A typical microwave oven has an optoelectronic device, called, magneton which holds a thermionic electron emitter (cathode) and an electron collector (anode), both fitted into an evacuated cylinder under a magnetic field. The ejected and accelerating electrons take a curved path to reach the destination (anode) while transmitting microwave radiation. It is this radiation that is dispersed into the cooking compartment by appropriate electromechanical design.
When foodstuffs to be cooked by microwave oven are kept in this kind of kettle, the majority of the microwave power reaches the water and tends to hasten the molecular rotations.
When we heat a cup of water alone in a microwave oven, the water gets all the microwave energy and becomes hot. But the upper part of the water is hotter than the lower part because of the following two main reasons (in case, the water is not heated to boiling). Though designed to distribute the microwave energy homogeneously (isotropic) into the cooking compartment, the top regions, usually receive a slight excess. The hot water moves to upper regions of the container because its density is less than normal water's. Therefore, as and when water is getting heated, the hot water moves upward due to lower density through convection. On this concept only do solar hot water grids on the terrace function. As the cup of water kept in microwave oven is hardly stirred, the temperature gradient is more obvious.