Microwaves interact with water molecules through a phenomenon known as "dielectric heating." Although photons themselves do not have a charge, they carry electromagnetic energy and can induce movement of charged particles, such as water molecules. Here's how the process works:
Polar molecules: Water molecules are considered polar molecules because they have a slight separation of electric charge due to the uneven distribution of electrons around the oxygen and hydrogen atoms. The oxygen atom attracts electrons more strongly than the hydrogen atoms, creating a partial negative charge (δ-) near the oxygen atom and a partial positive charge (δ+) near the hydrogen atoms.
Electric field: When microwaves pass through the food, they create an oscillating electromagnetic field. This field has an electric component that rapidly changes direction with the oscillation of the wave.
Interaction with water: As the microwaves pass through the food, the oscillating electric field interacts with the polar water molecules. The electric field causes the water molecules to align themselves with the field, attempting to follow its direction.
Molecular agitation: With the rapid oscillation of the electromagnetic field, the water molecules continuously attempt to align themselves with the changing field. This constant reorientation causes the water molecules to vibrate and rotate rapidly, leading to molecular agitation and generating heat.
Heating effect: The friction produced by the agitating water molecules transfers energy as heat to the surrounding food. This heat is what cooks or warms the food inside the microwave.
It's important to note that dielectric heating works best with polar molecules like water. Other substances with non-polar molecules may not be significantly affected by microwaves and might not heat up as effectively. This is why microwave-safe containers are often made of materials that are transparent to microwaves and do not significantly absorb the energy, like glass or certain plastics.