Wednesday, January 23, 2013

Lightning Part 2: Lightning is Electricity

Lightning is all about electricity. We need to understand (sometimes confusing) concepts like charge, current, voltage or potential difference, resistance and electric fields in order to pry open the mystery of how a bolt of electricity can appear all of the sudden out of storm cloud stuff.

Everything Is Electric

We tend to think of electricity as something out there somewhere, but it isn't. Every single building block of matter, every atom, comes with electricity built into it.

Every atom contains two kinds of charged particle - the positively charged proton and the negatively charged electron. Their electric charges are equal and opposite, so a neutrally charged atom contains an equal number of protons and electrons. Protons are relatively heavy and confined to the atomic nucleus, whereas electrons move about the nucleus, attracted by the positive charge of the protons, as shown in the Rutherford model of a lithium atom, below.

Tremendous force is required to remove a proton from an atom. Some electrons, on the other hand, can be sloughed off some atoms fairly easily (but other atoms hang on to their electrons much more tightly; check out Atoms Part 4A: Atoms and Chemistry - Atomic Orbitals and Bonding for the reason why). These free electrons are mobile. They will move in the direction of an electric field, a region of force that acts on charged particles and is created by charged particles. Each electron has a charge of 1.6 x 10-19 coulombs (C). A bunch of electrons moving together, a flow of charge in other words, is an electric current. Its flow rate is measured in amperes. One ampere (A) equals a rate of one coulomb/second.

Electricity is usually described as the activity of electrons. However, electrical phenomena are not limited to free electrons. Many molecules can split into positively charged and negatively charged parts, called ions. These molecules are called ionic compounds and they include many compounds that dissolve easily in water, such as table salt, NaCl, which dissolves into positive Na+ ions (11 protons and 10 electrons) and negative Cl- ions (17  protons and 18 electrons). Ionic compounds are explored in my article, Atoms Part4B: Atoms and Chemistry - Ionic and Covalent Bonds. Lightning, as we're about to see, involves not just free electrons but ions as well.

Using a Water Analogy To Understand Electricity

Electricity itself is invisible (the bright flash of lightning comes from another process as you'll see) so sometimes it helps to understand electricity by using a water analogy. We can think of current (C/s) as the rate of charge flow, in the same way as we would measure the flow rate of water in litres/second.

In order to understand voltage, let's use a water stream analogy: Like the flow of water down a stream, electricity is the flow of electrons "down" an electrical conductor, for example, a wire. A conductor is any material that lets electrons move through it. What makes a material a conductor has to do with what kinds of atoms are in it and how they are arranged.

Another Analogy: Comparing Lightning To A Battery

Like water flowing down a stream, the current in a wire must be continuously replenished or it will stop. Somewhere "upstream," something must generate the current. This can be a battery, for example. A battery acts like higher elevation does for a stream. Water won't flow down the stream unless its starting point is higher than its ending point, unless gravitational potential energy is available in other words. Likewise, a battery "pumps" electrons "upstream" to a higher electric potential energy. This is also called electric potential for short, or voltage. A battery stores electric potential energy by building up two separate regions of different charge. One region contains more free electrons than the other region. These electrons want to move back across the two regions to equalize the charges but they can't. Keeping them separate adds potential energy to the system.

The simplest battery is composed of a single electrochemical cell, shown below.

When the two electrochemical half-cells (the two tubs of solution with strips of metal stuck in them) are connected by a wire, the potential difference (voltage) causes charge to move through the wire. The black arrows show this current. We can also call this current electrical discharge, which means any flow of charge through a gas, liquid or solid, in this case a solid metal wire, although the term discharge generally implies the current is temporary.

The potential difference is created as zinc metal dissociates into zinc (Zn2+) ions and free electrons in solution. Electrons (e-, negative charge) build up at the anode as positively charged zinc ions build up in the solution. At the cathode, copper atoms are deposited on the copper strip, consuming free electrons (and copper ions, Cu2+) from the solution. The overall number of electrons in each tub stays the same but the number of free electrons changes. Sulphate ions (SO42-) move from right to left through a semipermeable barrier to balance the electron flow through the wire. This current flows from the anode, where free electrons accumulate, to the cathode, where free electrons are in short supply.

