Lightning in physics: explanation of the atmospheric phenomenon, the difference between lightning, thunderbolts and thunderbolts and the different interpretations given throughout history.
LIGHTNING IN HISTORY AND ANCIENT LITERATURE
“ Then Jupiter thundered from all the heavens, and hurled the fatal three-headed thunderbolt. First the lofty crest flew far away; then the bronzed shield fell to the ground, and the undaunted body is all fire .” (Statius’ Thebaid, Book X, the death of Capaneus).
“ And that same one, who had noticed that I was asking my leader about him, cried out: ‘As I was alive, so am I dead. If Jove tires out his blacksmith, from whom in his anger he took the sharp thunderbolt with which I was struck on the last day; or if he tires out the others one by one on Mount Etna at the black forge, calling out ‘Good Vulcan, help, help!’, as he did at the battle of Phlegra, and shot at me with all his strength, he could not have joyful vengeance .’ (Dante’s Divine Comedy, Canto XIV of the Inferno).
The two introductory passages represent only two of the possible examples taken from literature that illustrate what the ancients thought of the phenomenon of lightning that has always generated the curiosity of man in every age and in every place. Both passages (by Statius and by Dante, who takes up the same episode) speak of Capaneus, one of the seven fighters against Thebes who, having climbed victorious on the walls of the city, having challenged Jupiter sure of his strength and invincibility, was struck by lightning by the god.
In fact , lightning has always had a particular fascination for man, who, for centuries, thought it was a divine manifestation.
In Greco-Roman mythology , for example, lightning was considered to be the arrows of Jupiter hurled against mortals; the term saetta , synonymous with lightning, derives from the Latin word sagitta, which means arrow.
In Norse mythology, on the contrary, lightning was seen as the sparks produced by the striking of Thor’s hammer on an anvil. Not therefore an interpretation of merit, but rather an analogy with what the blacksmiths of the time must have observed when forging their tools and weapons. Curiously, with this similarity, the peoples of the North came closer to the real nature of lightning , that is, that of a large “atmospheric spark”.
“Natural philosophers”, over the centuries, made various hypotheses to give a real cause to lightning . The philosopher Empedocles (490-430 BC), argued that lightning was a part of the sunlight captured by the densest clouds which, with a crash, managed to free itself from its trap.
Anaxagoras (500 – 426 BC), on the contrary, argued that lightning was a part of the aether, a tenuous substance that filled the heavens where the planets were located , drawn down and made to fall into the material world. Aristotle (384 – 322 BC) argued that lightning was the result of a dry exhalation that was released from the clouds following the condensation of air into water.
This exhalation, said Aristotle, was “expelled from the densest part of the cloud downwards like seeds that spring from the fingers [when we try to crush them]”. The impact of the dry exhalation against the surrounding clouds was, again according to Aristotle, the cause of thunder. Lucretius (98-55 BC), in his ” De rerum natura “, embracing the atomistic theory of Democritus of Abdera , considered lightning to be due to the movement of very small and light particles that were able to pass even through material objects. With this hypothesis Lucretius also explained the fires sometimes started by lightning even inside houses. Thunder and lightning , again according to Lucretius, had a common cause but were independent: the impact between the clouds caused both the rumble (thunder) and the liberation of the light atoms that went on to form the lightning .
LIGHTNING: SIMPLE EXPLANATION – FRANKLIN EXPERIMENT
A decisive step forward in the explanation of lightning was made by B. Franklin (1706-1790) who was able to devote himself to his scientific passion, after having achieved a certain wealth following his typographic activity. He founded the American Physical Society and in particular studied electrostatic phenomena after having come into contact, in 1746, with the first electrical experiments of a Scotsman visiting America; from that moment he himself carried out continuous experiments developing personal theories that earned him the election as a foreign member of the Royal Society of London.
At a time when electrostatic knowledge was not yet fully understood and phenomena such as fire, combustion and lightning were confused, he supposed that lightning was nothing more than a gigantic electric spark. He demonstrated this hypothesis through an experiment: Franklin , in fact, wanted to demonstrate the electrical nature of lightning and, assisted by his son William, flew a kite equipped with a metal tip and connected to the ground by a silk thread. During the storm the tip of the kite became charged with electricity and Franklin verified its presence by bringing his hand close to a key tied to the thread at man’s height. In this way the scientist closed the circuit formed between the kite-(metal tip)-thread-ground and perceived the passage of current through his body. Only later did he realise the danger of his experiment, when he learned of the death of his Swedish colleague GW Richmann (1711-1753) who had attempted to repeat the experiment, being struck by an electric discharge much more intense than the one Franklin felt.
