What is a Plasma? (Jake Dickerman)

Title: What is a Plasma? How you see matter's 4th state on a daily basis

Principle(s) Investigated: Phase Change, Ionization, Charged Particles being attracted to EM fields

Standards : CA 9-12 Physics Standards 5.h, 5.i

Materials: 1-2 Tesla Coils, Taper Candle, metal Test Tube Stand (or any tall piece of metal that can stand freely). Tesla Coils may need to be purchased for a large sum, the Tesla Coil I am using can be find via this link.

Procedure: Plasma is the most common phase of matter in the universe, and yet since it is not a common phase on Earth, students have a great deal of trouble grasping what exactly a Plasma is. This demonstration is designed to show them that they experience plasma on a regular basis, they've just never known that.

The first step for this lesson is to remind the students what exactly the phases of matter are and to briefly go through the phase changes:

The students, in all likelihood, will understand what exactly a solid, liquid, and gas are, but it is worthwhile to ask them what a Plasma is. Collect their knowledge about Plasmas by using a quickwrite. Make sure that they do NOT use the other phases to describe a Plasma.

What distinguishes plasmas from other phases of matter is ionization. Plasma is the only type of matter composed of charged particles. Although, like with fluids, Plasmas shapes are variable, they are constrained by electromagnetic fields and do not have the range of motion that gases do.

Set up your taper candle so that the metal pole is directly behind it. If you are using a test tube stand, position the actual stand so that it is directly behind the candle. Light the candle. Tell the class that you are now going to investigate the candle flame. What is it?

Use the Tesla coils first on the metal pole on its own. Show the students that the tesla coil can only break down about a half inch of air (gases act as insulators, in order to create a spark the tesla coil must create a path of ions through the air, that takes a lot of energy!). Then, slowly bring the tesla coil near the flame. Students should see the flame push away from the tesla coil and towards the metal pole! What students should also see is that the electrical spark leading from the tesla coil to the metal pole is about twice the length as the original spark. In order to show them that the spark is coming from the tesla coil and not from the flame, remove the metal pole temporarily and show them that the tesla coil does not make a strong spark towards the flame.

Finally, blow out the candle. Use the tesla coil one more time to show, even the smoke from the flame pushes away from the tesla coil.

Student prior knowledge: Before this class, students should have some idea about phase changes and the phases of matter in general. They should know that plasmas exist but do not necessarily need to know what they are. They should know what ions are, however, and they should understand that ions will respond to electromagnetic fields.

Explanation: When the flame is ignited, it releases a huge amount of chemical energy, drastically increasing the enthalpy of the system and ionizing the particles in the gas released. Ionizing these gases turns them into a plasma, a mixture of positively charged particles and free electrons. The tesla coil generates a variable EM field, and the metal pole opposite of it, by virtual of those EM forces flowing into it, gains a charge equal and opposite to the tesla coil's charge. This charge differential on either side of the flame forces the flame to move. It will generally move away from the tesla coil, although I can speculate that it might be possible for the stretch to happen towards the tesla coil instead. Presumably, there should be particles moving in both directions, stretching out the flame. As of yet, I have not been able to create a strong enough charge to see that personally.

The tesla coil is able to make that greater electrical arc because, as the flame is a plasma (ionized), the tesla coil no longer has to break down the entirety of the air between itself and the metal pole. The air around the flame is already broken down into those hot plasmas, the tesla coil can take advantage of that to create a longer arc. In other words, because plasmas are conductors, unlike gases, the flame can almost act like a wire through the air.

Questions & Answers:

1. How does the flame as plasma theory help explain why flames do not diffuse in the same way as the gases being formed by burning hydrocarbons would normally diffuse?

-- Plasmas consist of charged particles. Because electrons cannot be somehow destroyed, an equal number of electrons that have left the now negatively charged ions must have joined the now positively charged ions. This means that the negatively and positively charged parts of the candle flame should in fact be attracted to each other, keeping the candle flame confined to a rather small volume.

2. Examine the video below. How does your understanding of plasma help you explain what is going on in this video? Is the lightning traveling through a gas? Why or why not?

-- Lightning can be described as a static discharge, however that discharge cannot simply happen. Gases are poor conductors for energy, for lightning to function, a path of ionized air must be formed all the way from the ground to the clouds. Once this path of ionized air has been formed, the energy can discharge from the negatively charged pole (be it the cloud or the ground) to the positively charged pole. It is important to realize that, when the energy is passing from one pole to the other, it is energizing ionized matter, meaning that lightning does not travel through a gas, but through a plasma.

3. Where else do you regularly see plasmas? Can you describe what they are used for?

-- The simplest answer here would be in fluorescent and neon light bulbs. Fluorescent bulbs are filled with gas. When a strong electrical current is put into them, the gases ionize and become plasmas. As plasmas, they give off spectra of light based upon their particular chemical composition. In most fluorescent bulbs, we tend to use mercury and argon gas, since they give off fairly close to white light.

Applications to Everyday Life: Plasmas can be seen everywhere. Having a better understanding of what they are allows students to understand what exactly is happening inside a fluorescent light bulb. Understanding how the tesla coil makes its spark gives students an understanding of how lightning works, and gives a place to explain how lightning has to make a path of ions from the clouds to the ground. It certainly helps explain why lightning does not only go from the clouds to the ground but from the ground up. The experiment helps students understand the magnetic and electrical fields emanating from stars, certainly the fields from our own sun.Finally, this demonstration answers the question of what exactly is a flame, a question that many students may have been wondering about for a long time. I know that when I was there age, it was a question that bothered me.

Photographs: Include a photograph of you or students performing the experiment/demonstration, and a close-up, easy to interpret photograph of the activity --these can be included later.

Videos: