Introduction
You have learnt in the earlier chapters that flow of electrons constitutes an electric current and current can be generated from a battery or a generator. Scientists later discovered that there are many other ways of producing the stream of electrons.
Thermionic Emission
The emission of particles from a hot filament was first discovered by Thomas Edison in 1883 but the effect was left unexplained until the discovery of the electron by J.J. Thomson in 1897.
Cathode Rays
When the pressure in the discharge tube is less than 10-4 mm of Hg, the discharge tube starts showing fluorescence. When this fluorescence was investigated, it was found that the fluorescence consisted of beams of negatively charged electrons. These electrons emanate normally from the cathode. As these emanate from the cathode, the rays are called the Cathode Rays.
Discovery of Radioactivity
The discovery of the phenomenon of radioactivity was purely accidental. In 1896, Henry Becquerel, a French scientist accidentally found that in presence of salt of uranium, photographic plates got heavily fogged even though they were wrapped in opaque paper. He concluded that the uranium salt must be giving off penetrating radiations similar to X-rays discovered one year earlier. Two years later, Madam Curie named the phenomenon radioactivity.
General Characteristics of Natural Radioactivity
It is independent of external factors such as pressure, temperature, state of substance, electrical field, magnetic field, catalyst etc.
Radioactive Decay or Disintegration
A radioactive nucleus or element emits an alpha or a beta particle and gets converted into a new nucleus or element.
Radioisotopes
A nuclide is any species of atom of which each atom has an identical proton number and also an identical nucleon number. Different nuclides, which have the same proton number (but different nucleon numbers) are called isotopes (isotopic nuclides).
Background Radiation
In astronomy, the low temperature microwave radiation that arrives at the earth's surface from all directions of outer space is called background radiation. It is so named because it forms a background to all the radio sources that have been detected by radio telescopes.
Nuclear Stability
Surprisingly, the mass of the nucleus is less than the sum of the masses of the individual protons and neutrons which makeup the nucleus. The lost mass (mass defect) has been changed into the energy necessary to bind the nucleus together.
Nuclear Reactors
A nuclear reactor is a device for obtaining and using the energy from a controlled nuclear chain reaction. Controlled means the rate or speed of the nuclear fissions can be changed by the operator. Most reactors are constructed in a similar manner. The six main parts of a nuclear reactor are fuel, moderator, control rods, coolant, heat exchanger, and safety shields.
Hazards and Safety Measures of Radiations
The first and only atomic bombs were used in war was in 1945 when Hiroshima and Nagasaki were devastated. Widespread destruction and damage were produced immediately by the blast.
Radioactive Waste
Waste products from nuclear power stations etc. are becoming a serious problem. They should be put where the radiation can do no harm. Unfortunately, there is no way of stopping a radioactive nucleus from emitting radiation.
Nuclear Energy and Chemical Energy
Nuclear energy is released when very small particles of matter split or combine. After the split or combination, they result in small particles or heavy particle. The rest of the mass is changed into energy. This process of changing mass into energy produces the energy.
Summary
When certain metals are heated to a high temperature, they emit thermions (electrons) and the phenomenon is called thermionic emission
