The reason a microwave cooks faster at first and then slower towards the end of its cycle is primarily due to the way microwaves distribute energy and the behavior of the food being heated.
When you start a microwave, it operates at its maximum power output. The microwave's magnetron generates microwaves, which are electromagnetic waves that pass through the food and cause the water molecules within the food to vibrate rapidly. This vibration generates heat, which cooks the food from the inside out.
At the beginning of the cooking cycle, the food is generally at room temperature, so there is a significant temperature difference between the food and the desired cooking temperature. As a result, the microwave's high power output can quickly transfer energy to the food, raising its temperature rapidly.
However, as the food starts to heat up, the temperature difference between the food and the desired cooking temperature becomes smaller. Additionally, the water content within the food begins to evaporate, leading to a reduction in the number of water molecules available for the microwaves to interact with. This means that the food becomes less responsive to the microwaves, resulting in a slower rate of temperature increase.
As the food approaches its desired cooking temperature, the microwave's sensor (if equipped with one) may detect this and start to reduce the power output to avoid overcooking. In microwave ovens without a sensor, the power output remains constant until the cooking time is complete.
In summary, the initial rapid cooking in a microwave is due to the significant temperature difference and higher water content, while the slowing down towards the end of the cycle is a result of the decreasing temperature difference and reduced availability of water molecules for microwave absorption.