Microwave ovens heat water quickly due to a phenomenon known as dielectric heating. This process is based on the interaction between the microwaves and the water molecules.
Microwaves are a form of electromagnetic radiation with a specific frequency range. When you place a container of water in a microwave oven and turn it on, the microwaves produced by the oven pass through the container and enter the water.
Water molecules have a positive side (hydrogen) and a negative side (oxygen), which makes them polar molecules. The alternating electric field of the microwaves causes these polar water molecules to rapidly rotate and align themselves with the field. This rotation leads to molecular friction and generates heat.
It's important to note that the microwaves themselves do not directly heat the water by bouncing around and creating a multiplicative effect. Instead, the microwaves continuously provide energy to the water molecules, causing them to rotate and generate heat. The process is efficient because microwaves are specifically designed to match the resonant frequency of water molecules, maximizing energy absorption.
This selective absorption of energy by water molecules is why microwaves are primarily used for heating water-containing substances. Other materials that do not have polar molecules, such as many plastics or ceramics, do not readily absorb microwave energy and are thus unaffected.
To summarize, the rapid heating of water in a microwave oven is due to dielectric heating, where the microwaves cause the water molecules to rotate and generate heat. This process is not a result of a multiplicative effect from microwave energy bouncing around, but rather a direct interaction between the microwaves and the water molecules.