The microwave heats the bottom of food first because of the way microwaves penetrate and interact with the food. Here's a simplified explanation of the process:
Microwaves work by emitting electromagnetic waves, specifically microwaves, that excite water molecules within the food. These excited water molecules generate heat, which then cooks the food. However, microwaves have difficulty penetrating deep into dense or thick objects, including food.
When you place food in a microwave, the microwaves are primarily absorbed by the outer layers of the food. As a result, the energy is initially concentrated on the surface, including the bottom of the food that is in direct contact with the microwave-safe plate or container. The plate absorbs some of the microwave energy and transfers it to the food through conduction.
As the food heats up, the heat gradually conducts through the interior of the food, but at a slower rate than the direct absorption and heating of the bottom surface. This slower conduction through the food's interior causes the bottom of the food to heat up faster than the top or other parts.
To achieve more even heating, it is often recommended to use microwave-safe containers or plates that allow for better heat distribution and stirring or rotating the food during the cooking process. This helps distribute the heat more evenly throughout the food and reduce the temperature gradient between the bottom and the top.
It's important to note that the exact heating pattern can vary depending on the shape, size, and composition of the food, as well as the specific microwave's power and cooking settings.