Jethalal S. Zaveri
Prof. Muni Mahendra Kumar
Fundamental thing to know about the subatomic particles is that every particle of the same species looks exactly alike. Every electron, proton and neutron looks exactly like every other electron, proton and neutron respectively.
Mass: Particles of different types, however, can be recognized by their distinguishing characteristics. Mass is the first distinguishing characteristic. A proton has about 1980 times more mass than an electron. (This does not mean that a proton is 1800 times larger than electron. A kilogram of iron has the same mass as a kilogram of cotton.) Mass of a particle at rest is called its rest mass. The mass of a moving particle increases with its velocity and at 99% of the speed of light it is seven times larger than the rest mass. At velocities above 997c of the speed of light, particle-masses increase dramatically. An electron volt is a unit of energy but it is also used for measuring a particle's mass (the energy that an electron gains from an electric field of one volt is called an electron-volt). Thus the rest mass of an electron is 0.51 million-electron volts (Mev) while the rest mass of a proton is 938.2 Mev. It is customary to use the mass of an electron as a unit. This arrangement makes the mass of a proton 1836.12 and the mass of a neutron 1837. By this system it is easy to sec how much heavier a particle is than an electron. A photon has zero rest mass, and is, therefore, called massless particle. ("Massless' is actually a clumsy translation from the language of mathematics to English language.)
All its energy is the energy of motion at the speed of light. It can neither be slowed down nor made to run faster.
Electric Charge: The second characteristic of a subatomic particle is its charge. Every subatomic particle has either a positive charge or a negative charge or is neutral. Its charge determines how the particle will behave in the presence of other particles. A neutral particle will be utterly indifferent to all other particles. Two positively charged or negatively charged particles will repel each other and will put as much distance between them as possible. On the contrary, a negatively charged particle and a positively charged particle will be irresistibly attracted to each other and will move nearer to it if they can.
A subatomic particle can have zero charge (neutral) or 1 unit, either positive or negative, or in certain instances 2 units of charge, but nothing in between. No particle can have 1.25,1.5 or 1.7 units of charge. In other words, electric charge is also quantized like energy, and all the charge quanta are of the same size. A particle with mass and charge emerges as a particle- personality.
Spin: The third characteristic of a subatomic particle is its 'spin.' A particle spins about a theoretical axis at exactly the same rate, neither slower nor faster. The spin of a particle is related to, but not identical to, our everyday concept of a spin of a top because it does not have any well-defined axis. Like every phenomenon in quantum mechanics, spin is also discontinuous i.e. quantized like energy and charge.
The spin of a subatomic particle is calculated in terms of angular momentum which depends upon the mass, size and rate of rotation of a spinning object. Although spin is a major characteristic, it is too complicated to explain in the present work, We shall try to explain spin in a slightly different way.
A particle of spin zero (o) is like a dot and looks the same from every direction. A particle of spin (1) looks different from opposite directions (like an arrow) but looks the same if turned round a complete revolution (360 degrees). A particle of spin two (2) is like a double-headed arrow, and looks the same if turned round 180°. But a particle of spin 1/2 has to be turned two complete revolutions for looking the same. Particles of spin 1/2 make up the matter in the universe (proton, electron, etc.); particles of spin 0,1, and 2 give rise to forces between matter-particles. In quantum physics, forces or interaction between matter-particles (spin 1/2) are all supposed to be carried by particles of integrated spins -0,1,2. These force-carrying particles can be grouped into four categories as we have seen. Graviton, a particle of spin 2, carries the force of gravity. Electromagnetic force is carried by photons, the massless particle of spin 1. The strong nuclear force is earned by another spin-1 particle called 'gluon' which interacts only with itself and with quarks.
