When you put more individual things in a microwave and need to microwave them for longer, it's primarily due to the principles of heat transfer and the way microwaves interact with food.
Microwaves work by emitting electromagnetic waves that are absorbed by water, fats, and certain other molecules present in the food. When these waves are absorbed, they agitate the water molecules, causing them to vibrate and generate heat. This heat, in turn, cooks the food.
When you place multiple individual items in the microwave, each item absorbs microwaves independently. The microwaves may not penetrate evenly throughout the entire stack of items, and the heat generated may not distribute uniformly.
As a result, the food items closest to the microwave's source might receive more energy and heat up faster than those further away. This uneven heating can lead to differences in cooking times for different items in the microwave.
Additionally, as the number of items in the microwave increases, more total mass is present, and it takes more time for the microwaves to transfer enough heat to raise the temperature of the entire mass of food.
To ensure even cooking, it's essential to arrange the food items in a way that allows the microwaves to penetrate more evenly. Rotating or rearranging the items during the cooking process can also help promote more uniform heating. Following the recommended cooking times in the microwave's manual or experimenting with different cooking durations can also help you determine the optimal cooking time for the specific combination of food items you're heating.