Metals cannot be heated as quickly in a microwave oven compared to other materials due to a phenomenon called "microwave reflection" or "microwave shielding." This occurs because metals have high electrical conductivity.
Microwave ovens work by emitting electromagnetic waves, known as microwaves, which are absorbed by food and other materials. When the microwaves encounter a metal object, such as aluminum foil or a metal container, they induce electric currents on the metal's surface. These currents flow through the metal and generate heat.
However, metals have a unique property: they reflect microwave radiation. The electric currents induced in metals by the microwaves rapidly redistribute the energy, causing the microwaves to bounce off the metal surface rather than being absorbed. As a result, the metal object itself does not heat up significantly.
In contrast, materials that are not highly conductive, such as food or certain ceramics, absorb the microwaves more readily, allowing them to heat up more effectively in a microwave oven.
It's important to note that certain exceptions exist. For example, microwave-safe metal objects, specifically designed for microwave use, may have a different composition or structure that minimizes reflection and allows them to heat up. However, it is generally advised to avoid placing regular metal objects inside a microwave oven, as it can cause sparks, arcing, and potential damage to the oven.