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A corona discharge is an electrical discharge that occurs when the fluid around a conductor ionizes. The strength of the electrical field must be great enough to cause the ionization but insufficient to cause actual arcing. This process requires an electrode with a high electrical potential within an electrically neutral fluid — usually air. A fluid in this context includes gases as well as liquids.
The electrical current creates charged particles known as ions from the neutral molecules in the fluid if the potential gradient is great enough. This is most likely to occur when the conductor has a sharp point. When the air around the conductor begins to conduct electricity, it effectively becomes part of the conductor. This makes the conductor less sharp and might prevent the ionization from extending beyond a certain distance from the conductor.
The ions eventually travel beyond this conductive region and become neutral molecules once again. The conditions around the electrode might also allow the area of ionization to continue growing and form a completely conductive path. This will result in continuous arcing, or sparking, rather than a corona discharge.
The ideal conditions for a corona discharge generally require a pair of electrodes. One electrode should be highly curved and typically is a small wire. The other electrode should be flat, such as a plate. This difference in curvature between the electrodes ensures a high potential around the curved electrode.
The polarity of the curved electrode determines the polarity of the corona discharge. This means that a curved electrode with a positive charge produces a positive corona, and a curved electrode with a negative charge produces a negative corona. These two types of coronas have very different behavior because of the difference in mass between positive ions and electrons. Positive ions do not collide with each other nearly as often as electrons do. This means that only negative coronas produce the ozone, violet glow and hissing associated with coronas.
There are a variety of applications for a corona discharge. It can remove an electrical charge from the surface of a flying aircraft. This can prevent an electrical discharge from damaging the aircraft’s electronic systems.
A corona discharge can also remove particulate matter from the air. It ionizes the air, which charges the particulate matter. The air then passes over a comb with the opposite charge. The comb attracts the charged particles, thus removing them from the air.
@allenJo - Yes, every time that I get on an airplane, I try to remind myself of the oft-heard claim that it's the safest way to travel. But I doubt that claim when I consider the lightning factor.
An interesting fact about those lightning strikes is that some of them are actually caused by the plane itself.
As it flies through the air it creates a disturbance in the nearby atmosphere and this can create electrical charges which attract lightning strikes.
It’s rare that the plane is actually destroyed however. The more likely threat is that it could screw up the aircraft’s computers and navigation systems, forcing the pilot to make an emergency landing.
I would hope that airplanes have sophisticated systems in place to deflect any type of coronal discharge. As they pass through the air the last thing they need is an electric shock of any sort.
I mention it because some time ago I saw a photo of an airplane that got zapped by lightning. The amazing thing was that this lightning strike didn’t destroy the airplane.
Then I heard that airplanes get quite a few lightning strikes each year and most of them survive. The electricity just passes through one part of the plane to another without going into the cabin.
If you want to hear what a corona discharge sounds like, you’ll find it in a nearby high-voltage power line.
Of course, I don’t recommend that you pay a visit to one to find out. We’ve had some pretty severe storms that took out some of our power lines, and even from a “safe” distance you could hear the cracking and buzzing of the power lines as they experienced the corona effect.
It’s at those times that you get a renewed and healthy respect for the power of electricity. Even though power lines are stepped down to deliver voltage to your home, they still pack a wallop and you can still hear them electrifying the nearby air.