I need some help with the physics of cooking, to be used when people start freaking out over the alleged danger of microwave ovens.

  1. It seems to me that hot food must be radiating into the infrared no matter how it was heated. In the case of microwave heating, a lower frequency is used to stimulate a frequency in the food that is higher than the frequency used to induce it. Correct?

  2. A stove burner transmits heat by conduction and an oven does it by convection. But how is the heat generated in the first place? Does an electric oven use electricity to generate infrared in the heating element, whose energy is then transmitted by conduction or convection? Or is something else going on? The heating element turns red so it seems that radiation is being generated in the visible spectrum.

  3. Does a gas stove generate infrared at all, or is the heat purely the result of the chemical reaction that occurs when we burn the gas? Do the flames look blue because they're generating blue photons, or because they're reflecting the blue photons that were already bouncing around the room?

  4. Is cooking over a fire different from cooking with gas, in terms of chemical heat versus electromagnetic radiation? I assume that a different chemical reaction is involved, since we're burning different materials.

Thank you for your help.


2 Answers 2

  1. Heated food emits radiation in the infrared, if it emitted radiation at higher/blue frequencies, it would be too hot to eat, (and burnt). The microwave radiation heats the water molecules in the food, to normal cooking temperatures. As regards frequency, here is a full explanation Microwave ovens

  2. I would naturally agree that when you see a red element, it is hot enough to emit red photons. But when you see those cop reality shows where the helicopter's infrared camera spots the "bad hombre" hiding in the trees, this shows you that infrared is not visible to human vision.

  3. Infrared radiation is heat, for cooking purposes, they are synonymous. The gas is blue because of the temperature it is at, which emits some light in the blue frequency of the electromagnetic spectrum. It does not reflect blue photons, it emits them. But the blue light you see is not necessarily the majority of the electromagnetic radiation, which is in the hard/impossible to see, infrared part of the electromagnetic spectrum.

  4. Obviously, you won't cook anything without adding heat to it. If you boil it, it's heat transfer through convection, hot water swirling around it, heated by either gas or wood. Gas is cleaner and more controllable. If you fry it, it's conduction, the pan conducts the heat from the gas or woodfire through the pan and onto the steak on the pan. If you roast something over a fire, or in an electric oven, it is cooked by both radiation and convection of the hot air. Burning wood is a different chemical reaction than than burning gas.

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From Cooking Temperatures

  • 1
    $\begingroup$ I argued with a colleague a few years ago about whether conventional ovens (i.e., not microwave ovens) transfer energy primarily through infrared radiation or using conduction between the heated air and the food. I am still not entirely sure which one dominates, but I am inclined to think that conduction has an advantage there. $\endgroup$ Mar 15, 2017 at 14:20
  • $\begingroup$ @honeste_vivere If you get it wrong, (and hunger/stupidity is my excuse), I will use the upper shelf and the element will burn the outside of the food before the inside gets done...so conduction and the bottom shelf works for me......but I can cook, in a way............"Hello, is that Dominos?" $\endgroup$
    – user146020
    Mar 15, 2017 at 15:42

Let's see:

  1. Anything produces some infrared radiation. The amount of infrared radiation increases with the amount of heat the object radiates. Even stuff at absolute zero still has molecules that "wiggle" (quantum mechanics uncertainty stuff)
  2. In the case of a stove burner or anything that works with gas/fuel(wood, coal, et Cetera) the gas is combusted to generate the initial heat and then conducted/convected/radiated. Whereas for electric appliances there are different ways that the heat can be generated:
    • For a microwave: A magnetron generates microwaves "tailored" for what you want to heat (food), these make the water molecules in the food "vibrate" or wiggle around which makes the food warm.
    • For an electric furnace/electric stove/water heater: Electricity is "sent" through a wire and the electrons make the wire get hot due to friction between the electrons and the conductor (wire), then the heat generated from that is conducted/convected/radiated into whatever is you want to heat.
    • Convection/Induction stoves: An electromagnet generates an electromagnetic field that heats up your cooking container(pot, pan, etc...). This happens because a very low-voltage but large current is induced in your cooking-container this current will heat up food but won't electrocute anyone because the voltage is too low to get throw anybody's skin.
  3. Anything that's hot irradiates heat and infrared light; the hotter, the blue-er until it gets white; then it's over the visible spectrum meaning that the frequency of the light emitted is too high for us to see. Furthermore, different fuel/oxidant compositions when there's combustion can generate different colors of flame because of other effects but for the case of just hot objects, it's always dependent on the temperature.
  4. Heating with fire and using conduction/convection/radiation is different from the microwave heating I mentioned because when using fire or something that's hot because of the electricity "flowing" through it, you just use the heat from whatever is hot (flame/heating-element) to heat up your food or container with food. On the other hand, for microwave ovens, the microwaves which are actually pretty big (100cm to 0.1cm) (but small compared to radio waves, 100km to 1mm) go through the food and make only the water molecules vibrate. Temperature is related to how fast whatever molecules you're talking about "vibrate". The faster, the hotter the thing you're touching/talking about...

Anyways, microwave ovens are completely safe, as the kind of radiation they produce is called non-ionizing because it cannot break chemical bonds due to the fact that these waves do not carry nearly as enough energy. (to break a chemical bond you need at least 5 eV/bond on average which is figures of magnitude smaller than what a microwave produces!) In other words, it can't do anything other than heat up stuff because it does not produce enough energy, so it just makes the molecules "wiggle" around. The reason why ionizing radiation such as the one produced by radioactive stuff is dangerous in large amounts is because it has enough energy to break chemical bonds, so it can break the bonds in your DNA effectively modifying it/messing it up. But in the case of Radiowaves or Microwaves, it won't even get close to it.


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