BALL LIGHTNING What is Ball Lightning? For hundreds of years there have been reports people have been reporting having seen objects between the size of grapefruits and footballs floating through the air, and then apparently disappearing. The balls sometimes appear as benign objects that just drift through walls, or else explode violently. These balls have been seen in a variety of colours although they very rarely give out much heat. This natural phenomenon, which as yet has no generally accepted explanation, is perhaps one of the most mysterious witnessed to man. It is known as Ball Lightning. Ball lightning has attracted a great deal of scientific curiosity. It has been known to appear in peoples houses, simply by just floating through the wall. The interior of aeroplanes has seemed to be a comfortable place for ball lightnig to endure it’s small lifetime. It sometimes has the ability to pass straight through windows with no apparant damage. Sometimes it appears greenish-blue in colour. Many attempts to investigate ball lightning scientifically have proved difficult and unfortunately fail most of the time. This is mainly because ball lightning does not occur regularly or to order. However, after searching through literature, it has been shown that ball lightning usually occurs during electrical storms, as with conventional lightning. It’s ability to float (usually as little as a few feet above the ground) means that ball lightning must have very little weight. Ball lightning has frequently been seen moving against the wind, at speeds up to 10 metres per second, and it’s ability to pass through some objects without causing damage, and not others means that it may not always be as hot as we would expect. As expected, all these observations and conclusions have led to numerous theories being pushed forward. Most of these theories involve electrical discharges or the presence of some fuel gas, such as methane in the atmosphere. Perhaps one of the most interesting theories would be that of the soviet Nobel prize-winner, Peter Kapitza, in 1955. He proposed that ball lightning might form at antinodes of interference phenomena among naturally generated radio waves. Theories behind Ball Lightning Theories which have been proposed in the past, apart from claims that that observations are an optical illusion (Argyle 1971), generally intend to assign an external power source to explain the constant illuminosity of the ball over periods for as long as 10s. Proposed power sources include: · a standing wave of electromagnetic radiation. This idea was first proposed by Kapitza in 1955. It was then brought up again in 1976 by Endean, and then again in 1993. · an electric cloud, powered by the electric field from a cloud. This idea was presented by Uman and Helstorm in 1966. · nuclear energy. This idea was pushed forward by Altschuler round about 1970. · antimatter. This idea was first proposed in 1971 by Ashby and Whitehead. · chemical combustion. This was suggested in 1981 by Fischer. However, it is difficult to see how these theories can explain how ball lightning can exist inside houses, and pass through glass windows. A theory that ball lightning is simply hot luminous air with trapped radiation fails because the ball would rise like a hot air balloon. One of the more recent theories is that ball lightning is a complex chemical phenomenom invloving water vapour. This was suggested by Turner in 1994. Investigations into Ball Lightning Until recently it has been difficult, even impossible to carry out definitive scientific experiments an to recreate ball lightning in the laboratory. Long lived balls that are independently mobile seem strangely reluctant to form. If they do form, they disappear again almost immediately. In the 1950s, a scientist named Nauer conducted a series of experiments into ball lightning. He tried to create fireballs by introducing an electrical discharge between a pointed electrode and a water surface. Bright orange spherical objects appeared above the water, but there structure was far too complicated to resemble ball lightning. Nauer also experimented with discharges inside a closed chamber containing hydrocarbons. He produced vague luminous shapes, but again, it was too dissimilar tom ball lightning. In the late 1960s, James Dale Barry from the University of California, Los Angeles repeated Nauers experiments to test whether the combustion of hydrocarbons might be the explanation for ball lightning. He used a bank of capacitors to produce sparks between copper electrodes, which were placed about half of a centimetre apart.He found that ay a concentration of propane of around 1.4 percent - less than that necessary for normal combustion a small yellow-green fireball appeared and whizzed around the tank before fading away. Now two Japanese physicists have described a series of experiments, which actually involves the creation of ball lightning in a natural atmosphere. This was a big step as it was soon realised that this artificial ball lightning did, in fact possess many of the properties of it’s natural counterpart. This includes it’s ability to pass through a wall intact and to move against the wind. The physicists were named YH Ohtsuka and H Ofuruton. Ohtsuka was working at the Waseda University, while Ofuruton was working at the Metropolitan College of Aeronautical Engineering. The two described a series of experiments using a 1.5kW source of microwave energy at 2.45GHz - which in effect could be thought of as a rather large microwave oven. When the power was fed into the empty cavity of the device, several different types of plasma fires were observed. Although most of the plasma fires were observed when the apparatus was switched on, they did not always occur at the expected electrical hot spots. On a few rare occasions , a glowing ball would punch a small hole in the aluminium foil that formed one end of the cavity. After emerging from the cavity it continued to glow for a few seconds, even after the power had been switched off. In another experiment, the Japanese team introduced a copper bar into the cavity. They saw that after turning on the microwave energy, a ball-shaped glow was observed moving along the bar. The team then introduced a strong blast of air in the cavity, and surprisingly enough, they found that the ball moved against the wind. All these observations suggested that they had created something very similar to natural ball lightning, and this led to the belief that the previously mentioned Peter Kapitza was on the right theoretical track 35 years ago. At the University of Bristol, a scientist named David Turner came up with a new theory. He claimed that physicists apply physics to the problem, and being a chemist, he would apply chemical thermodynamics. Turner’s speciality was the behaviour of salts in steam, which was very highly related to the theories of ball-lightning. During a thunderstorm, the air is very humid, and ions may be created when intense electric fields cause the air to “break down”. Turner used his knowledge of chemistry to model what happens around the air escaping into the surrounding cool humid air, which is electrically charged. Turner found that a structure similar to an onion’s is set up around the central plasma, with successive spherical shells dominated by different chemical processes.(See next page) The Onion-Like Structure of Ball-Lightning: Ball Lightning - Charge Motion in the Ground Charge motion during the Initial Breakdown Period Ball lightning is almost always seen immediately seen after a local lightning strike. Most lightning strikes are less than 3500A peak current and of duration of less than 1ms. In a typical lightning flash, approximately 20C of negative charge is delivered to the ground from the electric arc that constitutes lightning. The arc radius at these currents is only of the order of 1cm. This 20C produces an extremely large electric fields. The magnitude of these fields is sufficient for the electrical breakdown of most solids. The breakdown proceeds similarly to breakdown in a gas through the formation of filamentary discharges from the point of the discharge. The charge dispersion would not be spherical, but could be thought of as along cylindrical “fingers”. With this , the equation below shows how the electric field at the centre of the ends of a cylider can be calculated. where the cylinder is of radius , length ,and has uniform distribution of charge q Charge distribution following and during a lightning strike figure 1 Figure 1 shows the three principal changes in the principal changes in the distribution of electrical charges that are associated with an electric strike. 1) The development of strong negative electric charges in the base of the cloud, for example due to the interaction of wind and freezing supercooled drops in the cloud. 2) The rapid transfer of charge through the highly conducting arc of a lightning strike, positive charge going to the cloud and negative charges to the earth to distances of many metres. 3) The very much slower dispersion of negative charge along ‘fingers’ of relatively high electrical conductivity in the earth, in which the field at the head of the advancing charge will be less than 1MV cm-1. It is proposed that, in the air above this advancing charge, there will be occasions when the field will be greater than 5kV cm-1 and hence able to sustain bal lightning.

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