Energy is needed to move electrons through the wire and light up the bulb. That energy ultimately comes from the two chemical reactions in the tubs. The oxidation reaction of zinc releases more energy than the reduction reaction of copper uses up. The electrochemical cell will keep making current until the chemicals needed for the reactions are used up. If the wire is removed, the reactions in the two half-cells will quickly reach equilibrium states as their products build up in each solution.

The thundercloud is a little bit like a battery that is not hooked up to a wire, with some exceptions. Rather than chemical reactions, the movement of water molecules within the cloud builds up separate pockets of different charge. Water molecules have a unique ability to attract or lose an electron, so they can form both positive and negative ions. Like the battery, this builds up potential difference. The cloud has no electrodes, but water molecules both supply and attract electrons. In the top of the cloud, positively charged water ions (possibly ice pellets) build up. In the bottom, negatively charged water ions (possibly rain droplets) and free electrons build up. Like a battery that is not hooked up to a wire, there is no current flow, at least for now. And like a battery, there is a build-up of potential difference (voltage) and an electric field is created (wherever there is potential difference, there is an electric field). Unlike a battery, with a limited supply of chemicals, there is a huge supply of charge-building water molecules in a thundercloud.

A Water Pipe Analogy Helps to Explain Current and Resistance

Let's move on to a water pipe analogy to understand current and resistance. A thin water hose has more resistance to water flow than a hose with a bigger diameter. The flow rate of water molecules through the second hose is higher. Put another way, decreasing the resistance of the hose increases the water flow. In the same way, decreasing the electrical resistance of a conductor by using a thicker wire increases the flow of electrons, or current.

A hose that's twice as long will have half the water flow rate because we've doubled the friction that the water experiences. If you double the length of wire in the electrochemical cell shown above, you double the resistance against the charge flowing through it, and you reduce the current by half.

What happens if we use two identical water pipes but connect one pipe to a water source that's higher up than the other one? Here, we add gravitational potential energy to the water source of one pipe. There will be higher water pressure through the higher pipe. All other things being the same, the rate of water flow depends on water pressure, so the higher-end pipe will have a higher water flow rate over the lower end one. If we compare two wires the same way (both identical), the amount of current now depends on the voltage applied across the wire. If we increase the voltage across one wire, it will have more current through it than the other wire.

This is how Ohm's law, current = voltage/resistance, works.

Air Is An Electrical Resistor

Some materials allow electrons to move through them more easily than others. That is why copper is commonly used in home wiring. It is a good electrical conductor, like most metals are. In these materials, the outermost electrons are shared among atoms, and move around easily. To learn more about how electrons move in metals, try my article, Atoms Part 4D: Atoms and Chemistry - Polar Covalent and Metallic Bonds, and scroll down to Metallic Bonding: A special Kind Of Covalent Bond. Other materials, called resistors, tend to resist the flow of current through them. Rubber, glass and air are all good resistors. Outer electrons in their atoms tend to stay fixed in place.

Air is an excellent resistor. It will not allow current to flow through it, and this leaves the storm cloud with its ever-building pockets of charge, and ever-increasing electric potential, in a quandary. The system must eventually find a way to release all this energy, but how? It can't be conveniently hooked up to a wire like a battery can.

Comparing Lightning To a Short Circuit

Lightning is a temporary flow of current, an electrical discharge. Electrons rush from where there are too many toward where there are too few. It is a bit like a short circuit between two differently charged bodies. A circuit is simply a path for electrons to flow. The wire connected to the electrochemical cell provides a path for electrons to move. A short circuit is technically an abnormal connection, or path, between two sections of an electrical circuit that are at two different electric potentials. That accidental path is usually short and it offers little or no resistance to the circuit.

An electric circuit requires a continuous path for electrons to flow, left.

If you connect a wire to any one of the two terminals on a battery, nothing will happen. This is an incomplete or broken electric circuit. The same thing happens when you flip off a light switch in your home. The switch opens the circuit, so the current stops flowing to the light bulb. The two circles and line at the bottom of the circuit below left is a symbol for an open (off) switch. A lightbulb is an example of an electrical load. Sometimes people think that the light switches off so fast because the electrons themselves travel near the speed of light through the wire. They actually travel quite slowly, on the order of centimetres per minute. A light shuts off almost instantly because the energy of the circuit travels at almost the speed of light. This effect is similar to that observed in earthquakes and tsunamis - the energy (the wave propagation itself) travels very fast but the energy-carriers (electrons, soil/rock particles and water molecules, respectively) do not.

