Introduction: I read on Wikipedia's list of common misconceptions that microwaves work not by emitting the resonant frequency of water, but as a result of dielectric heating. As I understand it, this process heats a substance by emitting a constantly changing electric field, which makes the polar molecules in the substance attempt to align themselves with the field, thus introducing more molecular motion, that is, thermal energy. This makes me think that the heat introduced to the substance should be directly proportional to the polarity of the molecule. I then conducted a brief experiment:
Method:
I heated 25g of canola oil and 25g of water in plastic cups in a 1250 W microwave oven for 15 seconds each, measuring the temperatures before and after. I couldn't find the frequency of the waves emitted from the microwave, though I'm pretty sure it's the standard 2.45 GHz. If the frequency is necessary to know for sure, I think I could go back and partially melt a chocolate bar in it, finding the wavelength, and use the speed of light to find the frequency. (http://www.planet-science.com/categories/over-11s/physics-is-fun!/2012/01/measure-the-speed-of-light-using-chocolate.aspx)
Data:
| Water | Canola oil | |
|---|---|---|
| mass (g) | 25 | 25 |
| time in microwave (s) | 15 | 15 |
| initial temperature (C) | 25 | 24 |
| final temperature (C) | 51 | 33 |
| heat deposited (J) | 2718 | 472 |
Results:
Using estimated specific heat for canola oil (from https://www.sciencedirect.com/topics/neuroscience/canola-oil and https://doi.org/10.1080/10942910701586273) of 2.1 J/gK and 4.182 J/gK for water, it can be found that the change in energy of the water is:
$q = mc(\Delta T)=(25~\mathrm{g})(4.182~\mathrm{J/gK})(26~\mathrm{K}) = 2718 ~\mathrm{J}$
And the change of energy in the oil is:
$q = mc(\Delta T)=(25~\mathrm{g})(2.1~\mathrm{J/gK})(9~\mathrm{K}) = 472~\mathrm{J}$
So there was about six times more energy given to the water than the oil.
Discussion: This seems odd to me. First, if dielectric heating relies on polarity, why is the canola oil heating at all? Second, why is it heating as much as it is? I would think that the hydrogen bonds in the water are far more than six times as strong as the London dispersion forces within the oil. Is it because the oil is diluted with a polar substance? Is it because the lowered polarity makes it easier to move the molecules, and therefore impart heat? What should be expected to happen if a completely nonpolar material is microwaved?
Best Answer
In addition to the good answer from Gert, there's a problem in this step. The microwave oven is a metal box. The purpose of the metal box is to reflect whatever microwaves aren't absorbed by the food (either because they "missed the target" or because they passed through). This reflection isn't 100% efficient, but it's actually pretty close, so for the sake of discussion we can pretend that it is. What that means is that basically all of the power emitted by the magnetron goes into the food. If you put a sample in there with a lower dielectric loss it will absorb less energy, and heat up less, "on the first pass", but that just means that more of the energy will be available to bounce off of the walls and take another try, and another, and another — the field density will increase until it reaches the point where absorbed power equals input power.
Obviously there are practical limits to this (an empty microwave oven would either have to deliver all of its heat to itself, or else automatically turn off or reduce power), but for reasonable samples of food-like stuff it's close enough to true to seriously mess with the concept of your experiment.