Sparks in Microwave

Normally when we heat anyfood in the microwave we have to use, plastic or ceramic containers. we cant hse metal ones. even a cup with silver rim creates sparis in the microwave. but this morning a friend if mine placed a aluminium teapot accudently and it geated with no sparks whatsoever! how can tgat be? please explain.
- Ishrat (age 46)
UK

There are two issues here: 1) The use of metal containers, and 2) Small bits of metal (eg, decorative).

1) Metals (and conductors in general)" screen electric fields, so food enclosed in a metal container (e.g., wrapped in foil) won't be heated up.  This also happens, but to a lesser extent, with a metal cup even though the cup does not completely enclose the food,

2) If you have a container with some thin decorative metal, you will get sparks if there are small gaps in the metal, or often at sharp edges of the metal. Even if there are no gaps initially, if the metal is very thin, electrical currents will melt the metal, creating gaps.  This effect won't happen with a teapot, which is more difficult to melt, but you won't be able to boil the water that it contains.

jjt

(published on 03/02/2018)