Why does lightning push electricity through air, but common batteries do not?
Category: Physics Published: January 28, 2014
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and Assistant Professor of Physics at West Texas A&M University
Actually, a common low-voltage battery does push a small electrical current through the air. But this current is so small that in most cases it can be ignored. Nevertheless, an unconnected battery does slowly leak electricity through the air and will eventually end up uncharged if left sitting long enough (internal chemical effects also contribute to this loss of charge). People often say that air is an insulator until you reach the breakdown voltage, at which point it becomes a conductor. This statement is over-simplified. Air is a good insulator at low voltages, but not a perfect insulator. In fact, air is a lot more complex than most people realize.
In high school you may have learned that the electrical current I through an object and the electrical resistance R of an object are related through Ohm's law, I = V/R. This equation means that for a given applied voltage V, an object with high resistance will only allow a very small electrical current through. Naively applying Ohm's law to air, you may think that since air does not seem to carry any electrical current, it must have infinite resistance. But... the problem is that air does not obey Ohm's law. In fact, nothing obeys Ohm's law perfectly. Ohm's law is not a fundamental law of nature. Rather, it is a quick and dirty approximate equation that only works for certain materials within certain ranges of voltage. In science, we describe a material that roughly obeys Ohm's law at the voltages of interest as "ohmic" and all other materials and voltages as "non-ohmic". If you plot the current-voltage curve of a material, the ohmic range (if present at all) is the range where the curve is a straight line. Some materials are ohmic over a broad range of voltages. Other materials are ohmic over only a narrow range of voltages. Still other materials are not ohmic at all. Air is not ohmic at all. This means that there is no linear relationship between the electrical voltage applied to air and the resulting electrical current that travels through it. The behavior of electricity in air is far more complex. In order to create an electrical current, electrons must be ripped off of the air molecules so that they are free to move and form a current. Also, the ions left behind by the electrons are positively charged and can form part of the current. Ripping electrons off of air molecules is hard to do, but there are different mechanisms that can do this, leading to the complex electrical nature of air.
As the diagram above shows, there are three main types of electrical currents that flow through air:
1. Dark discharge is the electrical current that flows through air before reaching the first breakdown point of air. As its name suggests, dark discharge currents do not emit light and are invisible to the human eye (except at the tips of sharp objects where a dark discharge can form a visible corona discharge). In dark discharge, the voltage is not high enough to allow any mechanism to rip electrons off of air molecules. As a result, dark discharge relies on background radiation or normal thermal collisions to rip the electrons free. Once free, the electrons accelerate in the electric field of the applied voltage and form a current. At the low-voltage end of the dark discharge curve, all that is happening is that the electrons freed by background ionization are moving in the electric field. At the high-voltage end of the dark discharge curve, the recently-freed electrons accelerate enough to collide into other air molecules and rip more electrons off in the process. This process repeats, leading to avalanche ionization and a swift increase in electrical current at these higher voltages. Even with avalanche ionization, there is not enough energy present in the dark discharge setting to keep the ionization completely self-sustaining. Low-voltage dark discharge is the type of electrical current created by common batteries sitting in air unattached to anything. Note that the electrical current created in air by common batteries is about a trillion times weaker than lightning. The textbook Industrial Plasma Engineering by J. Reece Roth states, "The dark discharge receives its name from the fact that, with the exception of the more energetic corona discharges, it does not emit enough light to be seen by a human observer. The number density of excited species is so small in this regime that what little excitation light is emitted is not visible...radiation, from cosmic rays, radioactive minerals in the surroundings, or other sources, is capable of producing a constant and measurable degree of ionization in air at atmospheric pressure."
2. Glow discharge is the electrical current that flows through air after reaching the first breakdown point of air. Glow discharge is the type of current that makes a Neon sign glow and makes the gas inside a fluorescent light bulb emit UV rays (which are converted to visible light by the fluorescent coating). Once the first breakdown point of air is reached, the electrical current through the air becomes self-sustaining and does not rely on background ionization. A glow discharge is self-sustaining because the ions left behind by the freed electrons have enough energy when they hit the cathode (the negative end of the voltage source) to knock free new electrons, which then start the avalanche ionization process anew.
3. Arc discharge is the electrical current that flows in air after reaching the second breakdown point of air. Arc discharge is the type of current found in lightning bolts and is typically loud, bright, and hot. At the second breakdown point of air, the cathode becomes hot enough to directly eject electrons into the air, which then rip off more electrons in the normal avalanche pattern. The ions left behind are accelerated by the electric field of the applied voltage until they smash into the cathode, heat it up even more, and cause it to emit even more electrons. Arcing is a run-away process where the high electrical current causes high heat, which causes even more electrical current in a feedback loop. The run-away process continues and the current gets stronger until the applied voltage source is depleted of its charge.
In summary, common low-voltage batteries do drive electrical currents through air, but these currents are very weak and dark, they rely on background ionization, and they behave very differently from lightning.