Text e. The fundamental particles

All of the simple particles which exist in nature have been found to undergo reactions in which they are converted into or obtained from other particles or radiation. There are, then, no particles which can be said to be truly fundamental.

The electron was the first of the simple particles to be recognized, it being discovered by J. J. Thomson in 1897. The proton, the nucleus of the ordinary hydrogen atom, was observed as positively charged rays in a discharge tube. The nature of the rays was not at first understood. The next very simple particle to be discovered was the positron, found in 1932. The positrons were found among the particles produced by the interaction of cosmic rays with matter. They seem to be identical with electrons except that their charge is +e instead of —e. Their span of life as free particles is very short, it being less than a microsecond (1x10~⁶ sec).

The neutron is known to have been discovered by the English physicist J. Chadwick also in the year 1932. Neu­trons have been found to be particles with mass only slightly larger than that of the proton, and with zero electric charge. They having no electric charge, neutrons interact with other forms of matter only very weakly, and it is accordingly hard to prove their existence by direct method. On passage through solid substances they undergo deflection only when they approach extremely closely to nuclei, that is, when they undergo direct collisions with nuclei. Neutrons and nuclei being so small, the chance of collision is very small and neutrons are accordingly able to penetrate through great thicknesses of heavy elements.

Text f. Energy

The structure of matter cannot be explained completely in terms of material particles alone. It is necessary to make use of another concept which takes into account the relations of the particles to one another and the extent to which they are in motion. This concept is known to be Energy. A system is said to possess energy if it is able to do work. Work is done when the point of application of a force moves. If a force of 1 dyne moves 1 cm along its line of action 1 unit of work is done. This unit is known as the erg. Work is done in many processes, and may be calculated as the product of two factors if this product is expressible in ergs. For example, when a gas expands against a pressure, we state work to be done; the simplest conditions arise when the pressure is kept constant and the volume is changed, and the work then done is p (V₂—V₁) where p is the pressure (force per unit area),V₁ the initial and V2the final volume.

Energy which can be released from a system in the per­formance of work must previously have been stored in some

manner within the system. The energy of a system may depend either on the relative positions of parts of it: for instance, a raised weight has energy which can be released as work when it falls; or on the motions of parts of the sys­tem: for example, a moving body possesses energy which can be converted to work in the act of bringing the body to rest. Energy due to position is potential energy; energy due to motion is kinetic energy.