I know that microwaves only heat polar molecules but what about aluminum foil and graphitic carbon, which are both heated by microwaves even though they have no dipole moments? — EB
Aluminum foil and graphitic carbon are both conductors of electricity. When they’re exposed to microwaves, the electric fields in those microwaves causes currents to flow through them. If the aluminum were thick enough, it would be able to handle the currents without trouble. But aluminum is very thin and the current that flows through it may be more than it can tolerate, particularly if it’s only a narrow strip. It then becomes very hot. The effect is the same as would happen if you plugged the aluminum foil into an electric outlet and sent current through it that way. The same heating occurs in the carbon—the current that flows in it heats it up. In short, relatively poor conductors of electricity become hot in a microwave because they permit currents to flow in response to the microwave electric fields but then can’t tolerate those currents without becoming hot.