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Old 07-20-2013, 10:45 AM
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Microwave Ovens - Why Longer to Heat Multiple Items?

Musing this morning over coffee and a breakfast Hotpocket (don't judge me!) when I re-noticed something I've probably known for a long time. It takes longer for a microwave oven to heat multiples of the same item. For example, one Hotpocket - 2 minutes. Two Hotpockets - 3.5 minutes.

Why?

From my understanding, microwaves heat by vibrating the molecules in the food item, not by heating the item and the air around it like a conventional oven. I can understand taking longer to cook two "whatevers" in a conventional over, but as long as the Hotpockets are on the rotating platform, and both are being equally exposed to microwaves, shouldn't the warming time be about the same?
Old 07-20-2013, 10:49 AM
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In order for the items to heat up they must absorb some of the microwaves. The oven outputs a constant microwave power output so, if an item is heating up, it is using some of that power which leaves less for other things.

A conventional oven maintains a constant temperature and adjusts the heating power as necessary to achieve that.
Old 07-20-2013, 10:52 AM
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Consider the extreme example: Take the magnetron out of a standard microwave oven, and install it in a room-sized microwave chamber. Fill the chamber clear full of Hot Pockets. Run the magnetron (which uses perhaps 2000 watts of power) for 2 minutes. How much energy did the magnetron consume? By how much did the energy of all of the Hot Pockets increase?
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Old 07-20-2013, 10:57 AM
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the oven makes a certain amount of energy for a given setting and this is divided between all the items in an oven. more stuff is longer time.

ovens don't maintain a temperature. the air in the oven is not heated to be the heat transfer medium so air temperature is not measured for cooking purposes. some oven may have a temperature probe to insert in a food object, that is a different operating mode and the temperature is only at a single small spot.
Old 07-20-2013, 11:13 AM
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Still not certain I'm getting that though - the microwave puts out the same amount of power no matter if one or two items are in there, and I know the rotating platform is done to ensure even heating so it's not like the oven is targeting one Hotpocket if only one, or dividing the power between two if there are two - the rotating platform moves them both evenly through the beams, I would assume. So how do two items end up absorbing half the power if the beams are a steady stream (or steams) of radiation? Shouldn't the rotation carry one or two through the exact same amount of microwaves?

I mean, that must be the answer, it just seems odd that microwaves "know" to divide their power evenly between two Hotpockets taking almost twice as long to heat them.

Last edited by Werekoala; 07-20-2013 at 11:16 AM.
Old 07-20-2013, 11:27 AM
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any microwaves absorbed by an object are no longer available to that or any other object.

similar to having some objects in a room and a small light like a flashlight. the amount of light directly and reflected is limited. objects close to the light cast a shadow on objects farther away. you can see different objects as the light moves.
Old 07-20-2013, 11:31 AM
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Understood - I guess I must not understand how the oven works. I thought that there were actual beams that the food passed through rather than the whole oven being evenly filled with microwaves - if that were the case, you wouldn't need a turntable would you? What about ovens without a turntable - if you don't move the item around a bit during the heating, you get cold spots and burned spots, so I assumed that was a result of the item not being "centered" in a beam.
Old 07-20-2013, 11:56 AM
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There aren't really discrete beams as such. Rather, the microwave radiation has a wavelength of a few inches, and when the oven is on the microwave radiation reflects off the walls of the oven and creates complex patterns of standing wave, making spots inside the oven where the waves reinforce or cancel each other. The turntable is there to make the food cook evenly, but the waves are still present throughout the entire oven, and something absorbing energy in one spot will reduce the energy everywhere in the oven.
Old 07-20-2013, 12:03 PM
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Very roughly speaking, any "beam" that isn't immediately absorbed by the food will bounce off the walls, and continue to bounce until it is absorbed.
Old 07-20-2013, 02:11 PM
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Quote:
Originally Posted by AndrewL View Post
There aren't really discrete beams as such. Rather, the microwave radiation has a wavelength of a few inches, and when the oven is on the microwave radiation reflects off the walls of the oven and creates complex patterns of standing wave, making spots inside the oven where the waves reinforce or cancel each other. The turntable is there to make the food cook evenly, but the waves are still present throughout the entire oven, and something absorbing energy in one spot will reduce the energy everywhere in the oven.
Ah! Click - got it. One item absorbs not only the direct but reflected energy, therefore two will absorb the direct energy AND some of the reflected, reducing the total energy in the box and extending heating time.

Thanks much - now I know, and knowing is half the battle.
Old 07-20-2013, 06:28 PM
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I also feel like in a regular oven, there may be many things you could cook multiples of without it taking any longer at all! Certainly if the object is large enough, it could absorb enough of the heat for the cook time to go up, but one hot pocket in a traditional oven vs 2 hot pockets in a traditional oven is probably going to be about the same amount of time, no? Probably even up to a dozen of the suckers without much change in cook time in a traditional oven.

But maybe I'm way off.
Old 07-21-2013, 06:37 AM
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Originally Posted by drewtwo99 View Post
I also feel like in a regular oven, there may be many things you could cook multiples of without it taking any longer at all! Certainly if the object is large enough, it could absorb enough of the heat for the cook time to go up, but one hot pocket in a traditional oven vs 2 hot pockets in a traditional oven is probably going to be about the same amount of time, no? Probably even up to a dozen of the suckers without much change in cook time in a traditional oven.

But maybe I'm way off.
Depends on what you mean by a regular oven, old electric, electric convection or gas. In an old electric oven you heat the top and or bottom of the oven until the thermostat reaches a set temperature. The food is mainly heated by radiation and you usually preheat the oven, so adding more food means more of the heat is absorbed and the thermostat will fall faster below the set temperature causing the oven to consume more energy.
Old 07-21-2013, 08:53 AM
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Yeah, the oven can put out way more energy than a single hot pocket can absorb, and could keep up with a dozen pretty easily as well. It probably won't even take that much more energy, just because using an oven to heat a single hot pocket is pretty inefficient (due to having to bring the whole oven up to temperature).
Old 07-21-2013, 10:04 AM
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Fill the chamber clear full of Hot Pockets
You have dark, dark dreams, buddy.
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