Riding the Tornado
Over 20 years ago, I decided to play with a school science experiment, and placed a plastic cup salt on the top of a Van de Graf generator – just to see what would happen. I was expecting to see salt particles becomecharged from the generator, and then scatter as each grain of salt repelledevery other charged grain as, according to conventional electrostatic theory,the particles should always repel each other as they move away from thecharged dome.
Instead, to my puzzlement, frequently, a stream of salt grains leaves the pot of salt and forms a coherent stream, that suddenly breaks down and disperses at some distance from the dome.
This can be easily replicated, and demonstrated for the camera with the salt stream brightly lit against a black background. The experiment is best done in a large container to prevent contamination of cameras and equipment with the dust!
Since the salt grains had not behaved as I had predicted, I assumed that it was my own understanding of the electrostatic theory that I was teaching that was at fault, and I puzzled over this for some time before realising that I had made a simple mistake. At the time I assumed that the oversight was mine alone, and I modified my teaching of electrostatics to removethis error.
Some years later I was puzzled at the lack of a simple and effectiveexplanation of tornados in text books, and realised that maybe I was notalone in having this oversight. It was an oversight common to almost allour scientists! ( except plasma physicists )
It is easy for a teacher to see why this theory has been overlooked.Most of the world teaches its children science in the same way, using thesame apparatus and experiments. It is not easy for an adult to overturnthe compartmentalisation of their concepts of science, built into the foundationsof their understanding of electricity and magnetism.
Children learn about static electrical charge through rubbing rods of plastics and wood. They learn that there are two charges, positive and negative, and they learn that opposite charges attract each other and the same charges repel each other.
They may also have seen the charge build up on the top of a Van de Graf generator and discharge as a bright spark.
Then they go on to study electricity, and learn how electrons move in wires. They learn about conductors and non-conductors / insulators.
They learn how the electrons moving in a wire produce an electromagnetic field round the wire, and how the electromagnetic fields in two wires where the electrons flow the same way will attract each other. Next they maywrap the insulated wire into a coil round an iron nail to produce an electromagnet. All this is correct, but these children, and the adults they have growninto, scientist or laymen alike, have stored this information in two separateboxes. They understand the static electricity of charge, and they understandthe dynamic electricity of wires and electromagnetic fields.
But my theory is a simple observation of the dynamics of a mass flowcontaining moving charge.
Consider the demonstration that is actually used to give a definition of the unit of force: the Newton: a 1 amp current flowing through each of two wires 1m long and 1m apart produces a force of attraction of 2*10^-7N
Now suppose that the wires are replaced by two Perspex tubes, each 1m long and again 1m apart. Now blow negatively charged dust in the same direction through both tubes. If we charge the dust sufficiently and blow it withsufficient velocity, again we can produce a current of 1 amp through eachtube.
( You cannot actually do this- this is a thought experiment using valuesthat are easy to calculate!. Note also that the charge is carried on dust,not as free electrons. The velocity of the dust is equivalent to the driftvelocity of the electrons in a wire, and like the wire, there are also alot of positive charges.)
A 1 amp current flowing through each of two tubes 1m long and 1m apart again produces a force of attraction of 2*10^-7N
Now if you consider these two charge streams in isolation from anythingelse, you will calculate that the force of repulsion between them is manyorders of magnitude higher, and will dominate. The important aspect toremember that this is taking place within a cloud of similarly charged dust.For the sake of this exercise consider the cloud to be infinite in size.The force of repulsion between the charge flowing through the tubes is balancedby the same charge in all directions outside the tubes - the net electrostatic force is zero! The only force that remains is the force of attraction of 2*10^-7N
This is the point where a fundamental shift in perception has to be made. A static negative charge will repel another static negative charge. Thenegative charge is the result of extra electrons attached to the dust particle.
But a moving electron will create an electromagnetic field round itself, so a moving dust particle with its attached electrons will create an electromagnetic field. So a group of rapidly moving dust particles, all with a negativecharge, all moving in a similar direction, will create electromagnetic fieldsthat produce a force of attraction between the particles. (just as in parallel conducting wires).
In a plasma the movement of a strong current can cause pinching of the discharge because of the intense force of attraction produced (Z pinch). The strong forces can also be seen to attract parallel discharge threads.
