AIK            Static electricity

This is a pretty technical subject.  I would welcome any engineer or lecturer to pick holes in anything I have said below.  Call me on cdcnottm@aol.com if you find any howlers.

Static electricity is something of a law unto itself.  Quite a lot is known on a theoretical level, but the variables are so difficult to control that it is, as I have said, 'a law unto itself'.  However, I will tell you what I know about the stuff.

'Static electricity' and 'current electricity' obey the same set of laws, but the aspects of the two types of phenomena almost suggest that they are not the same thing.  You'll see what I mean if you read on. 

Normal current electricity flows through conductors and operates things like electric light, electric heaters and provides the power for motors.  Static electricity manifests itself by charging the surface of insulators and gives that annoying habit of making bits of plastic stick to you fingers like sticky tape.

As with magnetism; like charges repel and unlike charges attract.  as with magnetism, an uncharged body acts like an opposite charge to a charged body.  If you bring a magnetic north pole close to an un-magnetised piece of iron, that latter thing assumes a south polar magnetism.  The same happens with a static charge when a positive charge will make an uncharged body into a negatively charged one.  That's why bits of plastic sheet stick to your fingers.  The bit of plastic will have been charged by rubbing of its surface.  If the plastic had have been impregnated with, say, carbon; it would not have developed the charge in the first place.  Special plastic bags are made to carry integrated circuits, as these devices are extremely sensitive to a static charge and can easily be damaged by the static that forms with normal plastic sheet.

If you get a roll of sticky tape and unroll a piece of it in a darkened room while watching the place where the adhesive is being ripped apart, you will see a fiery glow where static electricity is being generated and forming an electric arc.  The voltages generated are in the order of thousand of volts, but the current is so small that no injury will occur.

If you have worked on electronic circuitry, as I have, you will find that your are required to wear an earthling strap to stop yourself from getting charged up.  It is so easy to get charged up by, for example, walking on a manmade fibre carpet.  Fibres like nylon and rayon are very good insulators and generator static charges so easily.  And because modern integrated circuits (ICs) have such minute gaps between different parts of the circuit, a thousand volts can cause a spark and damage the IC.  A thousand volts may sound a lot, but static charges of many thousand of volts can be generated by rubbing two pieces of plastic together.  Again I say that the actual power available is minute and not dangerous to the human body.

But if you take lightning, that is a different matter.  Very large volumes of air are involved, air molecules rubbing against each other can generate hundreds of thousand of volts between, for example, two clouds.  The unit of electrical charge is the coulomb, and that is quite a large charge.  A coulomb allows one ampere to flow for one second, so if it was dissipated in a milli-second, the current would be a thousand amperes (amps).  Actually, lightning can generate many thousand of volts at many thousand of amps.  Just think of a thousand volts at a thousand amps.  That's megawatt of power.  You can see why an oak tree struck by lighting may be split in two from top to bottom.  The mechanical forces could be enormous.  As one horsepower is 746 watts, a megawatt is 1340horsepower.  Quite a punch; quite enough to split a tree down the middle.  It probably boils the sap in the tree causing the explosive force.

Now if you are unlucky enough to be struck by lightning on a golf course, you probably won't be turned into a piece of fried steak as your contact with the ground will be poor, but you will probably still be killed.

At this point it is worth giving a few tips on how to avoid being struck by lighting.  If you are out in the open and you feel your hair being pulled by an invisible force, it is likely that a high static charge is doing the pulling.  Remember that high voltage-discharges generally make for pointed objects.  Get away from the place as quick as you can while not giving anything pointed in an upwards direction.   But, if possible, don't stand under a lone tree.  Get right into the woods where many trees are available to be struck.  With fair luck it won't be the one you are under.  People have stood under a tree that has been struck, and the high voltage current has not gone direct the earth, but flashed across the the human body before making its path to earth.  You will be a much better conductor than, say, an oak tree.

As I said above, pointed objects are the most likely to be struck by lighting.  That's why a lightning conductor has a pointed top.  But actually lightning conductors are not intended to be struck, they are intended to discharge the atmosphere around the building they are protecting.  And they work!  In the rare cases where a lightning conductor is struck by lightning, it is because of a rapid build-up of charge in an insulated environment.  That's why the conductor to earth is generally heavy copper strip capable of carrying the enormous current that the strike discharges to earth.

Remember Albert Einstein.  He developed a theory of "relativity".  His theory was much wider that the subject i am dealing with, but everything in life is relative.  While we have our feet on the ground, we consider the earth (or 'ground' as the Yanks call it) to be our reference point.  But if you go up in an aircraft, the body of the plane is generally the reference point.  The body of the aircraft will become charged relative to earth as it moves through the air, but the manufactures know this and the tyres will be slightly conductive, so by the time the plane had come to a standstill on the runway, the charge on the aircraft body will be discharged.

But think a bit wider.  What about a spacecraft touching down on another celestial body!  There may be an enormous difference in the potential of the spacecraft and the local 'ground' of the other planet.  I have heard nothing on this subject regarding the moon landings, but you can bet your bottom dollar that NASA considered this matter.  The moon soil would have been a poor conductor, so the feet of the lander would allow a slow discharge to occur.  Landing on Mars will be an even bigger challenge, but I'm quite happy that NASA will be up to it.

The effects of static electricity have been known since the earliest times.  Whether the Ancient Chinese related lightning to the effects seen when two pieces of amber are rubbed together, is unlikely.  But they knew of both phenomena.  So did the Victorians; an English engineer by the name of Wimshurst  (1832 to 1903) gave his name to a machine where two revolving plates are rubbed together, and the ensuing static charge fed away by a copper brush  and a wire connected to the brush.  Nowadays the device is only used as a demonstration machine, but it can give some pretty powerful shocks.

Steam passing through small orifices will generate static electricity.  There was a case many years ago when a steam-driven pile-driver killed an operator.  The pile-driver was mounted on a rubber base, and the operator stepped off the machine onto the ground.  He was instantly electrocuted.  You have probably heard of people getting shocks off a car door handle; this is a similar occurrence to the pile-driver as the tyres are good insulators.  I haven't seen it lately, but in the 1950s and 1960s, many cars dragged a small chain along the ground to help discharge the body of the car.

Static electricity has one definite use in the electrical world.  That is in capacitors (used to be called 'condensers').  Generally a capacitor consists of two sheets of metal foil with a sheet of  insulator between the metal foils.  The keep the volume of the device as small as possible, the sheet of foil is rolled up and encased in a envelope.  Capacitors come in all sorts of sizes and shapes depending on their requirement.  In the case of a capacitor, the insulator is known as a dielectric.  The area of the two sheets and the nature of the dielectric determine the capacitance.  In certain types of capacitor known as 'electrolytic capacitors', one of the two conductors is a liquid that forms a gas where it comes into contact with the other electrode.  This gas layer is very thin and an insulator, making for a very efficient capacitor in terms of size.

Capacitors have all sorts of roles in the electrical and electronic world.  The unit if capacitance is the Farad, and is an enormous unit.  A 'microfarad' (a millionth of a farad) is a  common practical unit.  In fact a 'picofarad' is quite common in electronics.  That is a million millionth of a farad.  The largest capacitor I have ever seen was in a transformer house of a large factory in Leatherhead.  It was used for power-factor correction purposes.  And if my memory is correct, it was about a quarter of a farad.