Microwaves with a frequency too high to be heard by humans can be used to heat food because they interact with water molecules and certain other molecules in the food through a process known as dielectric heating.
Dielectric heating is a method of heating that occurs when electrically polar molecules, such as water, are exposed to an alternating electric field, as is the case with microwave radiation. The microwaves used in microwave ovens have a frequency in the range of megahertz to gigahertz, which is much higher than the frequency range that can be heard by humans (20 Hz to 20,000 Hz).
Here's how dielectric heating works in microwaves:
Microwave Absorption: When microwaves pass through the food, they encounter electrically polar molecules, such as water molecules. These polar molecules have a positive and negative charge distribution, which makes them respond to the rapidly changing electric field of the microwaves.
Rotation and Friction: The rapidly oscillating electric field of the microwaves causes the polar molecules (like water) to rotate continuously as they try to align themselves with the changing field. This rotation generates friction between neighboring molecules, leading to kinetic energy and an increase in temperature.
Heat Generation: As the water molecules rotate and collide with other molecules, the energy is converted into heat throughout the food. This heat is what cooks or heats the food placed in the microwave oven.
It's worth noting that not all materials are equally affected by microwaves. For example, metals reflect microwaves and do not get heated, which is why microwave-safe containers are typically made of materials that do not interact strongly with microwaves, such as glass or microwave-safe plastics.
In summary, microwaves with frequencies too high to be heard by humans are used to heat food through dielectric heating, where the electrically polar water molecules in the food respond to the alternating electric field of the microwaves, generating heat through friction and increasing the food's temperature.