Electrostatics and the Electron

Background Information

Lightning

It is a well-known fact that when certain combinations of materials are rubbed against one another they become electrically charged. These materials may then attract small, light objects which are placed near them. Similarly, objects which are electrically insulated from their surroundings may become charged.

Two charged objects may either attract or repel one another. This may be explained in terms of two types of charge: positive and negative. Two objects with the same type of charge repel; two objects with different types of charge attract.

Electrical charges can exert forces on objects with which they are not in contact; their influence crosses space. This influence is called an electric field. Its strength at any point depends on the size of the charge and the distance from it.

Atoms are electrically neutral, because they have equal numbers of electrons and protons. Each electron carries one unit of negative charge, and each proton an equal unit of positive charge.

When two different materials are rubbed against each other, electrons move from one material to the other. In this way, one material becomes positively charged and the other negatively charged. There is then a potential difference (measured in volts) between them. If these two materials are subsequently joined by a conducting material, making a complete circuit, a current (measured in amps) will flow until there is no longer a potential difference between them.

Although the atmosphere consists mostly of uncharged molecules, there are always some charged particles (ions) and free electrons present. Ions are influenced by electric fields, and accelerated by them.

Strong fields can give these particles sufficient kinetic energy to 'knock' electrons off other molecules and atoms, creating even more positive ions and more free electrons. These in turn continue the process, which is accompanied by the release of energy as radiation and sound. The air is said to have become ionised; a current flows and a spark may be seen.

The electric field strength needed to produce this effect in dry air is of the order of three million volts per metre. Damp air is a better conductor, so slightly weaker fields suffice in this case. Very great quantities of electrical potential energy are converted into kinetic energy in a very short time, giving rise to intense local heating.

Temperatures can reach 3 x 10⁴ K, which is hotter than the surface of the Sun. The radiation released is the flash of lightning. Because the temperature increases so rapidly, the air expands explosively, generating a shock wave: the thunderclap.

Investigations of thunderstorms have often involved flying aircraft through them. Results have confirmed the presence of both large amounts of charge and very strong electric fields, but the mechanism by which the charge is produced is not fully understood.

One explanation suggests a process of charge transfer between relatively large pieces of ice falling through the cloud, and the mixture of ice crystals and water droplets through which they fall. These latter become positively charged, and the lumps of ice negatively charged. The bottom of the cloud becomes negative, the top positive.

A typical thundercloud might produce an electric field of about 20,000 volts per metre, which is less than that required for ionisation to occur. However, the distance between the ground and the cloud base will vary between high peaks and low flat regions. As a result, a larger gradient of potential, and correspondingly stronger electric field, is produced. The charge on a conductor is more concentrated where the curvature is greater. Hence fields become more intense near pointed objects, which is why trees and church steeples are often struck by lightning.

There is, though, film and photographic evidence suggesting that lightning begins inside the clouds themselves. Two stages have been identified: the 'leader' and the 'return stroke'.

The leader begins in a zone of high field strength when some air molecules become charged and an 'avalanche' of ionisation begins that creates a conducting channel in the direction of the local field. This intensifies the field at the end of the channel, causing further ionisation ahead. Thus the lightning is seen as a series of jagged strikes made up of short straight lines.

At the same time ionisation occurs at the points close to the earth where the field is strongest. Channels of charged particles moving upwards are created. If the upward and downward channels meet, about 30 coulombs of charge are transferred to the earth in about 20 milliseconds, giving a severe lightning strike (a current of 30/(20x10^-3) = 1500 amps flows).

The sudden transfer of energy produces intense local heating, the air rapidly expands, and the resulting shock wave is the thunderclap we hear.