﻿ Why does lightning push electricity through air, but common batteries do not?

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# Why does lightning push electricity through air, but common batteries do not?

Actually,a common low-voltage battery does push a small electrical currentthrough the air. But this current is so small that in most cases it can beignored. Nevertheless, an unconnected battery does slowly leak electricitythrough the air and will eventually end up uncharged if left sitting longenough (internal chemical effects also contribute to this loss of charge).People often say that air is an insulator until you reach the breakdownvoltage, at which point it becomes a conductor. This statement isover-simplified. Air is a good insulator at low voltages, but not a perfectinsulator. In fact, air is a lot more complex than most people realize.

Inhigh school you may have learned that the electrical current I through an object and the electricalresistance R of an object are related through Ohm'slaw, I = V/R.This equation means that for a given applied voltage V, an object with highresistance will only allow a very small electrical current through. Naivelyapplying Ohm's law to air, you may think that since air does not seem to carryany electrical current, it must have infinite resistance. But... the problem is thatair does not obey Ohm's law. In fact, nothing obeys Ohm's law perfectly. Ohm'slaw is not a fundamental law of nature. Rather, it is a quick and dirtyapproximate 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 interestas "ohmic" and all other materials and voltages as"non-ohmic". If you plot the current-voltage curve of a material, theohmic range (if present at all) is the range where the curve is a straightline. Some materials are ohmic over a broad range of voltages. Other materials areohmic over only a narrow range of voltages. Still other materials are not ohmicat all. Air is not ohmic at all. This means that there is no linearrelationship between the electrical voltage applied to air and the resultingelectrical current that travels through it. The behavior of electricity in airis far more complex. In order to create an electrical current, electrons mustbe ripped off of the air molecules so that they are free to move and form acurrent. Also, the ions left behind by the electrons are positively charged andcan form part of the current. Ripping electrons off of air molecules is hard todo, but there are different mechanisms that can do this, leading to the complexelectrical nature of air.

Asthe diagram above shows, there are three main types of electrical currents thatflow through air:

1.     Dark discharge is the electrical current that flows through air beforereaching the first breakdown point of air. As its name suggests, dark dischargecurrents do not emit light and are invisible to the human eye (except at thetips of sharp objects where a dark discharge can form a visible coronadischarge). In dark discharge, the voltage is not high enough to allow anymechanism to rip electrons off of air molecules. As a result, dark dischargerelies on background radiation or normal thermal collisions to rip theelectrons free. Once free, the electrons accelerate in the electric field ofthe applied voltage and form a current. At the low-voltage end of the darkdischarge curve, all that is happening is that the electrons freed bybackground ionization are moving in the electric field. At the high-voltage endof the dark discharge curve, the recently-freed electrons accelerate enough tocollide into other air molecules and rip more electrons off in the process.This process repeats, leading to avalanche ionization and a swift increase inelectrical current at these higher voltages. Even with avalanche ionization,there is not enough energy present in the dark discharge setting to keep theionization completely self-sustaining. Low-voltage dark discharge is the typeof electrical current created by common batteries sitting in air unattached toanything. Note that the electrical current created in air by common batteriesis 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 emitenough light to be seen by a human observer.

The number density of excited species is so small in thisregime 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 airat atmospheric pressure."

1.     Glow discharge is the electrical current that flows through air afterreaching the first breakdown point of air. Glow discharge is the type ofcurrent that makes a Neon sign glow and makes the gas inside a fluorescentlight bulb emit UV rays (which are converted to visible light by thefluorescent coating). Once the first breakdown point of air is reached, theelectrical current through the air becomes self-sustaining and does not rely onbackground ionization.

A glow discharge is self-sustaining because the ions leftbehind by the freed electrons have enough energy when they hit the cathode (thenegative end of the voltage source) to knock free new electrons, which thenstart the avalanche ionization process anew.

1.     Arc discharge is the electrical current that flows in air after reachingthe second breakdown point of air. Arc discharge is the type of current foundin lightning bolts and is typically loud, bright, and hot. At the secondbreakdown point of air, the cathode becomes hot enough to directly ejectelectrons into the air, which then rip off more electrons in the normalavalanche pattern. The ions left behind are accelerated by the electric fieldof 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 electricalcurrent causes high heat, which causes even more electrical current in afeedback loop. The run-away process continues and the current gets strongeruntil the applied voltage source is depleted of its charge.

Insummary, 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.