Why do microwaves heat things so quickly?
Microwaves produce a phenomenon called volumetric heating. Simply stated, they heat the entire mass of an object uniformly. This is because the microwave penetrated deeply into the object instead of just acting on the surface. Volumetric heating is unique to microwaves; almost all other forms of heating are conductive, where heat is applied to the surface of an object and it must travel to the interior by conduction. It takes a lot more time for the heat to move from the surface to the inside. This also results in a temperature gradient, where the outside is considerably hotter than the inside. This is why some skill is needed to properly grill a steak.
Temperature gradients are usually undesirable in industrial applications because they can result in a product with non-uniform dryness or overheated areas. How deeply do microwave penetrate? This depends on the microwave frequency and the characteristics of the material being heated. For 915 MHz systems the penetration depth is typically 10”, for 2450 MHz systems it’s about 3”.