ICSE Class 10 Physics Chapter 12 Radioactivity Revision Notes
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The free electrons in a metal if supplied with sufficient energy, from outside, leave the metal surface. The process of emission of electrons from the metal surface is called electron emission.
The minimum amount of energy required to emit electrons from a metal surface is called the work function (or threshold energy) of that metal.
It is generally expressed in electron-volt (eV). 1 eV = 1.6 x 10-19 J.
The emission of electrons from a metal surface when heat (or thermal) energy is imparted to it, is called the thermionic emission.
The electrons which are in orbits close to the nucleus are called the bound electrons.
They are tightly bound to the nucleus by the strong attractive force between the positive charge of the nucleus and negative charge of electrons.
When a large number of atoms come together to form a solid, the loosely bound electrons of the outermost orbit leave their individual atom and move freely inside the solid. These electrons are called conduction electrons.
Metals have a large number of free electrons, whereas non-metals have very few of them.
Cathode Ray Tube
The picture tube of the TV is a Cathode Ray Tube or CRT, an instrument that converts the input electrical signals into the output visual signals.
It is made up of three major assemblies:
1. Electron Gun: The electron gun has a tungsten filament, two batteries, one low tension and one high tension, a switch, variable resistance, cathode plate, control grid, and double anode cylinders.
2. Deflecting Plates: The deflecting plates consist of two pairs of flat plates. One pair of plates is kept horizontal. These are called Y- plates. The other pair of plates is placed vertically. These are called X – plates. All the plates are kept with their planes parallel to the electron beam.
3. Fluorescent Screen: The fluorescent screen is a flat glass plate placed at the end of the tube. Its inside surface is coated with a fluorescent material.
These three assemblies are placed in a long hollow evacuated glass tube.
Three principles on which hot ray cathode tube works:
- Thermionic emission
- Deflection of the electron beam by the electric and magnetic fields
- Fluorescence produced by the electron beam on a fluorescent screen
- The fluorescent screen is made up of fluorescent material such as zinc sulphide or barium platinocyanide or zinc silicate containing some traces of manganese etc.
The substances which disintegrate (or decay) by the spontaneous emission of radiations are called the radioactive substances.
Example: uranium, radium, thorium, actinium etc.
The isotopes of nearly all the elements of atomic number higher than 82 are radioactive and are called natural radioactive substances. Any physical changes or chemical changes do not alter the rate of decay of radioactive substances. Radioactivity is a nuclear phenomenon and cannot be predicted
Properties of alpha particles:
- An alpha particle consists of two protons and two neutrons.
- The mass of an alpha particle is roughly four times the mass of a proton.
- The speed of alpha particles is of the order of 107 m s-1.
- An alpha particle strongly ionises the gas through which it passes.
- An alpha particle rapidly loses its energy as it moves through a medium and therefore its penetration power is quite small.
- Alpha particles are positively charged, so they are deflected by the electric and magnetic fields.
- Alpha particles affect a photographic plate
- Alpha particles cause fluorescence on striking a fluorescent material.
- Alpha particles have large kinetic energy and momentum with them.
- Alpha particles destroy the living cells and they cause biological damage.
- Alpha particles get scattered while passing through the thin mica (or gold) foils.
Properties of beta particles:
- Beta particles are the fast-moving electrons emitted from the nucleus of an atom.
- The rest mass of a beta particle is 9.1 x 10-31 kg and its charge is – 1.6 x 10-19 C (or – e).
- Beta particles are given out from the nucleus whereas the cathode rays are given out from the orbital electrons.
- The speed of beta particles is of the order of 108 m s-1.
- Beta particles ionise the gas through which they pass.
- The penetrating power of beta particles is more than that of alpha particles.
- Beta particles are negatively charged, so they are deflected by the electric and magnetic fields.
- Beta particles affect a photographic plate.
- Beta particles cause fluorescence on striking a fluorescent material.
- Beta particles produce X-rays when they are stopped by the metals of high atomic number and high melting point such as tungsten.
- Beta particles cause greater biological damage as they can easily pass through the skin of our body.
Properties of gamma particles:
- Gamma radiations are electromagnetic waves like X-rays and light.
- The speed of gamma radiations is the same as the speed of light (in vacuum or air).
- The ionising power of gamma radiations is very low.
- The penetrating power of gamma radiations is the highest.
- Gamma radiations are not deflected by the electric and magnetic fields.
- Gamma radiations affect a photographic plate.
- Gamma radiations cause fluorescence on striking a fluorescent material.
- Gamma radiations are diffracted by crystals.
- Gamma radiations cause immense biological damage as they can easily pass through the skin of our body.
- Gamma radiations are very useful for the treatment of cancer.
- X-rays are emitted when there is a transition of elements in the inner orbits of an atom whereas gamma radiations are given out from the nucleus.
ICSE Class 10 Physics Chapter 12 Radioactivity Revision Notes PDF