# Current Affairs 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 machine called a spinneret. These filaments are extremely fine and pliable threads, even when they harden. Monofilaments are fibres made from molecules that have the same length as the fibre.   Rubber (Natural Rubber) We get rubber from "Hevea brasiliensis", a tree. The produce of this tree is "Latex", which is natural rubber. A natural polymer which possesses elastic properties, also termed as elastomer, is manufactured from rubber latex. Elastomer is a linear polymer of isoprene (2-methyl-1, 3 butadiene)   Vulcanisation of Rubber Natural rubber becomes soft at high temperatures (> 335 K) and brittle at low temperatures (< 228 K) shows high water-absorption capacity. It is soluble in non-polar solvents; is non-resistant to attack by oxidising agents. Raw rubber is heated with a mixture of sulphur and an appropriate additive, at a temperature range of 373 K to 415 K. On vulcanisation, sulphur forms cross-links at the reactive sites of the double bonds, and thus, rubber gets stiffened. For example, in the manufacture of tyre rubber, 5% of sulphur is used as a cross-linking agent.

#### 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 volume and 75% by weight. It is slightly lighter than air and only slightly soluble in water. It is used for filling electric bulbs, for making fertilisers, ammonia, nitrates, etc.
Ozone
• It is an allotropic form of oxygen containing three atoms in the molecule and is formed when oxygen or air is subjected to silent electric charge. It is bluish gas, very active chemically, and a powerful oxidizing agent.
• It is found in the upper atmosphere some 25 to 40 km from the Earth's surface, called ozonosphere. It is this layer which absorbs a large proportion of the Sun's ultraviolet radiation. Ozone is used for purifying air and water and in bleaching.
Carbon
• It is a universal constituent of living matter. It can be mainly classified into two forms allotropic form and amorphous form. Diamond and graphite are two of its allotropic forms whereas charcoal, lamp black, coke, etc. belong to amorphous form. Carbon atoms are capable of uniting with each other to form very large molecules upon which life is based.
Diamond
• It is the hardest naturally occurring substance. It is transparent to X-rays only. It is very costly and used in jewellery, drilling and cutting tools. It can be cut only by a diamond.
Graphite
• It is soft, easily powdered and gives a greasy feeling. It is a good conductor of heat and electricity. Used in lead pencils, electrical machines and as lubricant for heavy machines. Also used as a moderator in nuclear reactors.
Coal
• Over long periods of time, trees, bones get buried under the ground by violent geological changes. As a result of chemical reaction with clay sand, water, etc., these get transformed to coal in nature. This process is known as carbonisation. Due to this process, we get substances like peat, lignite, bituminous (soft) and anthracite (hard), depending upon degree of carbonisation.
Organic Compounds
• Constitute substances like petroleum, coal, food components (protein, fats, carbohydrates, vitamins), more...

#### 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 the mirror.
• The focal length of plane mirror is infinity because there is no divergence or convergence and run parallel forever. The focal length of plane mirror does not exist.
• Emergency vehicles are usually reverse-lettered so the lettering appears normal in the rear view mirror of a car.
• The minimum mirror height necessary to see one's full image is of half-length mirror.
• The point on the mirror surface, where the incident ray strikes is called point of incidence.
• The ray which is directed towards the reflecting surface is called the incident ray.
• The ray which is sent back into the medium is called the reflected ray.
• The perpendicular drawn at the point of incidence to the surface of mirror is called normal.
• Angle between the incident ray and normal is called angle of incident.
• Angle between the reflected ray and normal is called angle of reflection.
Spherical Mirror
• A spherical mirror is actually a mirror whose surface forms a part of a hollow sphere. The sphere can be made from any type of polished metal surface like glass.
• Convex Mirror: A mirror with a spherical surface and reflecting from the exterior of the curvature is called a convex mirror. A convex mirror is also known as Diverging Mirror as it diverges the incident rays after reflection.
• Concave Mirror: A mirror with a spherical surface and reflecting from the interior of the curvature is called a concave mirror. A concave mirror is also known as Converging Mirror as it converges the incident rays after reflection.
Reflection at Spherical Surface
• The geometric centre of a spherical mirror is called its pole.
• The centre of the hollow sphere for which the mirror is a part, is called the centre of curvature.
• The line joining the centre of curvature and the pole is the principle axis.
• A light ray incident of a spherical mirror, after reflection appears to pass through the principal focus in the case of a convex mirror and passes through the focus in the case of concave mirror. more...

