dsayers Posted February 17, 2016 Posted February 17, 2016 Say you have item X in a microwave for Y time and it ends up in state Z. If you want to achieve getting 2X to state Z, how much time is needed? I don't understand the way microwave ovens work to know if the answer is as simple as 2Y or not. Anybody have an idea?
shirgall Posted February 17, 2016 Posted February 17, 2016 Say you have item X in a microwave for Y time and it ends up in state Z. If you want to achieve getting 2X to state Z, how much time is needed? I don't understand the way microwave ovens work to know if the answer is as simple as 2Y or not. Anybody have an idea? Dielectric heating is caused by polar molecules being spun around by the electromagnetic fields in a microwave oven. However, fiddling with the frequency of the field is going to interfere with radios and everything else, no matter what your shielding. The 2.45 GHz frequency used is distant enough from most things (except Wifi) and is highly efficient for the materials and distances needed in a home kitchen. Instead increasing the power increases the effects on less conductive materials. Increasing the power instead increases the depth, and not necessarily the resulting time to cook. The bottom line is that 2Y is probably too long, as some of the wasted RF energy in the 1X case will get absorbed by the additional material in the 2X case. Per wikipedia: where is the angular frequency of the exciting radiation, is the imaginary part of the complex relative permittivity of the absorbing material, is the permittivity of free space and the electric field strength.
Wuzzums Posted February 17, 2016 Posted February 17, 2016 In a normal oven the way X is heated is by the oven heating itself (somehow) and then that heat is transferred to item X. Thus the more items in an oven, the more heat (or heat exposure) is required to reach state Z for each item. Thus 2X will require 2Y time to reach state Z. In a microwave oven, microwaves are generated that interact with the item's molecules (the water molecules to be more precise) thus generating heat within the item itself. There's the same quantity of microwaves generated each time in a microwave oven. You can have any number of X items in a microwave oven, in order to reach state Z it will still only take Y time.
Worlok Posted February 17, 2016 Posted February 17, 2016 A microwave emits electromagnetic radiation. Microwaves are just a certain amount of energy in the EM waves. Lower energy waves are called radio waves and higher energy waves are infrared, visible light, ultra-violet, x-ray, gamma. You take energy from the microwave and transfer it to the food. The increased energy causes particles to move very quickly, creating friction. It push your hands together and rub them together very fast. That's what a microwave does. you need 2x energy for 2x the mass. That said if the item in the microwave has parts that are thicker than others, the thicker parts will take longer to heat. twice the thickness, twice as long. That's why the edges melt fast on your quesadilla, while the thicker, inner area of cheese is still solid...
Mister Mister Posted February 17, 2016 Posted February 17, 2016 my understanding is that microwaves work by vibrating water molecules. so the rate at which something heats up is dependent on how much water is in it.
Koroviev Posted February 17, 2016 Posted February 17, 2016 also if there are multiple items in the microwave they both aren't going to get all of the microwaves all of the time one is going to absorb most of the waves at one point then as it rotates the other will get most of the waves. Also, front side vs back side needs to be taken into account. think about shining a light on what you are heating whatever is facing the light absorbs the most light waves
Anjin-san Posted February 17, 2016 Posted February 17, 2016 If the heating throughout the oven were smooth and even, with no hot or cold spots, it might be simple arithmetic, but it doesn't work that way. For a liquid in a single container, it might, but probably not for foods on a plate. Here's a nice video that explains the oven's principles - around 2:15 he shows a neat experiment that might clear it up a bit (it did for me): https://youtu.be/kp33ZprO0Ck (if this post shows up twice, it's because I tried to embed the video the first time, requiring moderator approval)
csekavec Posted February 18, 2016 Posted February 18, 2016 Every substance has harmonic frequency response. Microwaves are tuned to the frequency of water. Other substances have lower order harmonics to that frequency. Each substance absorbing according to its receptiveness. And all the rest heated by convection due to being in solution with stuff susceptible to microwaves.
WorBlux Posted February 18, 2016 Posted February 18, 2016 Say you have item X in a microwave for Y time and it ends up in state Z. If you want to achieve getting 2X to state Z, how much time is needed? I don't understand the way microwave ovens work to know if the answer is as simple as 2Y or not. Anybody have an idea? 2X is often too much. Microwaves are often overpowered and the time required is not a straight reflection of the energy needed, but also the time for the heat to get through the food. Also there are foods that need to stay at temp for a time. So the portion of time needed to bring the food to temp doubles, but the time you need to keep it at temp is the same. I usually go about 1 and 2/3's X and stir or flip halfway through the cook cycle.
rosencrantz Posted February 18, 2016 Posted February 18, 2016 Calculating the exact time is pretty hard. You have to use the Lorenz equation and adjust for a uniform source of heat https://en.wikipedia.org/wiki/Lorenz_systemWhen you heat water, there are turbulences in the water: hot water gets up, cools down, and gets up again. That is why the temperature of water you heat is not uniform, but different depending on the location of the thermostate.
Guest Gee Posted February 18, 2016 Posted February 18, 2016 Approximate item x as a thermal system in contact with a heat bath (the microwave) and use Newtons law of cooling. Discount that, I though you wanted that time for the same X to get to twice the temperature. fun question though, will stick up an answer in a day or two.
celticwar17 Posted February 22, 2016 Posted February 22, 2016 Every substance has harmonic frequency response. Microwaves are tuned to the frequency of water. Other substances have lower order harmonics to that frequency. Each substance absorbing according to its receptiveness. And all the rest heated by convection due to being in solution with stuff susceptible to microwaves. The phenomena with water and microwave absorption is slightly different than harmonic frequencies. The H20 molecule does get some special absorption spectrum's because it tends to wiggle and it has relatively high dielectric permitivity. H20 absorbs the microwave because it wiggles, not the H20 wiggles because it absorbs. From what I understand... I have a Bachelors Degree is Physics if that means anything... Say you have item X in a microwave for Y time and it ends up in state Z. If you want to achieve getting 2X to state Z, how much time is needed? I don't understand the way microwave ovens work to know if the answer is as simple as 2Y or not. Anybody have an idea? The hotter the water gets the less energy it absorbs.... which can be seen in this graph https://en.wikipedia.org/wiki/File:Dielectric_loss_water.png So technically, in combination with loss of water in the material(steam) AND the water in the food getting hotter, it gets heated less over time. The amount of time needed varies greatly depending on the initial temperature of the substance, it's mass and water content, and whether it's frozen or not(because Ice absorbs microwaves at a different rate). So the answer would be 2Y+something...... but you also have to factor in that microwaves heat things unevenly so one area of it might turn into molten lava, which would make you want to reconsider making the net temperature x2...
Guest Gee Posted February 26, 2016 Posted February 26, 2016 What Celtic said. I have more full solution, but as the joke goes, it only applies to a spherical chicken in a vacuum.
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