Microwave Ovens

微波炉

The microwave radiation of microwave ovens and some radar applications is produced by a device called a magnetron. Modern microwave ovens operate at the frequency 2,450 MHz.

微波炉和某些雷达应用产生的微波辐射是由一种称为磁控管的设备产生的。现代微波炉工作频率为2,450 MHz。

By federal regulation, microwave ovens are limited to 5 milliwatts (mW) of microwave radiation per square centimeter at approximately 2 inches from the oven surface. This limit is far below the level known to harm people. The microwave radiation would be expected to drop off according to the inverse square law, so at 20 inches it would be down by about a factor of 100.

根据联邦规定，微波炉在距离炉表面约2英寸处，每平方厘米的微波辐射不得超过5毫瓦（mW）。这个限值远低于已知会对人体造成伤害的水平。由于微波辐射预计会按照反平方定律衰减，因此在20英寸处，其强度应下降约100倍。

Considering the frequency of 2,450 MHz, the wavelength of microwave oven radiation is about 12 cm and the quantum energy of a microwave photon is about 1 x 10^-5 eV. The radiation interaction at such energies for free molecules can contribute to molecular rotation and vibration, but such resonant interactions are not a major factor in the heating of liquids and solids in a microwave oven. The average thermal energy at 20°C is about 1/40 eV, so any ordered molecular rotational or vibration created by the microwave interaction is quickly randomized by collisions with molecules of kinetic energy 2500 times greater than the microwave photon energy provided. The major mechanism for heating water in a microwave oven is described as dielectric heating. The microwaves exert torques exerted on the molecular dipole moments of the water molecules and other molecules and that would be expected to cause some heating. However, the detailed modeling of water heating described in Chaplin suggests that the major effect of the microwaves is from work done on the hydrogens rather than invested in the reorientation of the molecular dipoles of the water molecules.

考虑到2450 MHz的频率，微波炉辐射的波长约为12厘米，微波光子的量子能约为1 x 10^-5 eV。在这样的能量下，自由分子之间的辐射相互作用可以引起分子的旋转和振动，但这种共振相互作用在微波炉加热液体和固体时并不是主要因素。20°C时的平均热能约为1/40 eV，因此由微波相互作用产生的任何有序的分子旋转或振动都会很快被动能为微波光子能量2500倍的分子碰撞所随机化。微波炉中加热水的主要机制被描述为介电体加热。微波会对水分子和其他分子的分子偶极矩施加扭矩，这种扭矩预期会导致一些加热。然而，查普林在第几章中描述的水加热的详细建模表明，微波的主要效应来自对氢原子所做的功，而非水分子分子偶极矩重新定向所投入的能量。

Usually, the major heating effect of microwaves in an oven is the heating of the water included in the material. If you are heating pure water, the heating can be modeled more precisely. For pure water, the penetration of the microwaves into the water can be modeled and is charaterized by the depth of penetration which drops the microwave power by a factor 1/e, representing the absorption of 63% of the microwave power. For water at 25°C, this penetration depth is approximately 1.4 cm according to Chaplin.

Dangerous radiation from looking into your microwave oven? Even though you can see into the microwave oven when your food is cooking, the microwaves are effectively blocked from getting out into the room because the holes in the metal screen on the microwave oven door are about 1 mm in diameter compared to a 120 mm wavelength for the microwaves. The wavelength of the microwaves is about 120 times the size of the holes, and can't "see" the holes to get out. This is an application of the fact that you cannot image anything that is smaller than the wavelength of the radiation you use to image it. You can see through the holes because at 500 nm wavelength, the visible light wavelengths are about 2000 times smaller than the holes.

通常，微波炉中微波的主要加热效应是材料中所含的水的加热。如果你加热的是纯水，加热过程可以更精确地建模。对于纯水，微波进入水中的穿透情况可以建模，并由穿透深度来表征，该深度使微波功率降低到原来的1/e倍，表示吸收了微波功率的63%。对于25°C的水，根据Chaplin的数据，这种穿透深度约为1.4厘米。看向微波炉会有危险的辐射吗？尽管食物在加热时你可以看到微波炉内部，但微波几乎无法从炉子中逃逸到房间中，因为微波炉门上的金属网孔直径约为1毫米，而微波的波长约为120毫米。微波的波长大约是孔径的120倍，因此无法“看到”这些孔径从而逃逸出去。这是应用了这样一个事实：你无法成像任何比你所使用的辐射波长更小的物体。你可以通过孔径看到，因为在500纳米波长下，可见光的波长大约是孔径的2000倍。

Reference:

参考文献：

Water and Microwaves from "Water Structure and Science" by Martin Chaplin.

Transport of energy by electromagnetic waves

电磁波传输能量

水与微波来自 Martin Chaplin 的《水的结构与科学》

Index

Wave concepts

Electromagnetic wave concepts

Electromagnetic spectrum

索引波的概念电磁波概念电磁谱

HyperPhysics*****Electricity and Magnetism

HyperPhysics ***** 电学和磁学

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