Why can only food that contains water molecules can be heated using microwaves?
Answer:
This is a myth. The radio energy produced by a microwave oven will cause any molecule that is "polar" (has positive and negative ends) to try to turn back and forth in time with the microwave's alternating electric field. Oil, for example, can be heated by a microwave. Many solid food containers will also become hot in a microwave, not because the food they contain is hot but because the container itself has absorbed energy from the microwave.
Some may claim that the frequency at which a microwave operates (around 2.45 GHz) is near the resonant frequency of water; this is untrue for any physical state of water - solid, liquid or gas. The molecules in solid water (ice) are too tightly bound to resonate at such a high frequency. Liquid water has a fairly broad absorption band; it is somewhat "translucent" to microwaves at 2.45 GHz, so some energy passes through and some is absorbed, but there is no sharply defined frequency where this happens to maximum effect. Gaseous water (vapor) has a resonant frequency much higher than the 2.45 GHz of a typical microwave oven.
A side note: it is the translucence of liquid water that allows a microwave oven to cook food 'all the way through' rather than just heating the surface. If all the energy from the microwave oven were absorbed by the water near the surface of a food item, then the surface would burn while the interior remained cold. What really happens is that a great deal of the microwave energy is transmitted all the way through the food, so all parts of the food item absorb a bit of energy at the same time.
All this having been said, it's not easy to find foods that contain no water. Other than refined vegetable oils, most foods (even foods that seem dry to the touch) still contain some water.
because microwaves only cook water.
A microwave oven uses radio waves, usually in the 2,500 frequency range, to cook food. At that frequency, the waves are absorbed by water, fats, and sugars. When that happens, they are converted directly into atomic motion -- in other words, heat. When you cook something in the microwave, the radio waves penetrate the food and "excite water and fat molecules." The heat is everywhere at the same time, and the food is heated fairly evenly.
Hope this helps =]
Well, this is not entirely true, there are other molecules that can respond to microwaves; although for most intent and purpose, this is essentially right, if only because the microwave oven magnetron is tuned to the frequency that excites water molecule.
To understand how microwave ovens work, imagine you are listening to the radio. You are picking one and only one station, while hundred might be emitting. Why is that? Because your radio set is tuned to the frequency of the station you wish to listen to.
Well, water molecule can also receive microwave emissions of specific frequency; they capture that energy which excites them and makes them heat up. So the magnetron is tuned to produce that frequency that most excites water molecule because all foodstuff do contain water, and it was a natural choice for optimal efficiency.
Microwaves were built to increase the energy of water molecules and cannot increase the energy of other materials. Because it set at a wave length that will only make water molecules excited.
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