Re: Any information on kinetic gas theory - as relevant to spark plugs??

Hi Heather!
The formation of sparks is a fascinating topic. My website has a nontechnical description at http://www.amasci.com/tesla/spark.html Also see a previous MADSCI answer: http://www.madsci.org/posts/archives/oct98/909713179.Ph.r.html

To understand sparks, most people first need to rid themselves of a couple of misconceptions. Are leaping sparks a kind of electric current? No! Neither are sparks made of "electricity."

Here's an analogy which helped me greatly: a spark is similar to a crack in a solid object. What are cracks made of? They're not made of anything, instead a crack is a pattern in a material. Cracks grow because the stress in the material near the tip of the crack is large, and a crack can make "more crack" appear at its tip. If a crack takes a sudden turn, a new crack can begin growing from the jagged part of the old one. What then are sparks made of? A spark is not made of anything, instead it is a pattern, and it obeys some of the same rules that cracks do.

Air is made of molecules, and the molecules are made of protons, neutrons and electrons. Whenever a spark appears, those same protons, neutrons, and electrons are still there. There's a difference of course. In air, the protons and electrons are bound together, so they cannot flow separately. Within the spark the electrons and protons are no longer stuck together, and electrons can flow through the cloud of protons, and vice versa. Textbooks typically explain it like this: within the spark, the air has changed into a conductive plasma. Again, what is a spark? It is an altered state of matter: if air is like ice, then a spark is like a region of water within the ice. Sparks are simply regions in the air which has undergone a change in phase. They are not "made of" electric current. Sparks however are conductive, while air is not.

What causes sparks to appear at a certain voltage and pressure? How does voltage trigger the creation of plasma? To answer this we need to look at the microscopic details of spark creation.

When you apply a large voltage between the metal parts of a spark plug, you create an electrostatic field in the space between the metal parts. When this "e-field" is strong enough, it tears the gas apart into a conductive plasma, and a spark is born. "Voltage" essentially means "e-field", since the strength of an e-field is measured in volts-per-distance, and you cannot have voltage without having an e-field too. However, the net voltage placed across the metal electrodes is not the only thing that determines the strength of the e-field. The e-field will be strong near small, sharp electrodes, and it will be weaker near the surfaces of large flat ones. For this reason, as you raise the voltage across your electrodes, air will first turn into plasma near a sharp edge or small bump on the surface of the metal. The spark "ignites" at a sharp edge where the e-field is stronger than elsewhere. It then grows outwards. Sparks can start on the negative electrode and grow towards the positive, but they also can start on the positive and grow towards the negative. They can even start in the space between the electrodes and grow in both directions at once!

Why does air pressure play a role? This is caused by a phenomenon called ELECTRON AVALANCHE.

A strong e-field can turn air into plasma by pulling the electrons away from the protons of the oxygen or nitrogen molecules, but that's not what usually happens. Rather than simply yanking the electrons away, the e-field instead uses free electrons to bash the molecules, which knocks electrons away and provides even more free electrons which slam into more atoms.

OK, say you have a strong e-field in the air. The air moleucles are put under stress, since the e-field is pushing the protons in one direction and the electrons in another, yet these opposite particles are bound together. If one electron should pop lose, it will accelerate because of the e-field. If the air pressure is high, then the molecules are packed fairly densely, and the speed of the flying electron won't become very high before the electron hits an air molecule. If the speed of the electron is low, the air molecule will capture that electron, and no extra electrons will be knocked off. What happens when the air pressure is very low? In that case there are much larger spaces between the molecules, and any free electron will accelerate to a very high speed before it strikes another molecule. It can trigger an "avalanche" where one electron hits a molecule and frees a few more, and those free many more. Very quickly the gas changes from neutral molecules into a cloud of positively charged molecules immersed in a sea of freed electrons. It turns into plasma. Plasmas are a bit like metals: both are conductors because both are full of charges which are easily moved. Voltage can trigger the formation of a plasma in a gas, and if the pressure is low, less voltage is needed.

With your spark plug, as you raise the voltage across the metal electrodes, an electron avalanche will cause a region of plasma to "ignite." This typically occurs at a sharp edge, but if there is a dust-mote floating between the electrodes, it can start at the surface of the dust. The region of plasma grows. It grows in somewhat the same way that a forest fire grows: it causes the air adjacent to itself to "ignite" and form more plasma. Plasmas in strong e-fields tend to form into narrow filaments. They are very much like growing crystals: crystals can grow like "frost" rather than like bulk polyhedra. "Frostlike" growth occurs when the growth is very fast, and the tip of the crystal grows faster than any other part. With plasmas, if growth is slow, you get a region of glow-discharge or "saint elmo's fire", while if growth is fast, you get narrow spines or treelike fractal shapes where the tips grow the fastest. Remember that sparks are conductive. A narrow spark is like a wire, and when you place a high voltage upon a wire, you will see an electric discharge at its tip. When the "wire" is MADE of electric discharge, then the tip grows longer and longer by converting the air into "more spark."

As the "plasma tree" grows outwards, remember that plasmas are conductive. The growing "tree" is like an extending wire. When this "plasma wire" touches the other electrode, it explodes! It creates a bridge across the high voltage power supply, and the power supply suddenly creates an enormous current through the conductive spark. The flash and noise of a spark is the same as the flash and noise of a wire placed across a large battery: a spark is a short-circuit. Yet sparks can also grow outwards without bridging the gap between the electrodes. In this case they look like silent blue plasma fingers, not noisy incandescent explosions. In your experiments with sparks, try using large polished balls as electrodes, and also try using sharp needles. The sparking voltage will be very different because the sharp needles "attract" small regions of strong e-fields to their tips. And besides investigating the role of air pressure, you might also consider trying various gases. In particular Helium, Neon, and Argon will each create sparks at a voltage much lower than the nitrogen or oxygen of the air.