Posted: 9/1/2010 10:36:30 AM EDT
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This is probably a stupid question. If you're heating something by conduction, for instance frying an egg on the stove top, you're conducting heat from the burner, through the pan, into the egg, right? So if the burner is 400 degrees, the hottest the pan, and therefore the egg, can become is 400 degrees, correct? Now a microwave heats through radiation (I think), so what is the limit to how hot you can heat something in a microwave? Is it limited by the wattage of the machine? |
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Quoted: This is probably a stupid question. If you're heating something by conduction, for instance frying an egg on the stove top, you're conducting heat from the burner, through the pan, into the egg, right? So if the burner is 400 degrees, the hottest the pan, and therefore the egg, can become is 400 degrees, correct? Now a microwave heats through radiation (I think), so what is the limit to how hot you can heat something in a microwave? Is it limited by the wattage of the machine? There is more to it than that. I'll use water as an example because the more complex a food is the more variables there are. Water on a stove can only be heated to 212oF (at see level) it will stay there until all the water evaporates. Something like an egg is more complex because there are more changes that occur and things start to burn before you get to the stove temperature. But, the stuff in the pan will not get hotter than the stove. The microwave works by exciting water or other molecules and manifests as heat in the item. The same rules apply though, water in a microwave can not get over 212. However, other liquids or solids can get hotter but are limited by the temperature at which phase/structural changes happen as well. |
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Quoted: I know that if you put distilled or de-ionized water in a microwave for long enough, you can make it flash boil with the addtion of a nucleation site (you stick your spoon in the cup or add something to the water). That's what kinda got me thinking about it. Water refusing to boil and super heating until something is introduced. Like the opposite of making fake snow on a ski hill. |
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Quoted:
Quoted:
I know that if you put distilled or de-ionized water in a microwave for long enough, you can make it flash boil with the addtion of a nucleation site (you stick your spoon in the cup or add something to the water). That's what kinda got me thinking about it. Water refusing to boil and super heating until something is introduced. Like the opposite of making fake snow on a ski hill. temperature is the avg KE of the molecules in a substance. Heat is the total KE of the molecules of a substance. So if you keep adding heat by keeping something on the burner itll just get hotter and hotter until its emitting heat by convection/radiation at the same amount its receiving heat from the burner. |
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Quoted:
Quoted:
I know that if you put distilled or de-ionized water in a microwave for long enough, you can make it flash boil with the addtion of a nucleation site (you stick your spoon in the cup or add something to the water). That's what kinda got me thinking about it. Water refusing to boil and super heating until something is introduced. Like the opposite of making fake snow on a ski hill. mythbusters superheating water with a microwave |
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Quoted:
No expert but that sounds right to me. I believe microwaves only excite the water molecules though so if something has less water in it, it has less potential maybe? Like any other wavelength of light, some things are "white" to microwaves and some things are "black." Water happens to be one of the materials that is "black" to microwaves - meaning that water absorbs it rather than transmitting or reflecting it. It's the same idea as if you had a bowl of clear water and a bowl of water with black dye and put it in the sun. The water with the black dye would heat up must faster. As for the stove vs. the microwave, there are a lot of variables involved. Yes, the stove cannot heat up anything beyond its own temperature. But since the egg has unconfined water in it, it will not heat up beyond water's boiling point (the temperature at which water's vapor pressure equals atmospheric pressure) due to the cooling effect of evaporation*. But once your egg is dry, it will begin to burn as it is heated up beyond the boiling point of water. The same holds true for microwaves - you won't get water hotter than the boiling point (except in the rare occurrence of superheated water, which has happened with microwaves and is very dangerous**. But if you have a material that does not vaporize and readily absorbs microwaves, the only limit to how hot you can get the object is how well-insulated the object is. The hotter it gets, the faster it will cool due to radiation and conduction. If, theoretically, the object were perfectly insulated (which is impossible, or how could you get the microwaves to it?), there would be no limit to how hot you could get the object, no matter the wattage of the microwave. *Atoms bond to each other like magnets. When you cool them enough, they will stick to each other in a crystalline form, creating a solid. If you heat it up enough, you will break the crystal apart, but even though they cannot fully bond, they still stick, and turn into a liquid. It takes a big push to get an atom to completely break that bond and turn into vapor. Therefore, if an atom does manage to leave, it is only because another atom ran into it and gave it enough of a boost to knock it away. In a game of pool, if you knock a ball into another ball and give that other ball a boost, the ball that gave the boost slows down or stops. The energy was transferred. Similarly, the atom that knocked the other atom away now has less energy, because it expended its energy knocking the other atom away. In other words, it has cooled. On a large scale, this causes a general cooling of the entire mass. That is why evaporation causes cooling. **Superheated water occurs when there is no point at which the boiling can begin. Water bonds to itself strongly, molecule by molecule, and it requires a weak spot for a bubble to open. It's the same idea as a balloon. When you inflate a balloon, there's a lot of stress on the balloon, but it doesn't pop because the force is equally spread out over the entire balloon's area. It easily has enough force to pop, but the force is not concentrated enough. But if you poke it with a balloon, the balloon now has a weak spot to release its energy from. Similarly, if you touch or disturb superheated water, you have given it a weak point at which it can begin to release its excess energy. |
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Microwave ovens don't use "radiation". They use radio energy in the microwave frequency range, specifically 2.45 GHz, the same frequency band used by a whole lot of digital phones and wireless networks. The only thing different is the power level and the 50% duty cycle of the pulses transmitted into the cooking cavity. The microwave energy is absorbed into the food product and excites the product's water molecules into a higher level of energy. Think of it as molecular friction similar to rubbing your palms together rapidly. The faster and harder you rub your palms together the more heat you generate and you eventually "cook" the skin on your palms. There is NO nuclear radiation involved in microwave ovens. It's simply radio energy causing molecular friction in materials that are not transparent to radio waves. |
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Boiling points in a microwave are constrained to temperature and pressure, just like with conventional heating.
Water boils (releases heat as fast as it gets it) when it hits 212 at standard temp and pressure. It would do the same thing in a microwave. (I'm guessing  )
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Quoted:
I know that if you put distilled or de-ionized water in a microwave for long enough, you can make it flash boil with the addtion of a nucleation site (you stick your spoon in the cup or add something to the water). This. Water can be super heated (that is heated beyond the boiling point) in a microwave. I've even heard of this happening with regular water, not just distilled. I believe the mythbusters did this one. This posteris correct though. You super heat the water and then as soon as you put something in it, like a spoon, a scoop of instant coffe etc, it basically explodes. |
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Eggs are mostly water. It will not get significantly above the boiling point without serious burning. The browned margins are just slightly above the boiling point. The maximum possible temperature when cooking something that is mostly water is dictated by the Leidenfrost effect, this is where the temperature is high enough to vaporize the water, elevating the water above the pan. That is about 400-450 F and can be demonstrated by placing droplets of water on a hot pan and watching for the dancing beads of water. They do not touch the pan but are elevated on a cushion of steam. This limits heat transfer Now when you add oil, it can support temperatures up to 350 degrees without smoking and that is why deep frying is much quicker. Microwaves can superheat water because there are no hot spots. But it will only be several degrees of superheat, eventually it will boil. |