Current Affairs 8th Class

  Sound   Synopsis  
  • Sound is a form of energy that produces sensation of hearing in our ears.
  • Sound is produced when a body vibrates.
  • Sound needs a medium to travel. It cannot travel in vacuum.
  • Sound travels faster in solid medium (like wood) as compared to water and air.
  • All musical instruments have vibrating parts/which produce sound.
  • In human beings, sound is produced by the voice box or the larynx.
  • We hear through the vibration of our eardrum, whenever sound reaches our ears.
  • The number of oscillations per second is called the frequency of oscillation (f) and is expressed in hertz (Hz). A frquency of 1 Hz is one oscillation per second.
  • The time needed to complete one oscillation is called time period (T). Hence, the number of oscillations per unit time is the frequency of wave f = 1 /T.
  • The maximum displacement of the wave crest from the central position on either side is called its amplitude.
  • We differentiate sounds on the basis of their amplitudes and frequencies.
  • A human being can hear sound waves within a range of 20 Hz to 20,000 Hz. Some animals like dogs can hear sounds higher than 20,000 Hz.
  • The larger the amplitude of vibration, the louder is the sound.
  • Loudness of sound is proportional to the square of the amplitude of the vibration producing the sound. The loudness is expressed in a unit called decibel (dB).
  • Shrillness or the pitch of a sound depends upon its frequency. Higher the frequency, higher is the pitch and vice-versa.
  • Unwanted, unpleasant sound is noise. Loud noise produces noise pollution, which is harmful and may cause hearing impairment.
  The velocity of sound in air is approx.\[\text{330}\,\text{m}\,{{\text{s}}^{-1}}\]under standard temperature and pressure conditions. It varies with temperature, pressure, humidity and density of medium.  

  Friction   Synopsis  
  • Friction opposes the relative motion between two surfaces in contact. It acts on both the surfaces.
  • Friction depends on the following factors:
- Material of the bodies in contact. - Roughness of the two surfaces in contact. The rougher the surfaces, the greater is the friction.  
  • The force of friction is independent of the area of the two surfaces in contact.
  • The force of friction between two bodies is parallel to the contact surface and always opposite in direction to that of the relative motion.
  • Although friction is undesirable, it is important for activities like sitting, walking, braking, etc.
  • Friction in machines is reduced by using lubricants and smoothening the surfaces in contact (polishing).
  • Harmful effects of friction:
It increases wear and tear It produces heat It decreases efficiency  
  • The static friction (or limiting friction) is the friction between any two bodies when one of the bodies just tends to move or slip over the surface of another body. There is no actual movement of the body in static friction.
  • Sliding friction (or dynamic friction) comes into play when one of the bodies slides over the other.
  • When a body (like a roller or a wheel) rolls over the surface of another body (e.g., road surface), the friction is called rolling friction.
  • Rolling friction is much less than sliding friction.
  • Liquids and gases (fluids) exert much less friction as compared to solids.
  • The frictional force exerted by fluids (including air) is called drag.
  • The special shape of a body (object) to reduce drag is called streamlined shape. For e.g./birds and aeroplanes have streamlined shapes.
  • Air also exerts friction on a moving body, but it is much smaller as compared to solids and liquids.

Food Production and Management   Synopsis  
  • Agriculture is the science or practice of growing crops.
  • Plants of the same kind are grown and cultivated at one place on a large scale are called crops.
  • There are three main crop seasons -
(i) Kharif (June-September), e.g., rice, jute, maize, groundnut and cotton. (ii) Rabi (October-March), e.g., wheat, mustard, potato, barley and gram. (iii) Summer crops.  
  • The steps involved in cultivating a crop are as follows.
  • Ploughing, levelling and manuring the soil.
  • Sowing seeds at the correct depth and with right spaces between them. Some seeds are sown in nurseries and the seedlings are then transplanted to the main field.
  • Improving soil fertility by adding manure and chemical fertilizers and also by adopting methods like crop rotation and leaving the field fallow.
  • Ensuring irrigation at the right time.
  • Protecting crops from weeds, pests and diseases either by using chemicals or by using natural methods.
  • Harvesting, threshing and winnowing.
  • Legumes are often used in crop rotation, because the nitrogen fixing bacteria which live in their roots improve soil fertility.
  • Nitrogen fixation is a part of the nitrogen cycle, which is, continued cycling of nitrogen from the air to the soil and to living organisms.
  • Grains are stored in silos or god owns that have been fumigated. Buffer stock is maintained for emergencies.
  • Scientists have developed hybridisation processes to grow disease resistant varieties of plants. The earliest success is the production of high-yielding varieties of plants which led to increase in the production of food crops. This is often referred to as the Green Revolution.
  • The branch of agriculture dealing with the rearing of farm animals is called animal husbandry.
  • Animals give us milk, meat and eggs. Animal products are an excellent source of protein. Animal proteins are superior to plant proteins. Egg white contains the protein albumen.

