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8th Class

NCERT Summary - Polymers

Polymers   Polymers are high molecular mass compounds and their structure is composed of a large number of simple repeating units. The repeating units are usually obtained from low molecular mass compounds called monomers. Polymers have very vast spectrum of properties and that is why they are very important for different applications.   Examples of Plastics Polyethylene terephthalate-PET or PETE High-density polyethylene-HDPE Poly vinyl chloride-PVC Polypropylene-PP Polystyrene-PS Low-density polyethylene-LDPE   Fibres A fibre is a continuous polymer which, when spun into thread, makes fabric. The polymer strands line up close to one another, forming strong bonds between stands. If a fibre is pulled, it tightens the bonds, often making the fibre stronger and harder to break. Natural fibres, such as cotton or wool, come from plants and animals. Synthetic fibres, such as nylon and polyester, are chemically manufactured from petroleum-based substances by drawing softened plastic through small holes in a more...
Catgeory: 8th Class

NCERT Summary - Elements and Compounds

Elements and Compounds   Hydrogen
  • It is colourless, odourless, tasteless, inflammable and lightest known substance (gas). It is found in water\[{{H}_{2}}O\], organic compounds and all living things. It is neutral to litmus.
  • It can be produced in the laboratory by Bosch process and by electrolysis. It is used in balloons, ships, for ammonia and vanaspati ghee preparation, etc.
  Oxygen
  • It is colourless, tasteless, odourless, combustible, slightly heavier than air and somewhat soluble in water. Atmospheric air contains oxygen by about 21% by weight. It can not only be prepared in the laboratory but also in factories on commercial scale. It can be liquefied and solidified. It is employed in welding process and also used in hospitals for artificial respiration.
  Nitrogen
  • It is colourless, tasteless, odourless, non-combustible, inactive, non-poisonous gas, forming about 80% of the atmospheric air by more...
Catgeory: 8th Class

NCERT Summary - Reflection of Light

Reflection of Light  
  • A ray of light, the incident ray, travels in a medium.
  • When it encounters a boundary with a second medium, part of the incident ray is reflected back into the first medium.
  • This means it is directed backward into the first medium.
  Laws of Reflection of Light
  • The angle of incidence and angle of reflection are equal.
  • The incident ray, reflected ray and normal to the point of reflection lie in the same plane.
  Reflection of Light in a Plane Mirror
  • The image formed by a plane mirror is always virtual, upright, and of the same shape and size as the object it is reflecting.
  • When an object is placed in front of a plane-mirror, an image is formed behind the mirror at a distance equal to the distance of the object from more...
Catgeory: 8th Class

NCERT Summary - Pressure

Pressure  
  • Pressure is defined as the force acting normally per unit area.
  • Pressure on an area, A is the area of the surface on contact and F is the normal force applied.
    • Pressure is a scalar quantity.
\[P\,=\,\left( F/A \right)\]
  • If the force is due to the weight (W) of the object, the equation is then:
\[P\,=\,W\,/\,A\]
  • Hydrostatic pressure is the pressure that is executed by a fluid at equilibrium at a given point within the fluid, due to the force of gravity.
  • Hydrostatic pressure equation is P = hdg where P = pressure \[(N/{{M}^{2}}\,or\,Pa)\], h =height (m), d = density\[(kg/{{m}^{3}})\], g = acceleration due to gravity \[(9.81\text{ }m/{{s}^{2}})\]
    • The SI unit for pressure is the pascal. Pa, where \[1\,Pa=1\,N/{{m}^{2}}.\]
    • The standard atmospheric pressure is measured in various units.
1 Atmosphere more...
Catgeory: 8th Class

Notes - Stars and the Solar System

  Stars and the Solar System   ·                     The stars, the planets, the moon and many other objects in the sky are called celestial objects. ·                     The moon looks different everyday when seen from a particular place. The various shapes of the bright part of the moon as seen during a month are called phases of the moon. The phases of the moon occur because we can see only that part of the moon which reflects the light of the sun towards us. ·                     Phases of moon occur due to the movement of moon around the earth. ·                     Stars emit light of their own while planets do not have their own light. ·                     Celestial bodies are generally situated very far more...
Catgeory: 8th Class

Notes - Light

  Light   ·                     When light falls on a surface, a part of it reflects back. If light falls on a polished and shiny surface, most of its part reflect back in the same medium. There are two types of reflection of light ?
(i) regular reflection and
(ii) irregular reflection or diffused reflection.
·                     Regular reflection takes place when a ray of light is incident on a polished smooth surface like a mirror. Here the reflected ray of light moves only in a fixed direction. ·                     Irregular reflection or diffused reflection takes place when a ray more...
Catgeory: 8th Class

Notes - Some Natural Phenomenon

  Some Natural Phenomenon   ·                     Lightning is a huge electric spark. A lightning strike could destroy life and property. ·                     Benjamin Franklin, in 1752, showed that lightning is associated with electricity. ·                     Objects can be charged by rubbing with other objects. ·                     The electrical charges generated by rubbing are static. They do not move by themselves. When charges move, they constitute an electric current. ·                     There are two kinds of charges - positive charge and negative charge. ·                     When a glass rod is rubbed with silk, it is said to acquire positive charge. In the same way, when a glass rod is rubbed with wool, it is said to acquire negative charge. ·  Chemical Effects of Electric Current   ·                     The materials, which allow electric current to pass through them, are railed good conductors of electricity.   ·                     The materials, which do not allow electric current to pass through them are called poor conductors of electricity.   ·                     Some liquids are also good conductors of electricity and some are poor conductors.   ·                     Most liquids that conduct electricity are solutions of acids, bases and salts.   ·                     LED glows even when a weak electric current flows through it. There are two wires, called leads, attached to an LED, One lead is slightly longer than the other.   ·                     The passage of an electric current through a conducting more...
Catgeory: 8th Class

Notes - Sound

  Sound   ·                     Sound helps us to communicate with each other. It is produced by vibrating objects. In human beings, the vibration of the vocal cords produces sound. ·                     The to and fro or back and forth motion of an object is termed as vibration. ·                     Sound needs a medium to travel, it cannot travel in vacuum. ·                     We hear sound through our ears. ·                     Human ear has three parts ?
(i) Outer ear,
(ii) Middle ear and
  Friction   ·                     Friction is a force that opposes the motion. It acts on both the surfaces. ·                     Friction depends on the nature of surfaces in contact. ·                     The factors that affect friction are:
(a) the condition of the surfaces in contact and
(b) the weight of an object.
·                     For a given pair of surfaces friction depends upon the state of smoothness of those surfaces. ·                     Friction depends on how hard the two surfaces press together. ·                     The force required more...
Catgeory: 8th Class

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