The discussion centers on the design of microwave oven grids, specifically the size of the holes in the metal mesh. Microwaves have a wavelength of 12 cm, and holes smaller than half this wavelength effectively prevent the propagation of electromagnetic (EM) waves. The small holes reduce the extent of local fields associated with voltage and currents, minimizing potential radiation hazards. This design ensures that the microwave energy is contained while allowing visibility through the grid.
PREREQUISITESEngineers, physicists, and anyone interested in the design and safety of microwave technology, particularly those involved in appliance engineering and electromagnetic safety assessments.
The idea is to provide a mesh that is effectively a solid wall to microwaves but which can be easily seen through for convenience.this_tim said:Since the wavelength microwaves used in microwave ovens have a wavelength of 12cm, why are the holes in the grid behind the glass so small? If they were 12cm would the microwaves still stay confined? If the small holes are only to "play it safe" that seems irrelevant since microwave has a "fixed" wavelength. Very confused about all of this and can't seem to find a very clear explanation anywhere.
EM waves will not propagate through a hole that is smaller in diameter than half a wavelength. But there are still local fields that will exist on the outside. These are associated with the voltage across the hole and currents in the metal. By making the holes small, these fields are reduced in extent.this_tim said:Since the wavelength microwaves used in microwave ovens have a wavelength of 12cm, why are the holes in the grid behind the glass so small? If they were 12cm would the microwaves still stay confined? If the small holes are only to "play it safe" that seems irrelevant since microwave has a "fixed" wavelength. Very confused about all of this and can't seem to find a very clear explanation anywhere.
tech99, could you explain more about those "local fields" such as where they come from and why they would be a problem. I haven't much knowledge of physics, but am a programmer of 20+ years and though the two aren't exactly related, I "may" be able to follow along if you explain a bit more so I can better understand the size of the holes. It really does have me curious - Thanks!tech99 said:EM waves will not propagate through a hole that is smaller in diameter than half a wavelength. But there are still local fields that will exist on the outside. These are associated with the voltage across the hole and currents in the metal. By making the holes small, these fields are reduced in extent.
A hole in a metal sheet is a slot antenna. All antennas radiate by accelerating electrons, but to do this they need to have an accelerating voltage across them. This does not radiate energy itself but it does create an electric field close to the antenna in which electrical energy is stored. There is a similar magnetic field, which arises because, after being accelerated, the electrons have velocity. These fields are sometimes called the Reactive Near Field, and they often store much more energy than the antenna radiates each cycle.this_tim said:tech99, could you explain more about those "local fields" such as where they come from and why they would be a problem. I haven't much knowledge of physics, but am a programmer of 20+ years and though the two aren't exactly related, I "may" be able to follow along if you explain a bit more so I can better understand the size of the holes. It really does have me curious - Thanks!
. . . . and, for a (perfectly) conducting screen, the Energy is reflected (conservation has to apply). This is the same as for a continuous sheet.tech99 said:By making the holes small, both the radiated and Reactive Near Fields are kept very small.
Am I correct in guessing that a "perfectly" conducting screen is very difficult, if not impossible to achieve? And please say more about your "conservation has to apply" statement.sophiecentaur said:. . . . and, for a (perfectly) conducting screen, the Energy is reflected (conservation has to apply). This is the same as for a continuous sheet.
Pretty low resistance is ‘good enough’ and Conservation just means the Energy has to go somewhere. It has to be reflected if it’s not absorbed.this_tim said:Am I correct in guessing that a "perfectly" conducting screen is very difficult, if not impossible to achieve? And please say more about your "conservation has to apply" statement.
Makes sense. Thanks!sophiecentaur said:Pretty low resistance is ‘good enough’ and Conservation just means the Energy has to go somewhere. It has to be reflected if it’s not absorbed.