Coal and Petroleum   Synopsis  
  • Sources of energy:
All sources of energy can be divided into two categories: (i) renewable sources of energy and (ii) non - renewable sources of energy.  
  • Renewable sources of energy: The sources of energy which are found in nature and inexhaustible are called renewable sources of energy. Some renewable sources of energy are solar energy, geothermal energy, wind energy, tidal energy, bio-energy.
  • Non-renewable sources of energy: The sources of energy which have accumulated in nature over a long period of time and cannot be quickly replaced once exhausted are called non-renewable sources of energy. Some non-renewable sources of energy are fossil fuels like coal, petroleum, natural gas.
  • Coal and its products: Coal is a complex mixture of elements like carbon, hydrogen and oxygen. Coal is formed by the process of carbonisation. Coal is one of the cheapest and widely used fuels. Anthracite, bituminous, lignite and peat are different varieties of coal. The most important variety of coal which is used as a fuel is bituminous coal. The process of heating coal in the absence of air is called destructive distillation. Destructive distillation of coal gives products like coal gas, ammonical liquor, coal tar and coke.
  • Coke obtained from coal is used to make fuel gases like water gas and producer gas.
  • Coal tar is a black, thick liquid with an unpleasant smell.
  • Coal gas is a mixture of hydrogen, methane and carbon monoxide.
  • Petroleum and its products: Economy of a nation depends, to a great extent, on its petroleum wealth. That is why petroleum is called 'Black Gold'.
  • Crude oil is a complex mixture of solid, liquid and gaseous hydrocarbons mixed with water, salt and other particles.
  • Petroleum is lighter than water and insoluble in it. Petroleum is a dark coloured, viscous and a strong smelling liquid.
  • Petroleum is extracted by drilling holes in the earth's crust, sinking pipes and pumping out the oil. Petroleum is then refined by fractional distillation. Important fractions of petroleum are gasoline, kerosene, diesel, lubricating oil, petrol, etc.
  • Coal and petroleum resources are limited, so we should use them judiciously.
  Natural gas: Natural gas is stored under high pressure as CNG which is used for power generation and fuel. It is a cleaner fuel and less polluting. It consists mainly of methane (85%), ethane (10%), propane (3%) and others.                                       

Materials: Metals and Non-metals   Synopsis  
  • Metals
  • Metals occur in combined state or in the form of compounds.
  • Most of the metals are solids at room temperature except mercury which is a liquid.
  • Most of the metals have high densities because their atoms form close clusters.
  • Most of the metals shine and can be easily polished.
  • Metals are malleable, so they can be hammered into thin sheets. They are also ductile and hence, can be drawn into thin wires.
  • Metals are sonorous because they produce sound when struck.
  • Most of the metals are good conductors of heat and electricity.
  • Metals combine with oxygen to produce basic oxides. \[4Na+{{O}_{2}}\to 2N{{a}_{2}}O\]
  • Metals react with water to form oxides and hydroxides.
\[Mg+{{H}_{2}}O\to MgO+{{H}_{2}}\]and\[2Na+2{{H}_{2}}O\to 2NaOH+{{H}_{2}}\]  
  • Metals react with dilute hydrochloric and sulphuric acids to produce respective salts and hydrogen.
\[2Na+2HCl+{{H}_{2}}\]and\[2Na+{{H}_{2}}S{{O}_{4}}\to N{{a}_{2}}S{{O}_{4}}+{{H}_{2}}\]  
  • Metals displace one another from their salts.
\[Mg+2HCl\to MgC{{l}_{2}}+{{H}_{2}}\]  
  • Metals react with chlorine to form chlorides.
\[Ca+C{{l}_{2}}\to CaC{{l}_{2}}\]    
  • Non-metals
  • Non-metals occur in Free State as well as in combined state.
  • Many non-metals are solids or gases at room temperature.
  • Non-metals look dull. Many of them are coloured.
  • Non-metals are neither malleable nor ductile.
  • Non-metals do not produce sound when struck.
  • Non-metals are bad conductors of heat and electricity except graphite.
  • Non-metals combine with oxygen to produce acidic or neutral oxides.
\[2{{N}_{2}}+{{O}_{2}}\to 2{{N}_{2}}O\]and \[S+{{O}_{2}}\to S{{O}_{2}}\]  
  • Non-metals cannot displace hydrogen from dilute acids and water. Thus, they do not react with them.
  • Non-metals react with hydrogen to form covalent compounds. \[{{H}_{2}}+S\to {{H}_{2}}S\]
  • Non-metals react with chlorine to form covalent chlorides. \[{{P}_{4}}+6C{{l}_{2}}\to 4PC{{l}_{3}}\]
  • Corrosion of metals and its prevention
The slow destruction of some metals by chemical reactions is called corrosion. It can be prevented by (i) applying paint or grease to avoid the contact with air on corrosive metals. (ii)coating corrosive metal with non-corrosive metal. (iii) mixing corrosive metals with non-corrosive metals by alloying.

