After a quite technical post the other day I’d like to share with you some more microwave entertaining. In that post we said that a rough classification for the states of matter is: gas, liquid and solid but there are more, for example plasma. A plasma is a gas where some of the electrons have been detached from their atoms. In that way in a plasma one has free negative charges moving (the electrons) and also free positive charges moving (the ionized atoms). For this reason, some fun electrical phenomena happen such as the filamentation that one can observe in a plasma lamp:

Filamentation in plasma lamp

Filamentation in plasma lamp

As it turns out, you can create the same effect (at a smaller scale) in your own microwave. All you need to do is cut some grapes in halves, but not all the way through (just leave a small piece of skin holding together the two halves). Then dry the surface of the grapes with some kitchen paper and put them in the microwave for a few seconds.

Plasma grapes

Plasma grapes

Please beware that performing this experiment longer than a few seconds can be as dangerous as functioning the microwave empty and may result in permanent damage of your microwave oven. Also pay attention when taking the grapes out of the microwave because they will be very hot. This is the result:

Essentially what is going on is that the grapes are as little antennas, just like the light bulb was the other day. Their shape, composition (they are full of electrolytes) and size all conspire to make the electric field very strong near the grapes raising the temperature so much that the skin between the two halves burns and the air nearby is ionized creating a small plasma that provokes sparks.

Plasma grapes

Plasma grapes

You can actually see in the video that each grape bursts into flames exactly at the same spot in the microwave. This is because microwaves are not distributed uniformly and, since you need a high field to provoke this effect, you need to find the “hot spots” inside your microwave in order to create your own plasma.

Kids, don’t do it at home! Seriously, this experiment can be dangerous and must only be performed under adult supervision and with the permission of the owner of the microwave oven.

My friend Julia lend me an excellent book by the British physicists Peter Barham. It is titled “The science of cooking” and it contains very clear explanations of some kitchen phenomena alongside many try-at-home experiments. The first one I tried was this one, where he suggested cooking a light bulb in a microwave oven (only for a few seconds! less than 20 is a good safety estimate, otherwise the bulb might explode). This is the result:

To understand why this happens you need to know first how a light bulb works. Essentially, when an electric current circulates through the filament inside the bulb it heats it up until the metal becomes incandescent and emits light, just like iron becomes incandescent when heated up.

All you need to do to turn a light bulb on is to make an electric current pass through the filament. Normally this is done by plugging the light bulb to the electric network thus creating a difference of potential at both ends of the filament that makes electrons move from one end to the other creating an electric current. But there are other ways of inducing an electric current. For example, by exposing a piece of metal to electromagnetic radiation, the electrons in the metal will move inside it. This is the same principle that makes antennas work and it works better if the size of the metal is similar to the length of the radiation used. Microwaves have lengths between 0.1 – 1 cm, which is approximately the size of the filament in the bulb.

If you try this at home, you might notice how the intensity of the light changes as the bulb turns in the microwave. This is because microwaves are not uniformily distributed, but rather are more intense in some points of the microwave than others. More on this will follow in a future post where will talk about using a microwave oven for actual cooking.

I hope this post won’t contribute to the magical halo that electromagnetism has. The fact that some electromagnetic phenomena are counter intuitive or suprising should not lead us to believe that they can do anything (despite what you may hear in some TV shows and movies). Electromagnetic waves are everywhere around us and have always been: the light that comes from the Sun, radio waves, TV waves, the Earth’s magnetic field, wireless networks, GPS signals even the Cosmic Microwave Background it is all the same thing: photons. The only difference is the wavelenght they have.