Top 10 Demonstrations with the Plasma Globe
The plasma ball is an engaging and safe tool for studying high voltages and the electric field and can be used in middle school, high school, and college level physics courses. A very large voltage is created by a Tesla coil-like circuit and this creates a high electric field between the central electrode and the inner glass. The Field is strong enough to ionize the gases in the ball (it pulls their electrons off) and the freed electrons undergo collisions which liberate more electrons from other gas molecules. This process is known as cascade/avalanche or impact ionization. On first inspection, you will notice that the plasma ball responds to your touch. This is due to the polarization of your body (a decent conductor). As you approach the plasma ball you become polarized by the electric field and this attracts more charge to you.
1. Demonstrate plasma
Most physical science classes require that students have a cursory understanding of plasma as the "fourth state of matter." This title is misleading because plasma is the most common state of matter in the universe and plasma was in fact the first state to exist after the big bang. Plasma is a gas-like collection of atoms that have a large number of free electric charges. This means that newly created plasma has undergone ionization (the phase transition that is after melting and boiling). When the freed electrons are regained by ionized atoms the bonding energy is often released as visible light; therefore glowing is a signature of most plasma. Like a gas, plasma has no fixed volume and like other fluids it does not have a fixed shape.
Moving plasmas can usually be controlled by magnetic fields, but this will not be visible on the plasma of a plasma ball. In order to witness deflections of plasma, he charges must move for long enough times. A plasma ball operates on a high-frequency alternating voltage, and for this reason, the charges do not have much time to move in demonstrably measurable distances and get deflected.
Plasma is also an excellent conductor so, once one filament forms, it becomes generally stable allowing for more current to flow through it (similar to a lightning strike). This is more obvious when you bring a finger to the plasma ball. It is important to remember that plasma is very hot and it will slowly conduct heat through the glass.
2. Touch lightning
The very high voltages of the plasma ball can easily polarize a coin (or piece of aluminum foil) placed on top of the plasma ball. By bringing your finger only a few millimeters above the penny, you will be able to elicit a spark from the top of the coin. This spark will not cause pain, or electric shock, but will be hot and if you hold your finger their long enough it might begin to hurt. The tip of the finger will now show a few harmless burn marks that will rub off in a day. Let the students touch lightning too and use this sparking technique to explain how lightning forms due to the Electric Field ionizing the air. You can also have fun burning small pieces of paper with the spark. If you are too shy to touch the spark with your hand, you can touch a metal key (or any conductor) to the coin and the spark will still form while providing additional insulation. You should avoid touching the spark with your fingernail. Fingernails conduct electricity better than the skin and underneath it is a tissue that is dense lined with pain nerves.
3. Demonstrate convection
The plasma threads are very hot and they will rise due to their buoyancy in the other gases inside the plasma ball. For this reason, it is difficult to get a horizontal streamer to remain unbroken for more than a second – not unlike a Jacob's Ladder. However, a vertical streamer at the top will be stabilized by the buoyancy. With practice, you should be able to get just a single vertical thread. Once again, be cautious because the glass will heat up.
4. Investigate the oscillating electric field
The Electric Field created by the Tesla coil reaches beyond the glass dome and into the air surrounding the plasma ball. This Electric Field can easily be investigated with a small neon bulb or light emitting diode (LED). Bring either of these near the plasma ball and they will light up when aligned radially, but not circumferentially. This demonstrates that the voltages are decreasing with radial distance or (equivalently) that the Electric Field is radial. You will also notice no directional dependence of the diode because the field is oscillating rapidly. The circuit is providing a high-frequency alternating voltage which is necessary to "step up" the voltage to the levels needed to operate the plasma globe. Study the voltage directly by simply connecting a probe to one of the channels on an oscilloscope and you can probe the changing voltage spatially. Some experiments include determining how rapidly the voltage decreases with radial distance or whether the voltage differences are established radially or circumferentially (the answer is the former).
The LED bulb glows brighter as it approaches the plasma globe
For fun or if you don't own an oscilloscope, you can also use an audio cable as a probe and listen to the frequencies on an amplifier. These will sound louder up close and quieter far away or when probed circumferentially (along an equipotential line). The human body can serve as an excellent antenna for picking up the signal so be sure to touch the tip of the cable.
One last technique is to investigate the voltage differences directly by using a digital voltmeter set to read AC. Through this investigation, one can most easily verify the distance dependence of voltage as it decreases with radial distance.
