Rutherford

(From "Recollections of Lord Rutherford" by Academician P. Kapitsa)

Rutherford created the modern study of radioactivity. He was the first to understand that it is the spontaneous disintegration of the atoms of radioactive elements. He was the first to produce the artificial disintegration of the nuc­leus and finally he was the first to discover that the atom has a planetary system.

From research into radioactivity grew up an independ­ent science now called nuclear physics. Both nuclear ener­gy and the use of artificial radioactivity in science and technology are developing quickly and simultaneously. All this for the last thirty years grew out of one modest domain of physics which in the old times was called radioactivity, its father being justly called Rutherford.

There are numerous books and articles on Rutherford as a scientist. Everybody knows the simplicity and clarity of his thinking, his great intuition and great temperament to be very characteristic of his creative ability.

When at the beginning of our century Rutherford started studying radioactivity, these phenomena had al­ready been proved experimentally to contradict the most fundamental law of nature, the law of conservation of energy.

The explanation of radioactivity Rutherford gave, name­ly the disintegration of matter, at once provided not only the key to the understanding of these phenomena, but also led all investigation in right direction. The same thing hap­pened when Rutherford created the planetary model of the atom.

At first sight this model completely contradicted the laws of classical electrodynamics since in its circular mo­tion an electron was perpetually bound lose by radiation its kinetic energy. But the experiments of scattering the alpha particles definitely showed the existence of a heavy nucleus in the centre of the atom. Rutherford imagined the

collision of particles so clearly that even those contradic­tions could not prevent him from establishing the planeta­ry structure of the atom.

We know that only three years later N. Bohr, on the basis of developing quantum theory of light, evolved his brilliant theory of the structure of the atom, this not only justifying Rutherford's planetary model but also quantitatively explaining the spectra of atomic radiation.

Rutherford's finest and simplest experiments concerned the phenomena of scattering by nuclear collision. The me­thods of observation of scintillations by counters were worked out by Rutherford in collaboration with H. Qeiger.

The present development of nuclear physics is proceeding not by the invention of new experimental possibilities of investigating nuclear phenomena but thanks to the pos­sibilities of investigating nuclear collisions of a larger number of elements, these collisions being studied in the domain of larger energies reached mainly by the use of po­werful modern accelerators. But even now the way lead­ing to the knowledge of the nucleus is still the method dis­covered by Rutherford, and he was the first to appreciate its fundamental value. Rutherford always liked to say "Smash the atom".

Ohm's Law

Georg Ohm (1787—1854) was a German physicist. His enunciation of the law in 1827 aroused sucn bitter antagonism that he lost his position. Years later, when his work was corroborated by other scientists, he was honored by a profes­sorship in physics at the University of Munich. Ohm stated his law having no reliable voltmeters, ammeters or batteries. He employed thermocouples to generate currents.

What is an ohm? Every electrical conductor opposes the passage of electric charges through it. This opposition arises because of the moving charges colliding with the atomic nuclei and other particles of the conductor. In so doing, the moving charges give up energy, which appears as heat. According to Ohm's law, electrical resistance is the ratio of the potential difference to the current for a conductor at a given temperature. The ohm, the practi­cal unit of resistance, is defined in terms of the ampere and the volt, as follows:

One ohm is the resistance of a conductor through which the current is 1 ampere when the potential difference across the ends of the conductor is 1 volt.

One ohm equals 1 volt per ampere.

This is the well known and/fundamental law in electricity which makes it possible to determine the current flowing through a circuit when the resistance in the circuit and the potential difference applied to it are known? What Ohm discovered was that the ratio of the potential differ­ence between the ends of a metallic conductor and the current flowing through the metallic conductor is a constant. The proportionality constant is the electrical resistance.

Using Ohm's law is of great importance because of its being generally applied to so many electrical phenomena. One of its simplest applications is using a dry cell directly connected by wires to a small light bulb. The battery maintains a potential difference of 1.5 volts across the lamp. The electron current flowing through the circuit being 0.5 am­pere, the resistance of the circuit is

Although the resistance as found here is assumed to be the resistance of the light bulb, it really includes the re­sistance of the connecting wires, as well as the resistance of the battery. In practice one usually uses wires of sufficiently low resistance that they can' be neglected in most calculations. If they are not small, they cannot be neglected and must be added in as part of the R in Ohm's law.

Although electromotive force and potential difference are both measured in volts there is a real distinction be­tween them. Electromotive force is defined as the work per unit charge done by the battery or generator on the charges in moving them around the circuit. Potential difference between two points is defined as the work per unit charge, done by the charges in moving from one point to the other.

If any two of the three quantities: resistance, current and potential difference are known for a circuit, the third can always be determined by substituting in Ohm's law. In other words, any one of the three factors may be the un­known, and Ohm's law may be written in any one of three ways: