a hot spot.Secret tip #4
Most microwaves can be silenced by pressing and holding either Stop, Cancel, 0, or 1 for around three seconds. Some even have a "Sound" button right on the front that you can press to shut them up.That's all I've got for microwaves tips, but I'll leave you with a few warnings:
- Definitely do NOT put ivory soap in the microwave for a really cool effect like this guy.
- Under no circumstances cut a grape mostly, but not all the way in half like you can find instructions for in this video.
If you do go ahead and do these things, certainly don't cut a 1/4 inch hole in the grating so you can record it with a webcam (the waves are a few centimeters, so a 1/4 inch hole is just as good to contain microwaves as the tiny holes already in the mesh).
Cheers,
- Scott
P.S. I guess I'll try to make this more of a real thing, so I dinkered around in Inkscape for 10 minutes and made a logo, and I'll start putting some stuff up on Instagram for each post if you'd like to follow there.
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