Question

How are microwaves produced? Why is it necessary in the microwave oven to select the microwaves of frequency matching with the resonant frequency of water molecules?

Answer 1

Hint: Microwaves are electromagnetic waves that lie in a part of the electromagnetic spectrum which is sandwiched between radio waves and infrared waves. Its wavelength lies between 30 to 0.03 cm. These are used in microwave ovens to efficiently heat food. It heats the food evenly through the matter and it is a quick way to cook food.

Complete step by step answer:
As we know, that accelerated charge produces electromagnetic waves. In microwave ovens, microwaves are produced by vacuum tubes devices that operate on the ballistic motion of electrons. It means that electrons are emitted in the chamber using allow of tungsten ( since it has a high melting point ) and thorium (since it is a good source of electrons) and due to the large potential difference applied, electrons starts their motion which is controlled by applying magnetic and electric fields. Due to the motion of these electrons, the microwaves are produced. In addition, the chamber that heats food is designed in such fashion that the waves form standing waves inside the chamber.
Interaction with water molecules: Resonant frequency is the frequency which results in maximum amplitude of oscillation of water molecules. In order to design the microwave ovens, we have to make sure that the efficiency of the device is high. Hence we match the frequency of microwaves to the resonating frequency of water molecules to ensure maximum amplitude to produce maximum movement of the molecules to heat the food faster.

Note:
It must be noted that microwaves are not heat waves. They don’t carry heat along with them. They just increase the entropy of polar molecules present in food. Polar molecules mean molecules that have some dipole moment eg. Water. If we place plastic or ceramic utensils inside the microwave oven, it will remain at room temperature because of no polar molecules. This is also an advantage of using a microwave oven that utensils remain at normal temperature and food becomes hot.