ShawnD said:
I was always told that microwave ovens send out magnetic waves as well as microwaves. Things capable of soaking up a magnetic field (iron) will do so, and it induces a current. If the metal object in the microwave is uniform and well-connected, like a spoon, it will just heat up. If the object has lots of spaces and is poorly connected to dissipate charge, differences in charge will build up and the item will spark (steel wool).
Put a spoon in the microwave and see what happens. Put steel wool in the microwave and see what happens. Better yet, put steel wool in a mixture of sugar and drano; that's the most fun thing ever! I ruined a microwave diong that because the fire got out of control and it coated the microwave with carbon. It couldn't even heat up a glass of water after that.
Microwaves consist of a stream of photons, just like light or X-rays, but with a very much lower frequency of oscillation. All these are collectively described as electromagnetic radiation, and also include radio waves which are at even lower frequencies. This is because there is both an oscillating magnetic field and an oscillating electric field present. The propagating EM wave can be described mathematically by the Poynting Vector (look up Poynting Vector in Wikipedia, also vector cross product, Maxwells Equations, vector algebra)
If you have only year 10 Math and no knowledge of calculus, vectors, complex numbers or matricies, then the mathematical symbols and concepts used are going to present you with some problems.
I can aim you in the right direction if you post again with any questions.
Poynting Vector = the vector crossproduct of the electric field and the magnetic field. or in symbols :- S = E X B
The X symbol isn't a multiply sign, but the crossproduct operator symbol.
To put it is to plain English the Poynting vector points in the direction the EM radiation is going, and the electric and magnetic fields oscillate in phase with each other at right angles to this as well as at right angles to each other. The displacement current, and the magnetic field it produces, act like a mirror to the microwaves and reflect them - reflection of light or radio waves works the same way. It is impossible to have a propagating electric field without the accompanying magnetic field, and viceversa. The two are opposite sides of the same coin, like love and hate.
When an EM wave passes within a few wavelengths of a conducting surface it induces a Displacement Current in the metal close to the surface. The current density decreases with depth exponentially - this is known as the skin effect. The phase relationship of the current with respect to the EM wave also varies with depth and the current will be going in different directions at different depths.
The skin depth is the depth at which the current density is 1/e of that at the surface. Skin depth for copper is as follows
60Hz - 8.57mm
10kHz - 0.66 mm
1MHz - 66 micron
1GHz - 2.1 micron
Depth is proportional to the reciprocal of frequency squared
Skin depth for non-magnetic metals is only dependant on conductivity, shallower for better conductors. If the metal is ferromagnetic, then the skin depth is reduced significantly on account of the magnetic properties. Iron although it is a worse conductor than copper by a factor of 5 has a skin depth about 1/6 that copper because of this. Cobalt and Nickel will be similar.
Look up Skin Depth in Wikipedia - there is a full mathematical explanation and some graphs.
Very little displacement current flows deeper than a few skindepths. Waveguides that act as "pipes" for microwaves are often silver plated in order to reduce the resistance that the displacement currents see. The plating only needs to be a few microns thick to reduce the transmission losses by a worhwhile amount. Microwave heating of metal surfaces - eg the metalwork in a microwave oven - is minimal if the metal is thicker than a few skindepths, but if the metal is very thin - eg in the CDs mentioned in my post above - then the same total displacement current must flow, and so the current density higher. In the case of the CDs the aluminium coating is maybe only 10nm thick at best and therefore only ~1/100th the skin depth. The current density will be 100 times higher than if the metal were even only a few microns thick, and the joule heating quickly melts the metal and once the continuous original sheet is fractured by contraction due to surface tension moving the molten metal, the high voltages cause arcing and incandescense. The reason the wirewool mentioned in the above quote bursts into flame is because high voltages are produced between steel filaments that arnt quite touching and arcing ocurrs. Also if there are only small points of contact the high currents flowing there cause considerable heating. Similar sparking occurs if you put an aluminium foil plate in the microwave with a stainless steel spoon on it, but any single solid piece of metal doesnt.