While a battery maintains a constant voltage, an electrical load, such as a light bulb, resistor, or motor, increases the resistance and decreases the current through the circuit. If no load is present, the current could overload the wire.

We can think of a circuit with absolute minimum load, or resistance, as a very thick wire attached to a large battery. There is almost nothing to reduce current, so a very large flow of electrons goes through the wire and the battery itself. The wire will heat up and the battery will experience increased internal resistance, as too many electrons push into one electrode and out the other one.

This can cause the battery to heat up and possibly explode, shown right. This is the danger when you accidently cross the wires when you jumpstart a car battery. If you connect the electrodes positive-to-negative and negative-to-positive, you create a circuit of a high voltage battery connected by thick wire. If we use a too thin wire instead of a thick one, we still have a dangerous problem: The wire won't be able to handle all the current going through it. A smaller wire has higher resistance than a larger diameter wire, but too much current will cause it to break down and fail. It will create such intense friction, as too many electrons try to flow through it at once, that it may melt or explode, shown below.

A highly charged thundercloud will eventually discharge through any path of least resistance. As we will see, it will make its own "wire." And when it does, there will be very little resistance (load) to slow down the rate of discharge. The lightning channel that will eventually form is surprisingly small in diameter. It is like a very thin wire attached to a very high voltage battery with no load - a powerful, and potentially deadly, electrical phenomenon.

Lightning Is A Static Discharge

The thundercloud is building up charge but no lightning has struck yet. There is no electrical current. At this point we are really talking about the build-up of static electrical charge. Static electricity is a build-up of charge in a material. There is very little flow of electrons, or current, as the charge builds up. There is, however, significant voltage.

You have probably experienced a consequence of static electricity first-hand - an electrical discharge. When you walk along a carpet floor, rubbing your feet against it, and then touch someone else you might feel (and perhaps see) a tiny spark, especially in the winter when the air is dry. The spark is called an electrostatic discharge. Lighting is an example of a gigantic spark.

Static Electricity Is A Separation Of Electrical Charge

When you rub your feet in the carpet, some of the outermost electrons in the carpet atoms are removed from it and deposited onto your feet as your feet and carpet rub together. The rubbing increases the contact (electron exchange opportunity) between the two materials. Your feet (your whole body actually) build up negative charge while the carpet becomes positively charged. You are building up electric potential energy just like a battery. When you touch someone (who is neutrally charged) the electrons will travel out through the point of nearest contact (your finger for example) into that person because there is an electric potential difference between you two. You might even see a spark cross the tiny gap between your finger and the other person.

Air is a very good electrical insulator, a material whose electric charges do not flow freely, but the tiny bit of air between your finger and the other person's body reaches a point where it can no longer provide enough electrical insulation between the increasing potential difference between the two regions of charge. There is high enough electric potential energy to overcome the resistance of the air. Current (the spark) flows and returns the system to a state of electrical equilibrium (lowering the potential energy). Molecules of water vapour in air allow electrons to move through the air more easily. In humid air, your feet still build up negative charge but water is an excellent conductor so excess electrons on your body can enter microscopic water droplets in the air and be carried away, dissipating the charge before it builds up very much. This is why electronic equipment is more likely to be damaged by electrostatic discharge when the air is dry. When air is very dry, charge can build up to a damaging voltage level before it discharges. There is no "water droplet" release valve.

You might wonder then, why the humid air around a storm cloud doesn't allow the charge do dissipate in the same way, before lighting can form. Some charge certainly does dissipate this way, as it does around waterfalls. Water turbulence allows the mist around waterfalls to acquire a negative charge that dissipates as the mist spreads through the air. Water particle movement in the air around a thunderstorm is not sufficient to undo the enormous charge build-up inside the cloud. Powerful drafts around and within a developing storm promote charge build-up.

A thunderstorm dies when the charge-building updraft mechanism weakens and is overwhelmed by downdrafts. The thunderstorm loses energy and dissipates. Charged water particles carry off and dissipate any residual charge, as opposite charges recombine once again into neutral atoms and molecules.

Why some materials, like your feet, get negatively charged and other materials, like carpet, get positively charged has to do with how the electrons are arranged in the specific atoms and molecules in those materials.

