Sorry for brevity, but what is the exact physics explanation of why smaller quantities placed inside a microwave oven heat up faster than when you place a larger quantity of a similar material inside?
Thermodynamics – Why Do Smaller Portions Heat Up Faster in a Microwave Oven?
electromagnetic-radiationeveryday-lifemicrowavesthermodynamics
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The answer is this: You will need to cover the item you don't want to heat with a conducting material, creating a so called Faraday's cage, which is connected to an earth potential equal to the potential of the metal casing of the microwave oven.
The reason for this is threefold: The covering material needs to be a conductor, because conductors have the property that free charges can move free without significant resistance inside it. Because of this, the electric field inside the conductor is zero, for elsewise charges would move due to the electric field. But they move themselves in such a position that they cancel out the fiel again. Now, because the electric field inside the conductor material is zero, is also has to be zero in cavities of the conductor. For if it would not be zero inside the conductor, one could move electrons through a loop partially through the cavity, and partially through the conductor. The path integral through this closed loop would then be non zero. In onther words, one could generate energy without costs, obviously in contradiction to the law of conservation of energy. Therefore, wrapping an item in conducting material creates a so called Faraday's cage, whereby the electric field through the wrapped item is zero.
Now this is what we want, because the microwave oven heats items with the use of electric field waves. So, shielding the item off from electric fields, also shields it from heating.
However, charges inside the conductor start moving because of the electric field waves. This can create a potential difference with the shielding material of the casing of the microwave oven. It is because of this difference that arcing could occur. That is why you will also need te connect the conductive wrapping to the earth potential of the metal casing.
As already pointed out, microwaves in the oven have just the right frequency to heat water molecules. But this alone does not explain why chocolate with a tiny bit of milk heats up so much quicker than a glass of milk. The key is heat capacity.
With no milk the chocolate is almost transparent to the microwaves. Even a little bit of milk makes the mixture capable of absorbing a substantial portion of the energy the microwaves bring in. This energy is then spread out through the mixture of chocolate and milk. The heat capacity of the mixture is essentially that of chocolate if there is little milk, and the specific heat capacity of chocolate is much lower than that of water or milk. Therefore it takes less energy to heat chocolate to such temperatures that it burns.
If there is too little milk, you don't catch enough of the energy of the microwaves. If there is too much milk, it increases the heat capacity and slows down the heating. Somewhere in between there is a sweet spot where the milk acts as a microwave antenna for the chocolate but does not take up a substantial portion of the total heat.
I am not commenting on what the burning of the chocolate actually means. This answer only concerns the energy transfer process that leads to temperature change at different rates. The temperature around the boiling point of milk (≈water) is not enough for combustion but is enough for other processes that change the color and flavor of the chocolate. Also, sensitivity to microwave heating is not binary; plain chocolate heats up too but less so than milk, and it also depends on the variety of chocolate in use.
Best Answer
The magnetron injects microwave radiation at a certain rate. Ignoring losses, that radiation bounces around the walls until it’s absorbed by the food. If you put two burritos in there instead of one, on average there will be fewer bounces before absorption. That means that with two burritos, the average intensity of the radiation impinging on any point is less—some of photons, if you want to think of it that way, that would have been hitting the spot aren’t there because they’ve already been absorbed.
This is quite different from a regular oven—as long as there is enough power to keep the air temperature at the desired setting, it doesn’t much matter how many burritos you put in there, as long as there’s air space between them. They are heated by conduction from the air, which is unaffected by neighbors, and blackbody radiation from the surroundings which is only affected a bit.