Reading Assignment
“Microwave ovens: Dipole moments at work”
Figura, Kotz J, Química y reactividad química, pagina 367
Microwave radiation is a form of electromagnetic radiation, of a shorter wavelength than visible light. Microwaves are generated using a magnetron, which was developed as a variant of the klystron tube, during WWII, for radar systems.
Figura, Cutaway microwave magnetron, Wikipedia.org
The magnetron is a hollow cylinder encased in a horseshoe-shaped magnet. In the center of the cylinder is a cathode rod.
The walls of the cylinder act as an anode. When heated, the cathode emits electrons that travel towards the anode. The
magnetic field forces the electrons to travel in a circular path. This motion of charged particles generates microwaves,
which are adjusted to a frequency of 2.45 GHz for cooking. A waveguide directs the microwaves into the cooking compartment. Rotating fan blades reflect the microwaves to all parts of the oven.
The cooking action in the microwave results from the interaction between the electric field component of the radiation with
the polar molecules –mostly water—in food. All molecules rotate at room temperature. If the frequency of the radiation and
that of the molecular rotation are equal, energy can be transferred from the microwaves to the polar molecule. As a result,
the molecules will rotate faster. That is what happens in a gas. In the condensed state, a molecule cannot execute the free
rotation. Nevertheless, it will experiment a torque that tends to align its dipole moment with the oscillating field of the microwave. Consequently, there is friction between the molecules, which appears as heat in the food.
The reason that a microwave oven can cook so fast is that the radiation is not absorbed by nonpolar molecules and can
therefore reach different parts of food at the same time. (Depending on the amount of water present, microwaves can
penetrate food to a depth of several inches). In a conventional oven, heat can affect the center of food only by conduction
(that is, by transfer of heat from hot air molecules to cooler molecules in food in a layer-by-layer fashion), which is a very
slow process.
The following points are relevant to the operation of a microwave oven. Plastics and Pyrex glasswares do not contain polar molecules, and are therefore not affected by microwave radiation. (Styrofoam and certain plastics cannot be used because they melt from the heat of the food). Metals, however, reflect microwaves, thereby shielding the food and possibly
returning enough energy to the microwave emitter to overload it. Because microwaves can induce a current in the metal,
this action can lead to sparks jumping between the container and the bottom or the walls of the oven. Finally, although
water molecules in ice are locked in position and therefore cannot rotate, we routinely thaw food in a microwave oven. The
reason is that at room temperature, a thin film of liquid water forms on the surface of frozen food and the mobile molecules
in that film can absorb the radiation to start the thawing process.
Chang R,. Chemistry, page 383