Electrons in atoms are arranged in energy shells. Only electrons in the outermost shell can be exchanged between atoms. These electrons are furthest away from the positive nucleus so they are not so tightly electrostatically bound to it. Other nearby atomic nuclei may offer enough attraction to them to get them to "jump ship." In some materials (carpet), they can be removed, especially if another material, which tends to attract electrons (feet), is in contact. Some of the atoms in your feet have sparsely populated outermost electron shells that would be more stable if one or more electrons were added to them. They attract electrons from other materials. This effect is called the triboelectric effect.

The triboelectric series, shown left, tells you which materials tend to lose electrons (positive) and which materials tend to gain them (negative).

When your feet rub against the carpet, you increase your electric potential energy (voltage). Your body wants to shed its excess electrons and it will do that if something at a lower voltage gets close enough. A Van de Graaff generator, shown below right, present in many high school physics classrooms, generates electrostatic charge, just like your feet did.

(GDFL; Wikipedia)
A demonstration model like this one can attain a potential difference of hundreds of thousands of volts, but you can touch it harmlessly because the maximum current is very tiny. As a pulley drives a belt, a positively charged tiny metal comb at the bottom sloughs electrons off the belt (usually silk, which loses electrons easily), so that the belt is positively charged by the time it reaches the top. A tiny metal comb at the top allows electrons to flow away from the outer metal dome, leaving it positively charged. When a negatively charged wand is brought close enough to the dome, a spark will jump between it and the dome, as shown below.

Lightning is exactly the same phenomenon as the carpet and Van de Graaff generator examples, but on a much larger scale. In this case, charge separation occurs within just one kind of material - water molecules. Most researchers believe that as water drops or ice pellets fall through rising drops and pellets in a storm cloud, they experience a great deal of contact with one another. Although the mechanism(s) isn't fully understood, falling water molecules somehow gain electrons at the expense of rising water molecules. This builds up a large charge separation within the cloud, with the bottom of it becoming negatively charged and the top of it becoming positively charged.

Now that we have a handle on where electricity comes from and how it works, let's focus again on the thundercloud. Lightning is just about to strike.

Most researchers agree that water molecule-molecule contact is the root cause of charge build-up in a thundercloud. However, what kinds of movement are involved, and what kinds of objects (raindrops or rain against ice, snow or sleet) are involved, is not well understood. A scientific review paper by physicist and atmospheric physicist Clive Saunders (2008) offers several mechanism possibilities.

The top of a thunderstorm cloud, which is technically called a cumulonimbus cloud, climbs rapidly up to around 6000 m (here in Alberta) and to over 23,000 m in the tropics where the air column itself extends much higher in the atmosphere. A typical rising cumulonimbus cloud looks like the one below.


At this high altitude, any liquid water present freezes and the frozen portion at the top of the cloud tends to become positively charged versus the lower (liquid) portion. Water and ice molecules behave differently in a charged environment. Although water is just one material, two of its physical states - liquid water and ice - differ very slightly in their triboelectric nature. This means that water tends to accept or lose electrons depending on its physical state. This is well documented but it seems to be very complex. Researchers also know that the molecules in ice are less densely packed than they are in water. This is why water expands when it freezes. It also means that ice exhibits something called higher static charge permeability, between 10 and 100 times higher than water due to an effect called proton-hopping. This difference could be connected in some way to cloud charging, but the mechanisms involved are far from understood.

Cloud Charge Separation Generates An Electric Field

The entire thundercloud region contains an electric field, where the potential energy of the field at any point is the electric potential energy measured at that point. Charge separation always creates an electric potential and an electric field. The field is generally negative at the bottom of the cloud and positive at the top of the cloud, but it's not necessarily uniform or straight up and down. Cloud-to-cloud lightning may occur across a roughly horizontal electric field, for example. Electrical charge is concentrated around curved objects, as we'll see, and that may add curves to the electric field. Here we are focused on cloud-to-ground lightning so this field can be represented as a series of parallel lines, as shown below right (imagine the cloud superimposed on the diagram).

The direction of cloud's electric field is downward, shown by the arrows.
An electric field exerts a force on charged particles. The black arrows on the lines right represent the direction of the force on a (test) point of positive charge. As negative charge accumulates in the bottom of the thundercloud, electrons experience an upward force. If the cloud-charging mechanism suddenly stopped, they would simply migrate upward while positive ions migrate downward and the charges would neutralize. But the charging mechanism is going full-blast, so the electrons can't move upward. Eventually, however, these electrons are going to go somewhere. They will stream down to the ground instead. Why? The electric field is negative at the bottom of the cloud, so it induces a positive charge on the surface of the ground, as shown below left. Electrons here rush deeper underground, leaving an over-abundance of positive ions at the ground surface.

