Problem
A microwave oven heats water because the electromagnetic wave makes water molecules vibrate and rotate, entering resonance with the oscillating field (frequency ). So why don’t dry porcelain plates heat up? What about metals? What frequency would an oven need to heat oil well too?
Solution
Why water heats up. The water molecule has a strong permanent electric dipole moment. The oscillating electric field of the wave forces it to reorient back and forth billions of times per second; collisions between molecules turn this agitation into heat. This is absorption due to dipolar relaxation, not a sharp single-molecule resonance.
Dry porcelain. Porcelain is a ceramic material whose atoms are locked in a rigid lattice: it contains no dipoles free to rotate. It therefore absorbs very little energy from the microwaves and stays (almost) cold. If the plate warms up a little, it is only from heat transferred by the food. A wet plate, on the other hand, heats up because it contains water.
Metals. In a metal the conduction electrons are free: the wave’s field sets them in motion, generating surface currents that reflect almost all the radiation (the metal behaves like a mirror for microwaves). At points and edges the field becomes concentrated and can cause electric arcs and sparks: this is why metal objects should not be placed in the oven.
What about oil? Oil molecules have a weak dipole moment and, above all, a relaxation time different from that of water: at the standard frequency of they absorb little. To heat it effectively a different frequency would be needed, generally higher, tuned to its dipolar relaxation time.
Connections
Topics: Electromagnetic waves Concepts: Electromagnetic wave