TYPES OF LIGHTNING IN PHYSICS
In the Earth’s atmosphere there are electric charges whose origins, characteristics and concentrations vary according to altitude and originate from an ionization effect of atmospheric molecules.
Ionization consists in the production of an electron, negative, and a positive ion of much greater mass and the ionizing agents, that is, those responsible for this process, are: ultraviolet radiation and X-rays from the Sun, cosmic rays, high -energy particles present in the solar wind or from external sources, the planetary system and natural terrestrial radioactivity. In the lower atmosphere, free electrons are rapidly captured by neutral molecules, forming negative ions. Up to an altitude of about 70 km, the negative electric charge is predominantly made up of ions. As the altitude increases further, the percentage of free electrons increases considerably until around 100 km the negative charge is almost entirely made up of electrons.
As a first approximation, in a given volume the charges of opposite signs are present in equal numbers and thus neutralize each other. However, in small volumes there may be significant deviations from neutrality. The presence of electric charges gives the atmosphere a certain conductivity σ, i.e. the ability to conduct electric current, which varies with altitude.
In the upper part of the atmosphere, where the rarefaction of the air and its ionization are greatest, the resistivity drops to extremely low values, and in comparison to the resistivity of the intervening column of air, the surface and ionosphere can be considered as two perfectly conductive surfaces.
There is also a difference in charge between the Earth’s surface and the upper atmosphere, which, together with the variations in conductivity, is responsible for both an electric field , whose value, averaged over the entire Earth’s surface, ranges from 120 to 360 V/m depending on atmospheric pollution , and for a current along the vertical. It is estimated that there is a constant current of 1800 A between the atmosphere and the Earth.
DIFFERENCE BETWEEN LIGHTNING, LIGHTNING AND LIGHTNING BOLT
During thunderstorms, however, this situation varies considerably and lightning can be generated . These are sudden and violent electrical discharges that occur between a cloud and the Earth’s surface, or between two clouds, due to large potential differences that are created, mainly during thunderstorms, in the atmosphere . The phenomenon manifests itself with a light effect ( lightning ) and a sound effect ( thunder ) that are not perceived simultaneously by the observer due to the different speeds of propagation of light (300,000 km/s) and sound (vs = 340 m/s): the lightning is seen almost instantaneously, while the thunder is heard after a time interval Δt proportional to the distance S of the lightning from the observer. In fact, we will have that S = vs Δt.
LIGHTNING IN PHYSICS: THE ELECTRIC PHENOMENON
In general, a lightning bolt is composed of a main branch and many secondary branches , with the characteristic zigzag appearance due to the fact that the electrical discharge moves along the path with less electrical resistance. The length can reach 2-3 km, with peaks of 5 km. When they occur between clouds, they can instead reach 10-15 km.
It is now known that large thunderclouds (cumulonimbus clouds) are positively charged in their uppermost part and negatively charged in their lowermost part. There are several theories that attempt to justify this situation. The most widely accepted hypothesis is that these charges are formed by the continuous friction between the ice particles, present in the upper part of the clouds where the temperature is lower, and the water particles in the lower layers that are pushed upwards by internal convective motions (warmer and more humid air upwards and colder air downwards). It is believed that the smaller particles tend to acquire positive charges, while the larger ones acquire negative charges. These particles tend to separate due to the effect of ascending currents and the force of gravity , until the cloud assumes the electrical state previously described (positive at the top and negative at the bottom). Because of this separation of charges between the top and the bottom of the cumulonimbus, enormous potential differences are produced both inside the cloud and between the cloud and the ground, which by induction tends to become positively charged.
If, for simplicity, we assume that this induction is complete, then this situation can be represented schematically with the plates of a spherical capacitor with an insulating material inside. A potential difference, indicated by ∆V, is therefore created between the plates for the accumulation of charge Q. The proportionality relationship exists between these two quantities: Q = C ∆V, C is called the capacitance of the capacitor. For a spherical capacitor, if we call R1 and R2 the radii of the internal and external plates respectively, we have Co = 4πε0 R1 R2/( R2- R1).
