The operation of a microwave oven

 

microwave oven

Fig. 1   Mircowave oven

Traditionally, we cook by fire. Heat is transmitted via a pot to the food, which is heated up slowly from the surface to the inside. This method has two disadvantages: it takes a long time; the inside and the surface of a bulky piece of food cannot be done at the same time, thus it happens that the inside is still raw while the surface is already burned. The heating method of the microwave oven can overcome these two disadvantages.

Microwave is a kind of electromagnetic waves, its wavelength ranges from 1 mm to 1 m. For domestic microwave ovens, the international standard wavelength is 122 mm, corresponding to a frequency of 2450 MHz. This wavelength is chosen to avoid interference with telecommunication. Why does microwave generated by the oven heat up food much faster? Microwave can penetrate insulators readily, but when it reaches food that contains water, it sets water molecules into vibration at the same wave frequency. In the oscillations, friction between the molecules generates heat. The power of the ovens is rather high; generally speaking, it ranges from 600 W to 2000 W. In the oven, water molecules oscillate 2.45 billion times per second, these oscillations happen almost simultaneously in all parts of the food, and that is why the oven can heat up a large piece of food so quickly.

However, microwave has another property: it can be reflected by metals. So we cannot use a metal container in a microwave oven, otherwise the food cannot be heated up and the oven will even be damaged in the long run. It is because if the microwave is totally reflected by the container without any dissipation, it will heat up the microwave generator to a high temperature and finally damage it. We should only use containers made of insulating materials, such as pottery, heat-resistant glass or plastics, in a microwave oven.