#### 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 = 760 mmHg = 101.3 kpa
• The mercury barometer and other manometer devices are reliable pressure measurement devices. A "rising" barometer indicates increasing air pressure i.e. calm weather, a "falling" barometer indicates decreasing air pressure, i.e., inclement weather or harsh weather.
• Factors Affecting the Magnitude of Pressure
• Magnitude of the force: The larger the force, the higher the pressure.
• Contact area: The larger the contact area, the lower the pressure.
• The pressure at any level in the atmosphere may be interpreted as the total weight of the air above a unit area at any elevation.
Pascal's Principle
• Pascal's Principle is used to quantitatively relate the pressure at two points in an incompressible static fluid.
• It states that pressure is transmitted, undiminished, in a closed static fluid.
• The total pressure at any point within an incompressible, static fluid is equal to the sum of the applied pressure at any point in that fluid and the hydrostatic pressure changes due to a difference in height within that fluid.
Applications
• The underlying principle of the hydraulic pressure.
• Used for amplifying the force of the driver's foot in the braking system of most cars and trucks.
• Used in artesian wells, water towers, and dams.
• Scuba divers must understand this principle. At a depth of 10 metres under water, pressure is twice the atmospheric pressure at sea level, and increases by about 105 kPa for each increase of 10 m depth.
Surface Tension (T)
• Surface Tension (T): It is the property of a liquid by virtue of which it behaves like an elastically stretched membrane with a tendency to contract so as to occupy a minimum surface area.
• Mathematically $T=\frac{F}{L}$
F = Force per unit length L = Length over which force acts
• I. Unit is : $N{{m}^{-\,1}}$ and Dimensional formula is ${{M}^{1}}{{L}^{0}}{{T}^{-\,2}}$
Surface Energy
• The energy responsible for the phenomenon of surface tension may be thought of as approximately equivalent to more...

#### 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 from each other. Such large distances are expressed in light years. For example, the distance between the earth and the star named Alpha Centauri is 4.3 light years (approximately 40,000,000,000,000 km). ·                     All stars appear to move from east to west but the pole star appears to be stationary from the Earth as it is situated close to the direction of the axis of rotation of the Earth. ·                     The groups of stars that appear to form recognisable shapes are called constellations. Ursa Major, Ursa Minor. Great Bear, Orion, Cassiopeia are some well-known constellations. ·                     The solar system consists of eight planets, numerous moons, and lots of asteroids, comets and meteors. ·                     A body revolving around another body is called a satellite. Satellites are of two types - (i) natural and (ii) artificial ·                     Moon is the natural satellite of the Earth. Some other planets also have natural satellites. ·                     The satellites made by man are called artificial satellites. These artificial satellites revolve around the earth. They are used for weather forecasting, remote sensing and various other purposes. ·                     Mercury is the planet nearest to sun. ·                     The brightest planet in the night sky is Venus. It is also the hottest planet of our solar system. ·                     The largest planet of the solar system is Jupiter. ·                     Saturn is the sixth planet from the sun and the second largest planet in the solar system, after Jupiter It has a prominent ring system. It is less dense than water.

#### 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 of light is incident on a wall or wood, which is not smooth or polished. ·                     In case of irregular reflection, the different portions of the surface reflect the incident light in different directions and thus no definite image is formed, but the surface becomes visible. It is commonly known as scattering of light. ·                     Light always follows the law of reflection, whether the reflection occurs off a curved surface or off a flat surface. Two laws of reflection are: §  The angle of incidence is equal to the angle of reflection. §  Incident ray, reflected ray and the normal drawn at the point of incidence to the reflecting surface, lie m the same plane. ·                     Image formed by a plane mirror is laterally inverted i.e. the left of the object appears in right and the right of the object appears in left. ·                     Multiple images are formed when two plane mirrors inclined to each other ·                     A kaleidoscope is a cylinder made with two or more mirrors. Light reflecting between these mirrors produces multiple virtual images of stunning beauty. ·                     Sunlight, called white light, consists of seven colours. ·                     When white light is made to pass through prism, it splits into its constituent colours. This phenomenon is known as dispersion. ·                     Eye is an important organ which helps us in seeing the beauty of nature. The important parts of the eye are cornea/ iris, pupil, lens, retina and optic nerve. ·                     Retina contains two types of nerve cells ?
 (i) rods and (ii) cones.
·                     Cones more...

#### 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. ·                     Like charges repel and unlike charges attract each other. ·                     An electroscope is an early scientific instrument that is used to detect the presence and magnitude of electric charge on a body. ·                     The process of transfer of charge from a charged body to the earth is called earthing, ·                     The process of electric discharge between clouds and the earth or between different clouds causes lightning. ·                     Earthing is provided in buildings to protect us from electrical shocks due to any leakage of electrical current. ·                     Lightning conductors installed at the top of the buildings protect them from lightning by allowing the discharge to go through them to the earth. ·                     A sudden shaking or trembling of the earth is called an earthquake. It is caused by disturbances occur deep inside the earth's crust. ·                     Despite of the advancement of scientific technologies, it is not possible to predict the occurrence of an earthquake. ·                     Earthquakes generally tend to occur at the boundaries of earth's plates. These boundaries are called as fault zones. ·                     A seismograph is the device that scientists use to measure earthquakes. ·                     The power of an earthquake is expressed in terms of magnitude on a scale called Richter scale. It is a logarithmic scale. ·                     The earthquake measuring 0-2 is considered as mild while an earthquake having intensity 7 or more can cause severe damage to life and property. ·                     We should take necessary precautions to protect ourselves from earthquakes.

#### Notes - Chemical Effects of Electric Current

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 liquid causes chemical reactions. The resulting effects are called chemical effects of currents.   ·                     Deposition of precipitate, change of colour and formation of bubbles are some of the chemical effects of electric current.   ·                     Electroplating is one of the most important applications of electric current.   ·                     The process of depositing a layer of any desired metal on another material, by means of electricity, is called electroplating.

#### 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 (iii) inner ear.
·                     The sound enters our ear through outer ear and travels down to eardrum present in the inner ear through a canal. The eardrum senses the vibrations of sound and sends the signals to the brain, in this way we hear the sound. ·                     The frequency of sound is expressed in hertz (Hz). The number of oscillations or vibrations per second is called the frequency of oscillation. ·                     Loudness of sound is proportional to the square of the amplitude of the vibration producing the sound. Thus larger the amplitude of vibration louder is the sound. It is expressed in a unit called decibel. ·                     The frequency determines the shrillness or pitch of a sound. Higher the frequency of vibration, the higher is the pitch, and shriller is the sound. ·                     Low frequency vibrations produce low-pitched sound which is loud to hear For example, a drum vibrates with low frequency and thus produces low pitched sound and thus it gives loud sound. ·                     High frequency vibrations produce high-pitched sound which is feeble to hear For example chirping of a bird. ·                     Human ear can detect the sounds of frequencies ranging from 20 to 20,000 Hz. These sounds are called audible. ·                     The sounds above or below this frequency level, are not audible to human ear. Such sounds are called inaudible. ·                     The sounds of frequencies below 20 Hz are called infrasonic sounds. ·                     The sounds of frequencies above 20 Hz are called ultrasonic sounds. ·                     Some animals can hear sounds inaudible to human ears. The animals like dolphins and elephants communicate to each other through the sounds of frequencies below 20 Hz. ·                     The sound which is pleasing to our ears is called more...

#### Notes - Friction

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 to overcome friction at the instant an object starts moving from rest is a measure of static friction. The maximum value of static friction, when motion is impending, is sometimes referred to as limiting friction. ·                     The force required to keep the object moving with the same speed is a measure of sliding friction. It comes into play when an object is sliding over another. Sliding friction is smaller than static friction. ·                     When one body rolls over another body, rolling friction comes into play. Rolling friction is smaller than the sliding friction. ·                     Friction is a necessary evil. If friction force is not present, it becomes almost impossible to stop a moving object. ·                     Friction is sometimes useful and sometimes it is undesirable. ·                     Friction can be increased by
 (i) treading (ii) scratching (iii) making a surface rough.
·                     The sole of the shoes and the tyres of the vehicle are treaded to increase friction. ·                     Friction can be reduced by
 (i) using lubricants (ii) using wheels (iii) using ball bearings in machines.
·                     Fluids exert force of friction on objects in motion through them. The frictional force exerted by fluids is called drag. ·                     The frictional force on an object in a fluid depends on three factors ? more...

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