5. Illuminate a fluorescent lamp
This demonstration is normally done with a Van de Graaff generator but often results in you getting mild shocks. However, there is no pain or danger if you simply use the plasma ball! Borrow a long fluorescent tube from your overhead lights, or buy one from the hardware store and bring it near the plasma ball. You will notice that once a part of the mercury gas in the tube gets glowing that it can stay glowing even as you extend it. There is essentially no limit to how far you can pull the tube. It also works on the household small tubes. Emphasize that the fluorescent tube holds ionized mercury (plasma) and that plasma is a conductor (because of the free charges) and for this reason, the tube's light can be drawn with no apparent increase in resistance (no decrease in brightness).
Also, note that the starting point of the tube must be close to the plasma ball where the Electric Field is largest (the voltage is changing the most rapidly). This can be demonstrated by moving the tube closer then further radially to the globe. At certain distances, the tube will not glow. There is a minimum Electric Field required to ionize the mercury gas and if the field is not strong enough the tube will not light.
Explain also how the fluorescent light is produced: the low pressure, ionized mercury gas releases mostly UV and violet light when it regains its electrons. This light falls on the fluorescent paint that coats the inside of the tube. The paint then glows white. The UV light is blocked by glass, so harmful UV light does not escape the glass tubes. Thus, the process does not work in reverse: if you shine UV light on the tube from the outside the paint won't fluoresce.
6. Create a human short-circuit
While you have the fluorescent tubes out, demonstrate that the Electric Field can be diverted to a grounded, shorter circuit if a lab-partner grabs part of the tube. This will reinforce the idea of lightning and currents (perhaps later on) taking the path of least resistance. It will also awaken students to the reality that their bodies are paths through which electricity can flow. (A valuable lesson in electrical safety!)
Touching the fluorescent tube diverts the current.
7. Analyze the spectrum of the gases within globe
When it comes to analyzing the spectrum of the gases in your plasma ball, a good place to start is to analyze the point where your finger touches. Looking straight at the plasma globe, place a finger as far to one side as possible. This should create what looks like a vertical (pink?) stripe. Analyze this with your diffraction grating and compare the spectrum to known inert (noble) gases. Since there is often more than one gas, this can be difficult but is worth the effort.
To analyze the (bluish white) streamer filaments, it is helpful to create the vertical streamer from experiment #3. This vertical column will be ideal for analyzing its spectrum. It is best to have a partner supply a free hand and beware once again of the plasma heating up the glass. This may also be a good time to break out the digital spectrometer or other spectrum-analyzing equipment to get specific wavelengths measured. Different plasma globes use different gases and in different amounts, but they are almost always noble gases.
8. Hold ionized gases in the palm of your hand
Ionizing gases and observing their spectra is normally associated with dangerous, high-voltage equipment that only instructors can handle. But now you can put ionized neon tubes in the hands of eager students because your plasma ball ionizes them safely. No longer is a black box needed to confuse students as to what is happening. The plasma ball's strong Electric Field rips the electrons off their atoms and unique colors are produced as electrons are reacquired by the various orbitals. Teaching about the emission spectrum of ionized gases can now become a hands-on activity.
Neon gas spectrum tube near the plasma globe
9. Power up your cathode ray tube
A plasma ball provides a safe source of high-voltage that can allow you to investigate the properties of cathode rays safely. A typical concern with doing cathode ray tube experiments is that you have to connect your CRT to a dangerous high-voltage source. Teacher and student alike can now safely and easily demonstrate the magnetic deflection of electrons and relive the discoveries of J.J. Thomson thanks to their marvelous plasma globe.
10. Demonstrate an absorption spectrum
A plasma globe provides a rare chance for you to demonstrate that light is absorbed by ionized gases. Send a beam of collimated, white light into the plasma housing and you will be able to observe the absorption spectrum. Collimated light is produced by sending a bright beam through two holes on either side of a box; this guarantees that the light that emerges is a narrow column. Note that projectors that mix RGB will not suffice as a white light source – the light has to be a full rainbow. The best source is a bright incandescent flashlight or an overhead projector. Focus the beam so it passes through the plasma, then separate it with a diffraction grating or prism and project the rainbow on a screen or wall. When the plasma globe is off, the white light will split into a full rainbow. When the globe is on, some of the colors will be missing as thin bands. Most notably will be the yellow and reds observed in the emission spectrum from earlier. This will verify that emission and absorption spectra have the same wavelengths.
In conclusion, the plasma globe is an under-utilized and relatively familiar piece of lab equipment. I strongly recommend that every physics teacher include one in his or her laboratory and use them to make electrostatics as hands-on as possible.
Tarbut V' Torah High School
Irvine, CA, USA
James Lincoln teaches Physics in Southern California and has won several science video contests and worked on various projects in the past few years. James has consulted on TV's "The Big Bang Theory" and WebTV's "This vs. That" and the UCLA Physics Video Project.