(Foundations of Electromagnetic Theory, R. Milford)
We make effective use of these properties to control moving electrons in the beams that produce the picture in a TV tube - but here the elctronsare not moving attached to a particle, they are moving on their own, andas their is no relative motion between the electromagnetic fields producedby these electrons, they do not interract in the same way
The second phenomenon that we need to consider is the way that dipoles behavein an electric field. In a water droplet in an electric field, some of thecharge is able to move in reponse to an external field. Negative charge movingtowards the more positive direction of the field. A particle that isnot a good conductor will turn to bring its most negative part to face thepositive of the field.
A cloud of dust that behaves as dipoles in a charge field will develop aforce that pulls the positive end of one dipole towards the negative endof adjacent dipoles. This can cause water droplets to aggregate into largerdroplets, but where the dipole particles are fine, the friction (collisions)with the surrounding air molecules limits their ability to travel throughthe air - they will take the air with them when they try to move. A cloudof dipole dust in an electric field should try and contract laterally asthe dipoles adopt a least energy configuration.
A very effective demonstration is of salt particles streaming between charged plates – best in a Perspex container to prevent contamination of cameras. It looks quite spectacular with occasional sparking between the plates. The charge from one plate is transferred by contact onto the saltgrains. On the salt grain the charge forms a dipole with the charge fromthe plate furthest from the plate. When there is enough charge on the grainfor the repulsion of the same charge on the far surface of the grain to begreater than the attarction of the opposite charge on the near surface ofthe grain, the particle flys away towards the opposite plate. As the saltgrains stream away, these dipoles orientate themselves so that the positivecharge of one dipole is attracted to the negative charge on the other dipolesnearest it. This attraction keeps the charged grains from flying apart asthey move. This requires an external electric field to maintain the dipoles.If the external field weakens, the grains are no longer dipoles, and theslight negative charge of each now repels the surrounding grains- the grainsnow fly apart.
So to summarize this basic law of electrodynamics:
1 ) Two stationary particles carrying the same electrical charge willrepel each other, but within an electric field they will form dipolesthat will experience an attractive force.
2 ) In a large charge cloud where the repulsion between the charges is thesame in all directions, this force will cancel out
3 ) Two streams of dipoles with the same field charge movingin parallel in a large charge cloud will also develop a force of attractionthrough their electromagnetic interaction.
When the two streams of same-charged dipoles are moving within a chargecloud they will move together until the electromagnetic attraction due totheir motion in parallel is balanced by the electrostatic repulsion thatresults from their closer proximity. As the particles are moved faster, the dynamic forces of attraction will again be balanced by the electrostaticrepulsion, and the particles are bound in a rigid balance of force betweenthe opposing forces of attraction and repulsion. The faster the particlesmove, the more tightly bound, and closer together the particles become.They effectively form a semi-rigid lattice. (A lattice well reported fromthe literature of plasma physics!)
Most of us are familiar with an electromagnet produced by coiling aninsulated wire many times round a pencil, or a soft iron nail. When electricityflows through the coiled wire, an electromagnet is produced.
Technically the coiled wire is called a solenoid, and the magnetic field produced by the rotating charge is equal to: permeability*turns/m*current.
A similar electromagnetic field can be produced by spinning a mass of charged dipole particles, all with the same charge, round a common axis– just like a large wheel turning round the hub. We can increase the strengthof the field by increasing the density of charge or by increasingthe rate of rotation.
When a conducting wire is coiled into a helix to form a solenoid, the magnetic fields reinforce each other inside the solenoid, and counteract each other outside the solenoid.
In the same way when similar charged dipole particles move in a circularmotion, the magnetic fields will also be reinforced inside the circle ofmotion but counteract each other outside the circle of motion. The opposingforces of the tube walls moving in opposite directions repel each other,and prevent the tube collapsing in on itself, but within the charged sheathof moving particles the magnetic forces are attractive and maintain thetube. They also prevent it extending in length unless there is a greaterforce at work – the tube will have a well defined end.
The magnetic field produced by a solenoid is largely confined withina solenoid.
In the same way the magnetic field produced by similar charged particles in a circular motion will also largely be confined within the circle ofmotion.
This information is copyright Peter Thomson 2001-2004