Why does a microwave oven heat food unevenly?

Question

some is burning hot, some is still ice cold!

Answer 1

Best Answer - Chosen by Asker

A microwave oven heats food by bombarding it with Microwave energy (a portion of the electromagnetic spectrum, much like visible light or radio waves). When water molecules are hit by a photons in the microwave frequency, they become excited, and move around faster - this is what causes them to heat up.

Since the outer surfaces of the food are the ones exposed to the most microwaves, they tend to heat up faster than the insides.

Basically, for a microwave to excite a molecule on the inside, it has to 'miss' all the molecules on the outside to get to it. The thicker your food is, the less likely that is to happen.

What ends up happening is that the outside of the food gets heated by direct radiation, while the inside has to be heated by conduction - the transfer of heat from the outer surfaces to the inner ones directly. This process is much slower, and so you end up with food that is hot on the outside, and cold on the inside.


That was longer than I anticipated, but hopefully it helps!

Asker's Rating:

Asker's Comment:

simple, yet clear. thanks!

Answer 2

It has something to do with how the rays bounce of the sides of the microwave

Answer 3

maybe your tray doesn't spin

Answer 4

Cuz ur microwave sucks, mine never does that.

Answer 5

The microwaves excite water molecules. Thus, food items with comparatively less H2O will heat slower than those with more.

Answer 6

Because you don't follow the directions.

Answer 7

The microwave is based on the premise that it uses x-rays to heat the water in a substance. Now, if that substance has low moisture then it wont cook quickly but if it is high in moisture, then it will cook readily. But, as it cooks from inside out, it has to cook outside in, because of density. So if you want it to cook faster, cover it, and add a little water to it.

Answer 8

it is the way the thing works, if the turntable does not turn you will get "hotspots". This is quite useful actually, (KITCHEN SCIENCE COMING UP). You can work out the speed of light using your microwave, a 15cm ruler and a bar of chocolate.

1 IMPORTANT. Unwrap the Chocolate.
2 Look at the back of your Microwave, find the label that tells you what frequency it works at.
3 Remove the Turntable
4 Place the chocolate as close to the centre as you can
5 Turn it on for 10 second "blasts", until you see the "hotspots" where the chocolate melts.
6 Measure the distance between them
7 Multiply this distance by 2 and then multiply that figure,(the wavelength) by the frequency and you should come close to 299792458 metres per second , or the speed of light.

Source(s):

Kitchen Science

Answer 9

The food is heated by absorbing the radiation. The center of the food will get less radiation because the food outside of it has absorbed some. The usual dodge is to zap the food for while, stir it, and then zap it some more. Since ice does not absorb microwaves nearly as well as liquid water, it is clever to add just a bit of water to something that you are thawing.

Answer 10

A microwave oven heats by using a radio frequency field in the microwave band to induce an electrical current in the food. Most food is a homogenious mixture so has varying areas of electical resistivity and magnetic permiability. Not to mention the wide range of geomectric shapes. Basically the food is a big resistor but its resistance is not consistant throughout its mass, therefor the areas of least resistance get hotter than the areas of greater resistance.

Also the mositure content of the food is important. Dry foods are much harder to heat becauses of increased resistance. The more water in the food, the easier to heat, and the more uniform the heating

Answer 11

First because microwaves heat water molecules, so dry things won't get hot as quickly as moist things. Second, because microwaves have to penetrate from the outside in (like any heat), and so they don't get to the center. As said above, the center has to be heated by conduction of heat from the outside of the mass, which is as slow as a regular oven. For best results, slice your food and spread it out, so microwaves can shine on all surfaces, and make sure it's moist.

Answer 12

First of all, someone said that microwaves are X-rays. This is absolutely untrue. The typical u-wave oven operates at a frequency of around 2400 MHz. That has a wavelength of 0.125 m. An X-ray has a wavelength of around 50e-12 m; that's 50 PICO-meters. As for why a u-wave heats unevenly, it's because the u-waves enter the cooking area in a relatively narrow beam, which then bounces around. Some areas end up with a greater energy concentration than others.

Answer 13

Most of the people that have answered are correct about the microwave energy exciting the water to heat the food. How well the food is heated is largely dependant on the water content of the food and how evenly distribued the water is.

What people are forgetting is that the energy emitted is a wave. If you remember anything about waves from physics, then you'll remember that waves will interfer. The waves bouncing off the walls and the new waves being emitted will either constructively interfer (to get a hot spot) or destructively interfer (cold spots). So in summary because the wave nature of the energy emitted you can get hotter and colder spots in the mircowave oven.

If I recall correctly, the box is sized so the waves form a hot spot in the middle, but towards the edges that are some cold spots.

Answer 14

This happens in many old microwaves, as well as problems with the food itself. Be sure to stir the food well. Also, make sure you uncover it by any plastic or paper towels in the colder areas, and cover it in the wamer areas.
Do you have a turn table in the microwave? Those are very helpful, as the food is revolved and the areas being heated are switched very often. If you microwave did not come with one, microwave tables may be bought at kitchen and general stores (such as Wal-mart). If you already have one, make sure it is working properly.
Is your microwave old? I have a very old microwave, and it has the same problem. The device in the machine that is used to provide the heat may be old, or malfunctioning. Check it, if possible.
In closing, you problem is somewhat natural for microwaves. The best quick-solve thing to do is to remove the parts that are burning hot, and continue to cook the food, or cover the warm areas with thick plastic and continue to cook.
Good luck!

Answer 15

Your microwave sux get a new one!

Answer 16

you use microwave rays to heat it up. not real heat. thats y it cools down 1000 times faster then a stove. the microwave rays bounce off of some surfaces just like radar bounces off of certain surfaces.

