[Petrochemicals (OP)]

Why when metal is placed inside a microwave oven does it spark and arc?

[evan_au]

Metal acts like an antenna; it picks up microwave energy and turns it into voltages and current.
- If the voltage is high enough, it will exceed the breakdown voltage of the air (about 700,000 Volts/meter) and cause a spark.
- If the current is high enough, it will melt the metal film on a decorated plate or cup, and this causes a small gap (with hot metal in the middle), which can also cause a spark.

[Petrochemicals (OP)]

Metal acts like an antenna; it picks up microwave energy and turns it into voltages and current.
- If the voltage is high enough, it will exceed the breakdown voltage of the air (about 700,000 Volts/meter) and cause a spark.
- If the current is high enough, it will melt the metal film on a decorated plate or cup, and this causes a small gap (with hot metal in the middle), which can also cause a spark.

Is this in a similar way to metal through a magnetic field? I notice new metal fast food containers have painted metal that is microwavable.

I found this also.

https://www.thenakedscientists.com/articles/questions/why-dont-microwaves-spark-themselves

[evan_au]

Is this in a similar way to metal through a magnetic field?

Yes, similar. A microwave has an oscillating electromagnetic field.
- A conductor in a varying magnetic field will set up "eddy currents", which will heat the material. But if the metal is continuous (no gaps), it won't produce enough of a voltage to produce a visible spark. The classic magnetic antenna is in the form of a loop; quite high currents can be induced if the size is comparable to the wavelength of the radiation.
- A conductor in a varying electric field will induce a voltage in the conductor, which will heat the material. But if the metal is continuous (no gaps), it won't produce enough of a voltage to produce a visible spark. The classic voltage antenna is in the form of a straight line; quite high voltages can be induced if the size is comparable to the wavelength of the radiation.

If you put two straight metal lines close to each other in the microwave, the induced voltages can be opposite, and produce a visible spark.