Beneath the cloud, the electric field direction is reversed. Electrons want to go downward. The strength of the field's force directly depends on the amount of charge build-up. Under a storm cloud, the electric field and the electric potential become very powerful. This is why you might feel the hairs on your arms standing up. Your hair is like the carpet mentioned earlier. In an electric field, it tends to become positively charged, so it will "repel" the ground and be attracted to the underside of the cloud above you, and lift up. This is also a warning sign that you are about to be struck! It means you are in a region of extreme positive charge and lightning could strike you within seconds.

Air Resists Lightning

When charge separation builds up, the electric potential energy grows. Like any system in physics, there is a built-in push toward finding the lowest energy state possible, and nature wants to find that now. It can do it by creating an electrical discharge from the cloud to the ground. All the excess electrons in the bottom of the cloud can simply flow down into the ground, neutralizing both the cloud and the ground and eliminating the electrical potential. All this would be far less dramatic (and there would be no lightning) if air cooperated with this plan, but it doesn't.

The gas molecules in air strongly resist any movement of charge through them. The reason they resist is because the molecules created when they bond exhibit very stable electron configurations. 99% of air is composed of the diatomic gases nitrogen (N2) and oxygen (O2). Electrons are not free to move between these molecules, and they are not easily transferred between them, as they are between water molecules.

An electrical discharge is a temporary flow of electrons (or current) through a material. There is no way for electrons to flow through air when its electrons stay stubbornly fixed in place within the molecules. This makes air an excellent electrical insulator. Resistivity is a measure of how strongly a material resists electric current flowing through it.

This picture can change, however. Like any insulator, air can experience electrical breakdown if it is subjected to a sufficiently intense electric field.

The figure shown below illustrates what happens when a block of Plexiglass ® is subjected to intense voltage (an intense electric field).

(Berk Hickman;Wikipedia)

Plexiglass ® is normally a good insulator. It has a resistivity of 1 x 1013 ohm metres, versus air which varies between 1 and 3 x 1016 ohm metres. Plexiglass ® strongly resists any movement of charge through it. When high enough voltage (a strong enough electric field) is applied to it, it begins to break down. The electrical breakdown of the Plexiglass ® creates a beautiful hair-like discharge pattern that is thought to grow finer and finer, extending all the way to the molecular level. All the white "hairs" are where the Plexiglass ® has broken down into conductive material. It is also where an electrical discharge eventually flowed. Notice the similarity between this pattern and the fine branching of many lightning bolts.

When air is subjected to a sufficiently intense electric field, finely branched paths within it break down and become conducting paths. It does this through a series of remarkable steps.

Changes in Air Set The Stage For A Lightning Strike

Air is an electrical insulator and it exhibits a high dielectric strength. The dielectric strength of a material is similar to, but different from, its resistivity. It is a measure of the maximum electric field strength a material or gas can withstand before it breaks down and its electrical insulating properties fail. The dielectric strength of air is affected by dust, temperature, pressure and water vapour. The dielectric strength of the typical warm moist air under a thundercloud is about 3 million volts/m. Increased air pressure, for example, increases this value, while humidity (water vapour content) decreases it (remember that water is a good conductor). Tiny differences in the dielectric strength of air due to dust, humidity fluctuations, etc., might account for the intricate jagged path that lightning often takes, as if it is stepping from region to region across the sky (while generally traveling in the direction of the electric field) where resistance to its discharge flow is minimized.

We can call the cloud and the ground - two charged bodies separated by distance - electrodes. The shape of the two electrodes involved affects electric discharge. Pointy objects, like your finger and someone else's sharp elbow, make good electrodes, while flat surfaces don't. The cloud bottom and the ground are both generally poor electrodes due to their flatness, but tall pointy trees, lightning rods and the wing tips of planes, for example, change the picture. They encourage electric discharge by allowing charge to concentrate in one spot, as shown right.

This rough diagram demonstrates how a pointy or curved surface locally enhances the strength of the electric field, and the voltage, in that region and it encourages electric discharge - a spark or lightning - to take place there.

Now we're ready to get to the most exciting part,  in Lightning Part 3: The Lightning Bolt.


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