I did a forum search and noticed you've corrected some errors in old threads!Before I answer let me tell you that I am a microwave scientist and have worked microwaves and microwave ovens for nearly 50 years - I also teach this stuff. …
This is a fascinating question and not easy to answer because microwave ovens are extremely complex and not as well understood as you might think. Anyway, here are some thoughts.Would metal, completely submerged, in water still arc or would it help concentrate energy into the heating of the water?
Water molecules are dipolar – two hydrogen atoms and an oxygen atom. The heating of pure water is essentially due to the dipolar rotation phenomenon as you’ve probably been taught (actually more complex than that but OK for now). But add a pinch of salt to the water and a new mechanism dominates: ionic conduction – the acceleration of ions such as sodium, chloride, hydronium and hydroxyl ions, by the alternating polarity of the electric field, and that causes these ions to collide with non-ionized molecules like water and the collision impact translates to heat. So, there are many different heating mechanisms.The "why" part of the original poster's question is still pending. It's about how to reconcile -- on one occasion the schools teach that water molecules are polar and oil molecules nonpolar -- on another occasion they say that microwave ovens work on the principles of making polar molecules spin around to align themselves. So why does the microwave oven heat oil?
Thanks for your answer,This is a fascinating question and not easy to answer because microwave ovens are extremely complex and not as well understood as you might think. Anyway, here are some thoughts.
First, you have to understand that microwaves are a form of electromagnetic energy, just like visible light, TV, radio, X-rays, etc. Each of these is distinguished by its frequency and associated wavelength. For the microwave oven the nominal (average –it’s really a range) frequency is 2450 Megahertz (MHz) or million of cycles/second, or 2.45 Gigahertz (GHz) billions of cycles/second; and this has an associated “free space” wavelength (effectively air or vacuum) of 12.2.cm. The wavelength decreases as it passes through an absorber such as water where the wavelength at room temperature is a bit over one cm. It’s called an electromagnetic wave because it has an electrical component and a magnetic component – both have to be there – one can’t exist without the other. It is the electrical component that heats things like water, food, oils, etc. The magnetic component heats magnetic materials such as iron (ferrous) containing materials.
The answer to your question is very complex because there are so many factors (I originally wrote an answer early this morning but, because I am new to this site, it got lost in never-never land. Since then I’ve thought of more things). The thing that stands out in my mind is the relative volume of water & metal. Let’s say you have a liter of water in a glass beaker and you place a small magnet on the center bottom. It is unlikely the magnet would get any microwave energy because the water would have absorbed all of it since the penetration depth of microwaves into water at room temperature (the distance at which 63% of the energy is absorbed and causes heating) is a little more than a centimeter. I’m neglecting microwave energy coming through the center bottom of the beaker – very complex! So, all the microwaves heat the water….. But now, let’s assume the magnet’s volume is large enough to come within a centimeter of the glass side wall – first microwaves would be absorbed by the water, but there is likely to be sufficient microwave energy left to heat the magnet. It sounds good, but there are lots of complications. For example, if the magnet is that large, then there isn’t much water so the magnet will be heating a small volume of water and that represents a particular case of the problem. I suggest you consider how you would experiment to learn the answer. A warning – I’ve been thinking about how to run the experiment and it is very difficult. I’d be happy to try t help you. Good luck! By the way, it couldn't possibly arc - the electric field energy would be far too small because of the water.
Hi mikelpore - thought you might enjoy this factoid about microwave heating.microwaveguru, thank you for your helpfulness!
I chose a magnet because it is magnetic and easily understood regarding the interaction with the magnetic field. It would be the same with any material with magnetic properties.Thanks for your answer,
I was thinking of a solid ring of metal, like copper, and am not quit sure why you mentioned a magnet.
I'm always too close to the edge on this forum, so I won't go into any detail, but a couple of thoughts involved a coil of tubing, with the first part (two or three rings) being cermic and the rest copper, submereged in water and capable of rotation. principles of induction heating along with the principles of action, in a bubble jet printer.
Again Thanks for your time.
The grey-colored materials are called “microwave susceptors”. They are specially manufactured microwave interactive packaging materials that get very hot – 375 F – very quickly and provide a hot surface for the pizza to crisp. A susceptor is made by coating a thin film of plastic (PET: polyethylene teraphthalate) with vacuum deposited aluminum (that looks like minute island of aluminum under an electron microscope). That film is then bonded (glued) to paper or paperboard – depending on whether you want a flexible or rigid susceptor – your pizza tray is rigid. That then gets packaged with the pizza and it’s placed under the pizza when microwaving it.While we're on the subject of microwaves, and since we have a resident expert, I have a question for you. How do those grey-colored materials that are packed with some microwave foods work? What exactly do they do to help heat something such as a pizza?
Fascinating. I didn't know that the material itself was getting so hot, since when you are done cooking they don't feel particularly hot to the touch. I thought they just reflected microwaves back into the pizza or somethingThe grey-colored materials are called “microwave susceptors”. They are specially manufactured microwave interactive packaging materials that get very hot – 375 F – very quickly and provide a hot surface for the pizza to crisp. A susceptor is made by coating a thin film of plastic (PET: polyethylene teraphthalate) with vacuum deposited aluminum (that looks like minute island of aluminum under an electron microscope). That film is then bonded (glued) to paper or paperboard – depending on whether you want a flexible or rigid susceptor – your pizza tray is rigid. That then gets packaged with the pizza and it’s placed under the pizza when microwaving it.
There is a lot of complicated physics here but to make it as simple as I can: You may remember this heating equation:
P = I2R , where
P = Power in watts – that tells us how hot it gets
I = current flow (amperes)
R = resistance to current flow (ohms).
Aluminum foil will let a current flow though it with no resistance to the flow – so P is zero – no heating
Paper has lots of resistance, but it won’t allow a current to flow though it, so again P = zero.
In a susceptor those tiny aluminum islands allow a current to flow but there are gaps between the islands, which represent a resistance, and if the current can jump the gap you get Power – watts – heat. It’s sophisticated though – the island size and the gap size is critical. If he gap is too large = too much resistance so no power; too small and there is not enough resistance = not enough power to heat.
Actually they get very hot - 375 degrees - and then they stop heating, so it probably lost a lot of its heat to the pizza. I know they get hot because I touched one that didn't ave any pizza on it and burned my hand - on television!Fascinating. I didn't know that the material itself was getting so hot, since when you are done cooking they don't feel particularly hot to the touch. I thought they just reflected microwaves back into the pizza or something
No -it shouldn't - wax is interesting (I gave a paper in Japan last year that involved heating wax). Wax is almost microwave transparent when it is solid and can stay like that for minutes of microwaving. Then, once the wax melts it becomes very microwave absorptive and heats like mad - it exhibits "runaway heating" - see my response to another question. Now, what happens when the wax isn't absorbing microwaves and why doesn't it blow up the oven? Microwave ovens are constructed to protect the magnetron from lots of reflected power when it is empty or has something like solid wax in it - in this case, the glass turntable is made to absorb the microwaves (technically it is called a "matching load"). Also, the steel that makes up the walls of the oven are slightly magnetic, so the walls will also absorb some power. The trick in microwave oven design is to get most of the energy to the food when you need it. Who needs hot walls?But wax will.