Everyone knows that when you put something metal in the microwave it can cause a lightning storm. Most people will assume that this is something to do with the fact that metal conducts electricity, but will leave it at that. So in this article I will explain just what is going on.
Microwaves cannot penetrate metal in the same way they can penetrate food, ceramic bowls or plastic. Instead they reflect off the surface, but when they make contact with the metal they induce an electromagnetic field in the surface of the metal which causes electrons to move, and it’s the ability for electrons to move so freely which makes them such good conductors. These electrons will accumulate and concentrate in bends and points of the metal – the tighter the bend/ sharper the point the more tightly electrons will accumulate. It is this grouping which will cause the lightning spark, so things with very tight bends, such as forks, or sharp points like scrunched up foil will be much more reactive than a metal bowl.
Electrons have a negative charge, and so the accumulation of electrons in an area like this will create a strongly negatively charged area. This strong negative charge will repel electrons in nearby air molecules and strip them from the atoms. This creates what is called plasma, and the electrons in the air are able to flow freely, much like they do in the metal, and allow a current to flow. This creates a positively charged area of air, and a negatively charged bit of metal which is perfect conditions for a current to flow. Electrons from the metal will flow up to the positive atoms in the air creating a current which releases energy that we perceive as little bolts of lightening/ explosions, and sometimes ‘popping/ banging’ sounds.
It is possible that the stripping of electrons can continue through the air until an object is met (probably the wall of the microwave oven) and then the lightning will arc between the two through the plasma rather than just spiking out into the air.
This is very similar to what happens in lightning storms. In these storms clouds become negatively charged (from winds and varying temperatures). This negative charge attracts positive charges from the ground (and repels negative charges) until the positive charge of the ground and the negative charge of the clouds becomes so strong the air becomes ionised, which allows the negative charge to create a current to the ground. This current releases energy that we see as lightning, and hear as thunder.
If you look at the impressive animation on the right from scijinks of a slow motion lightning strike, you can see the air ionising to connect the positive ground and the negative clouds. Once a connection is made, then the lightening will strike, discharging the current. A similar mechanism occurs in the microwave.
So metal causes a small lightning storm in a microwave for the same reason that it can conduct electricity – free electrons. Microwaves cause these electrons to accumulate in tight points of the metal, which means forces the electrons off the molecules in the air. This creates what is called ‘plasma’, which means electrons are free in the air, much like they are in metal. With electrons being free like this, they can create a current and discharge the imbalance of charges that the concentrations of electrons causes. This discharge is what we see as lightning.
Image courtesy of Garrett Coakley
This Youtube video will give an overview of the information found on the article tab. If you want to know more about the topic, or want to see where the information came from, have a read of the article after you watch the video.
Microwaves induce an electromagnetic current in metal objects, which causes electrons to accumulate in points and corners. This causes large positively charged areas to build up against large negatively charged areas, and eventually, the electrons will ‘discharge’ and cause lightening as they rush towards the positivly charged area.