Asymmetrical and Dust Charging
Electrical phenomena associated with sand and dust storms, blizzards and falling snow have been reported for centuries. Explosions from coal dusts in mines, flour, chocolate powder and grain dust in silos pose hazards. And in some volcanic eruptions lightning in dust clouds, that is, in largely non-phreatic ejectamenta has received some investigation.
The electrification of what I will collectively call dust clouds was noticed in the 18th century when the study of static electricity became a primary interest. Amongst the investigations of the Rev Abraham Bennet, using his new invention of the gold leaf electrometer was what happened when a small pile of dust placed on the cap of the electrometer was blown away. The leaves diverged.
On the hypothetical side various charging mechanisms were proposed to account for how friction between materials produced a separation of electrical charges.
In the early twentieth century Frictional Electrification was given the name Tribo-electrification. ‘A rose by any other name would smell as sweet’ W. Shakespeare A peculiarity of human nature. If you cannot explain it –change its name.
Throughout this Electro-magnetic Miscellany I have for the most part deliberately excluded Mathematics. This qualitative approach is to give you both the results of some experimentation and to help you either repeat experiments, devise your own OR take up an investigation to take further something which has taken your fancy.
I will now précis one series of static electrification experiments made
in the early twentieth century by Professor Peter. S. Shaw of
One basic notion which came from the goodness knows how many frictional experiments over two or more centuries was that unlike materials were needed to produce positively and negatively charged objects; glass and silk, ebonite and fur, nylon (carpet) and shoe leather, chocolate and tooth enamel and on and on.
Attempts were made to form a Tribo-electric series. To as it were parallel static electrification of various materials in a similar manner to the series of metals in electro-chemical cells.
This was in part successful but the temperamental behaviour of electrostatics brought, and brings, in controversy as to which materials should be placed where in the series i.e. in the list.
Peter Shaw set out to make yet another attempt at a Tribo-electric series and discovered something of basic importance. Namely that like materials rubbed in a particular way will also generate unlike polarities of electrical charge. (It may be that this had been seen earlier but Shaw made an intensive study of the phenomena.)
This is my version of Shaw’s apparatus. A conductor/ tin with ends removed and smoothed, rests on two vertical Perspex insulators. A charged sample – in this case illustrated by a glass rod – is placed inside but not touching the conductive cylinder. The cylinder is connected to an electrometer – (in my work –it was a modern Kelvin Quadrant Electrometer made by WPA Instruments of Cambridge, England).
The whole of the above is placed, when in use, inside a Faraday cage to prevent extraneous charges from outside affecting the experiment.
Shaw investigated glass, ebonite and ice as his main materials. I used Polystyrene, Perspex and glass.
It is important to prepare clean material from the same sample, in the form of two rods or bars.
One piece is rubbed asymmetrically on the other.
Hold a rod in each hand. Rub a small area of Rod A along most of the length of Rod B. Do this several times. Carefully place a rod inside the conducting cylinder and note polarity. Do this again with the other rod.
The polarity on one rod is opposite to that on the other.
REMEMBER this is electrostatics! Most of the time things work but often, when you least expect it, they do not; usually when you are demonstrating an effect to someone else as I have just theoretically done for you.
If you examine the surfaces which have been in contact wear is apparent. I found glass to be very temperamental, due I suspect to humidity. Shaw prepared his glass samples meticulously as you may read in his paper. Evenso he had his moments as we might say!
I dare say that this experiment would, within error of measurement limitations, demonstrate that equal but opposite charges exist on the rods.
What is important is the fact that:-
Like Materials will, when in asymmetric contact, generate unlike electrical polarities of charge.
P.E Shaw & C.S.Jex. Tribo-electricity and friction. Proc. Roy. Soc.A Vol 3,758.1926
P.E.Shaw. Experiments on tribo-electricity. 1. The tribo-electric series. Vol 94, 656,
P.E.Shaw & C.S.Jex.Tribo-electricity and friction.II. Glass & Solid Elements. Vol 118, 779, March 1928.
As above III. Solid Elements & Textiles. Vol.118, 779, March 1928.
P.E.Shaw. The Electrical Charges from Like Solids. Nature, Letters, Nov.6 1926.
P.E.Shaw. Electrical Separation Between
Identical Solid Surfaces. Physics & Physical Soc. Papers.
and friction.IV. Electricity due to Air Blown
Particles. Proc.Roy. Soc. A. Vol 122. 789.
Douglas Rudge. On the Electrification Produced during the
Raising of a Cloud of Dust. Proc. Roy. Soc. Vol. 90. 618.
I will mention just these two papers but there must be very many more by now. Back in the 1970s Kamra was investigating Indian Dust Storms and Mills used dry sand in an evacuated flask to demonstrate the possible source of lightning on Mars.
A reasonably simple set up for investigating dust electrification is this one:-
This was my initial apparatus – shown without its Faraday cage.
The horizontal copper tube is 2 inches diameter, which gives you an idea of the scale. The small copper funnel, on the left, was in a stand at the left hand end of the tube. Air was supplied by a reversed vacuum cleaner. A measured weight of powder was poured into the funnel and blown along the pipe into the large vertical cylinder. Air left at the top and dust settled to the bottom which could be on an earthed metal plate or a sheet of insulator.
Various experiments were tried. For like on like material the whole apparatus was lined with paper first coated with the kind of powder to be used. For simple trials it was used as shown here.
Some kinds of powder used.
Graphite Aluminium Zinc Copper Titanium dioxide
Silicon Silicon dioxide Sand Pumice
Dust from Vesuvius Coal dust Flour Silicon carbide
The basic powders were ANALAR from a chemical supply company, others were collected in the field.
( Be very wary of metal powders and flour they can ignite. Wear a mask to prevent inhalation.)
In the first trials the simple leaf electrometer worked to show that it was worth all the effort! Later and using very small quantities of dust a quadrant electrometer was used in place of the simple leaf electrometer.
Using like on like demonstrated that in practice asymmetrical charging occurred and in consequence of which if a dust cloud were to be constituted of only one kind of powder it will charge with electricity.