# Materials and Eletromagnetic Waves

by tbfranks
Tags: eletromagnetic, materials, waves
 P: 2 Hello everybody! I have an question, that seems silly, but i really need know that. When a material is subject to waves, he starts to oscillate in the frequency of the wave and this material becomes a source of secondary waves, this justifies including the reflection proccess of the waves, ok? finaly my question: when we are warming the food in the microwave we are putting the food to vibrate at a frequency of microwave. This food would be a source of secondary waves? Are we eating eletromagnetic waves? How i can prove if is a lie? thanks, []'s
 PF Patron Sci Advisor P: 1,717 The currents that excite the secondary waves only exist due to the incident electromagnetic waves. As soon as you cease inpinging the material with an incident field, the material will cease emitting the corresponding secondary waves.
 P: 2 ok, but when the waves are focusing on the material, it is nothing to vibrate? oscillation must permenacer awhile before the damping is not it? Where does this energy? thanks
PF Patron
P: 1,717

## Materials and Eletromagnetic Waves

I do not understand your statement here.

When microwaves inpinge upon a food item, the incident electric fields cause molecules in the food to polarize into weak dipole moments. These dipole moments align with the incident electric field. Since the field is oscillating, the dipole moments oscillate too, causing the molecule to rotate and thus vibrate. The movement of these dipole moments gives rise to what is called a displacement current (as opposed to conduction currents which are conduction electrons moving in response to the fields). This displacement current gives rise to the secondary fields.

It is true that there is a relaxation time for the molecules when we turn off the field. A more technical explanation can be found by looking into Debye relaxation. Debye relaxation, amongst many other models like Cole-Cole, is a way of estimating the time-dependent permittivity of a medium in response to a time varying signal. So if we turn on a source and polarize the material and then turn it off, the polarization of the material will persist and slowly go away. Of course there are many complicated methods that this can happen and the Debye relaxation is probably one of the simplest.

However, needless to say, these relaxation times are very fast. I do not recall offhand what the relaxation time of water is even though I just saw a presentation on this the other week. But it is rather quick and the momentary displacement currents that arise as the polarized molecules resettle would die out very quickly after you turn off the microwave source.

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