how does microwave-oven warms up the molecules of water?

Answer 1

A microwave oven works by passing microwave radiation, usually at a frequency of 2.45 GHz (a wavelength of 12.24 cm), through the food. Water, fat, and other substances in the food absorb energy from the microwaves in a process called dielectric heating. Many molecules (such as those of water) are electric dipoles, meaning that they have a positive charge at one end and a negative charge at the other, and therefore rotate as they try to align themselves with the alternating electric field induced by the microwaves. This molecular movement creates heat as the rotating molecules hit other molecules and put them into motion. Microwave heating is most efficient on liquid water, and much less so on fats and sugars (which have less molecular dipole moment), and frozen water (where the molecules are not free to rotate). Microwave heating is sometimes explained as a rotational resonance of water molecules, but this is incorrect: such resonance only occurs in water vapour at much higher frequencies, at about 20 gigahertz. Moreover, large industrial/commercial microwave ovens operating at 915 MHz also heat water and food perfectly well.

Answer 2

The water molecules begin to resonate exited by the microwaves

Answer 3

Radiation causes molecules to become excited.That kinetic energy then causes heat.