We indicate with Co, the capacitance of the capacitor if there is a vacuum between the plates and it depends only on the geometry of the system. If, however, an insulating material is placed between the plates, we observe that, for the same Q, a smaller ∆V is recorded. This implies that the capacitance of a capacitor with a dielectric inside, Ci, is greater than Co. The ratio Ci/Co is called the relative dielectric constant of the dielectric (εr) considered and therefore provides the value by which the electric potential, and the electric field, are less due to the presence of the insulating material itself. In general, at normal pressure, the constant εr for air differs little from 1. The decrease in potential in a capacitor occurs because in the absence of an external electric field E the dielectric is overall neutral, but when it is in the presence of a potential difference the phenomenon of polarization occurs.
The electric field E exerts a force on the internal charges of the atoms of the dielectric causing a deformation of the electron clouds rotating around the nucleus transforming it into a small dipole (strain polarization).
In some cases, some substances have molecules that already have a non-symmetrical charge distribution and are therefore already small dipoles, however, since these dipoles are oriented randomly, the overall dielectric is neutral.
The presence of an external electric field, however, orients these small dipoles all in the same direction, producing an overall dipole moment other than zero (polarization by orientation). In both cases, the electric field produced by polarization in the dielectric is arranged so as to oppose the electric field generated by the accumulation of charges on the plates. The result is that if this capacitor is inserted into a circuit, a smaller flow of electric current is recorded. This polarization is directly proportional to that of the electric field E, however there is a maximum value of E beyond which the dielectric loses its insulating properties. Beyond this value, which is called dielectric strength, is measured in V/m and is different for each material, an electric discharge occurs which can also lead to the destruction of the substance.
In the case of clean and dry air, the value of the dielectric strength is approximately 3 MV/m, which drops significantly, to values lower than 0.3-0.4 MV/m, in the presence of humidity, atmospheric dust or other impurities. Lightning is the atmospheric equivalent of the phenomenon previously described; in this case the dielectric is the air and the two conducting bodies are the cloud and the ground, or two different clouds or two different parts of the same cloud. The potential differences in this system can reach tens or hundreds of millions of volts, causing the dielectric strength of the air to be exceeded: at this moment, lightning strikes .
HOW MANY KW DOES A LIGHTNING STROKE DISCHARGE?
The discharge mechanism is however quite complex and can be divided into two parts:
- Initially, a weak and invisible discharge (pilot discharge ) composed of negatively charged particles descends from the cloud towards the ground at a speed of about 100 km/s, which moves along successive paths of short length (about 50 m). Along this zig-zag path, an intense ionization is created which predisposes to the second phase.
- When the pilot discharge approaches the ground, a “return” discharge starts from the ground, which can last a few tens or hundreds of microseconds, directed upwards and composed of a flow of positive charges present on the Earth’s surface. When the two discharges meet, the cloud-ground circuit is closed and an electric current passes along this path toward the cloud at a speed estimated at about a third of the speed of light. The current illuminates the ionized channel that had remained dark until then, thus generating the classic luminous streak. During the passage of the current, there is a sudden change in temperature and density in the ionized channel left by the pilot discharge. This sudden change originates a pressure wave that propagates and is perceived as thunder. The ionized charge channel has a diameter of a few centimeters, while the temperature reaches 30,000 K.
THE PHASES OF A LIGHTNING STRIKE
The various phases of a lightning strike are illustrated schematically . From left:
- fig a) the pilot discharge of negative charges starts from the base of the cloud and heads towards the positively charged ground;
- fig. b) the return discharge of positive charges starts from the ground;
- fig c) when the two discharges meet the circuit closes and a current flows and the actual lightning is produced.
The conducting channel, created by the guiding discharge, can branch into several branches, along which several return discharges can occur, giving the lightning an appearance similar to the roots of a plant.
Each cloud-to-ground lightning flash is actually made up of multiple components. Often, in fact, after the first discharge, another leading discharge can occur, in the same wake of the first, which triggers a second lightning flash. This can occur multiple times in one or two seconds, causing the flickering effect in the flash light. The individual components last a few milliseconds and are called strokes. In some cases, a process similar to lightning can occur but with reverse directions: a flow of positive charges starts from below (ascending channel) and meets the charges coming from the cloud (descending channel), helping to close the path. In some extreme cases, the ascending channel can reach the cloud before meeting a descending channel, thus generating an ascending lightning flash.