Source(s):

my brain

Answer 17

maybe you should try rotating the food when the time is half way through and it might heat the food all the way. but if you have one that rotates by its self then i dont know what it would be

Answer 18

It depends on the type of microwave you have. If it emits microwaves towards the center of foods then it'll only be warm in the middle but in most cases the edges get cooked too much while the center is left raw.

In some cases the microwave transmiter will be on the top of the microwave oven (remember, microwaves are literally waves of energy that your food obsorbs to heat up) and so the warm particles and waves will only be directed at the center of your food.

Then in most and all other cases the beams come from the side (hence the spinning motion of the bottom plate in your microwave if you have this type). The waves are directed at the sides of your food instead of the center so in this case your food is warmed from the side.

There is also a way to correct the side-transmitting problem though. If you have a stove (I'm assuming you do) just throw your food on the stove if it can be put on the stove. This works for foods like bacon, burgers, hot dogs, eggs, and other foods similar to these because stoves warm food from the center out. This could possibly balance out the uneven distribution of warmth from your microwave.

Another thing you can do to evenly distribute heat through your microwave-ovened liquids (ex. soup) you can take it out and stir it. Actually most soup labels will direct you to stir the soup for a while and then put it back in the microwave. This distributes the heat throughout your soup so that the top doesn't get scalding hot while the bottom is left at room temperature.

Answer 19

the microwave didn't heat them unevenly, some foods just cook at different rates.

Answer 20

Do you have the kind of micro with a spinning carousel? If not, that could be the reason.

Answer 21

it focuses and agitates molecules inside an object. rotary plates inside the oven help.

Answer 22

Your tray. If your using a glass tray then the middle will not cook as fast as the edges because the glass will obsorb heat from the sides and that's where you eventually burn the sides and the middle will be uncooked. What i subjest is that you use rubber. Since rubber doesn't get burn or get as hot as glass in the oven, it will be able to spread through out your pastry/food/ what ever your cooking and the middle and sides/edges will be perfectly cooked. And unlike glass, heat goes through rubber, not against it. The heat will run through the rubber and cook your food evenly from top to bottom and side to side.
*Note* A spinning plate in your oven will not do much. Will it get the middle cooked a but more? Yes, but only a bit more. Because as it rotates, the heat isn't concentrated one one spot and will spread alot faster. You will have to wait extra longer if you rotate your main food you are cooking in the oven.

Hope I Helped!

Source(s):

Science Fair Project Back In High School 5 Years ago.

Answer 23

Because the thing in the microwave oven that makes the micro-waves is on one side, so the food that is closer to that thing is hotter than the food on the other side of the oven. Notice that the hot side of the food was always on the same side in the oven. That's why most newer microwaves have a turntable to automatically rotate your food while it's cooking. If your microwave doesn't have a turntable, you can just pause the cooking every couple minutes and rotate your plate 180 degrees, then let it continue cooking.

Answer 24

well some times the microwave could loose heat on one side and some theft on the other

Answer 25

Doesn't matter why it does. The problem is how to get around it. Most directions call for mixing the food in the middle of the cooking, For foods that can't be mixed (like steaks, fillets,etc.) try cooking part way, flip and rotate nuke some more, and so on till it is done.

A microwave with a turntable is very helpful. If yous does not have one, you can buy then at Discount or Dept stores.

There are also many cookbooks available for cooking in a microwave.

Good luck.

Answer 26

The beam of microwaves come out in a straight beam like a flashlight. They are supposed to hit a little slow moving fan in the top and reflect onto the food. Also some microwave ovens have this platter to rotate to put the food under the beam. It would help if the walls of the oven weren't parallel, but you wouldn't buy it because it don't look right. Microwave ovens are a compromise, but not too bad so no one bothers to improve them.

Answer 27

A microwave will heat food evenly if used properly. Also it depends on the food and content being cooked along with other varibles as well.

Answer 28

It is because microwaves emerge from the magnetron as a beam which spreads out so is not the same intensity throughout the cavity. Also more of the radiation is absorbed near the surface of the food than inside. A turntable helps the food heat more evenly. It also helps to let the food stand a few minutes after microwaving, to let the heat disperse evenly.

The cavity also contains standing waves which interfere constructively in some spots and destructively in others. Microwaves can heat by inducing currents in food which is conductive, but mostly it heats by agitating water molecules, which are dipolar.

Answer 29

You have some amazing answers here. The ones talking about the moisture are the funniest. The frequency that microwave ovens operate at are roughly the same as those used in radar. Radar doesn't penetrate water well, folks. That's the reason you get a radar return from rain. The RF energy bounces off. Neither does your answer have anything to do with the food being an electrical resistor. The moisture does enter into the equation in that the more moisture, the less RF energy that can reach the interior of the food being heated. The klystron tube emits the RF from the top of the unit and directs it down. The only direct RF the food gets is from above. The remainder is bounce. Those who said the RF excites the molecules of the food, making them rub against each other creating friction, therefore heat, are correct. In my days with aerospace, I used to check airborne radar transmitters by placing my hand over the RF feed horn. If my hand got warm after a few seconds, I knew the unit was transmitting. That was a long time ago and since I'm still alive, guess it didn't hurt me, contrary to the opinion of many. I'm an MSEE (electronics) with a lot of experience over the years. Unless they've changed the laws of physics while I wasn't looking, I believe you'll find this information accurate.

Answer 30

The foods have different specific heats. Also, unless the microwave is a convection-style oven, the heat does not circulate evenly.

Answer 31

Your heating with radiowaves,so something directly in line with the transmitter will get hot. I have also heard that the heat is so high that it cooks all the nutrition out. Better, put the frozen meal in a pan of hotwater on your stove,cover it and eat after the water boils for a few minutes. I do because I threw my microwave out.