When a grape is cut in half except for its peel, and placed inside the microwave oven and microwaved for a few seconds, a plasma discharge occurs between those two pieces and under certain conditions, the plasma could be sustained. This is one observation that you might not have observed before, unless you felt bored to microwave a grape. So why a grape produces plasma inside a microwave and what happens in that grape?
Plasma as we know is a state of matter where the molecules in the hot gas breakdown into ions due to very high temperature caused by the strong electromagnetic field. In this case, the electromagnetic field is produced by the microwave oven in the form of microwaves.
How to do that?
The grape has to be cut in two halves with just the skin connecting the two parts. This piece of grape about 1.5 cm to 2 cm lies inside the wavelength of the microwave, when it is inside the oven. When you switch on the microwave for about 10 to 20 seconds, you would see a plasma arc rising to the top of the microwave.
What happens inside the microwave oven?
Here is what exactly happens when you turn on the microwave. The microwave is generated inside the oven and the microwave energy is concentrated towards the grape. That’s because, the grape now acts like a small dipole antenna (with a very thin skin between the two grapes), as it lies inside the wavelength of the microwave. Due to this dipole ‘grapetenna’s’ concentration of the electromagnetic waves, the electrons are forced back and forth through the narrow skin from one-half of the grape to the other (just like the dipole antenna). When the electrons are pushed back and forth through the narrow thin skin due to the electric field, it heats up and burns the skin due to the resistance. Also, this movement is so rapid at high temperature and it causes the electrons to be knocked off from the molecules, forming the electron, ion cloud. At high temperatures, this forms plasma.
Besides, the grape juice is rich in electrolytes. This increases the ion concentration at the thin skin, which also aids in heating up the skin, adding more ions in the process. The plasma continues to sustain as long as the energy is transferred back to the surrounding gas by releasing heat and light, as the free electrons recombine to form gases again.
You can also sustain the plasma by covering the grape with a glass cup or container. Several other objects like a candle, a matchstick or berries can also be used to produce plasma, but you will probably end up with a messy disaster. So please don’t try this anywhere.