  Light   Synopsis  
  • Light is a form of energy. It is an electromagnetic radiation which can travel through vacuum with a speed of \[3\times {{10}^{8}}\,\,m{{s}^{-1}}\]. Light travels along straight lines. This property is known as the rectilinear propagation of light. We can see an object only when it reflects or scatters the light falling on it.
  • Any polished or shining surface acts as a mirror and regular reflection takes place.
  • When there is no regular reflection, sharp images are not obtained and so they cannot be seen by us. This happens because of scattering of light due to irregular reflection.
  • An image which can be obtained on a screen is called a real image.
  • An image which cannot be obtained on a screen is called a virtual image.
  • The image formed by a plane mirror is erect. It is virtual and is of the same size as the object. The image is at the same distance behind the mirror as the object is in front of it. In an image formed by a plane mirror, the left side of the object is seen on the right side in the image, and right side of the object appears to be on the left side in the image. This is known as lateral inversion.
  • If two plane mirrors are placed at an angle between them, a number of images are formed. If the angle between the mirrors is 6, then the number of images formed "n" is given by the formula,
\[n=\left[ \frac{360}{\theta }-1 \right],if\,\frac{360}{\theta }\] is even and \[n=\frac{360}{\theta },\]if it is odd.  
  • White light is composed of seven colours.
  • The splitting of light into seven colours is known as dispersion of light. Rainbow is a natural phenomenon showing dispersion.
  • Human eye consists of structures like cornea, iris and pupil.
  • Visually challenged persons (blind) can read and write using the Braille code.

Stars and the Solar System   Synopsis  
  • Universe is a vast collection of stars, planets, nebulae, galaxies, intergalactic matter and everything that exists in them. It is believed to have been formed 15 billion years ago from a huge explosion called 'Big Bang’
  • To measure distances on astronomical scale, the unit light year is used. It is the distance travelled by light in one year. 1 light year \[=9.46\times {{10}^{12}}\,km.\]
  • Stars are self-luminous, astronomical bodies which produce energy from nuclear fusion reactions. They form from a large collection of gases called nebulae.
  • A galaxy is a group of billion stars and other celestial bodies. Galaxies exist in different shapes such as circular/elliptical, ring and irregular. They usually revolve around themselves and move away from each other at tremendous speeds.
  • A group of stars which seem to form a pattern is called a constellation. e.g., Big bear, Orion etc.
  • Sun along with its nine planets/their satellites, asteroids and comets are together named as solar system.
  • Planets like Mercury, Venus and Mars have internal structures similar to that of the earth. So, they are called terrestrial planets. Jupiter, Uranus, Saturn and Neptune are mainly made up of gases. They are called gas planets.
  • Asteroids are irregular, rocky bodies that revolve around the sun between the orbits of Mars and Jupiter.
  • A comet is a small body of rocks, ice and gases that revolve around the sun in a highly eccentric, elliptical orbit.
  • A man-made spacecraft orbiting around an astronomical body is called a satellite. If the orbit of a satellite is synchronised with the earth's rotation/it is called a geostationary satellite. If a satellite seems to be stationary from the point of view of the sun, it is called the sun synchronous or polar satellite.
  • Satellites are used for communication, weather forecasts, remote sensing, conducting scientific experiments and for various other military uses.
  • The various shapes of the bright part of the moon as seen during a month are called phases of moon.
  • A planet has a definite path in which it revolves around the sun. This path is called an orbit.
  • The axis of earth is inclined to its orbital plane at an angle of \[66.5{}^\circ \]. This inclination helps in the change of seasons on the earth.
  • The time taken by a planet to complete one revolution around the sun is called the period of revolution, while the time taken to complete one round around its own axis is called the period of rotation.

  Force and Pressure   Synopsis  
  • A force is a push or a pull. It can cause any one, two or all of the following when applied to a body:
(a) cause movement or stop motion of an object (b) change its speed/direction of the motion of a body (c) change its shape and size of a body  
  • Forces that are in contact with a body are muscular force, frictional force. Forces that are not in contact with a body and act from a distance are magnetic force, electrostatic force, gravitational force.
  • Force is due to the interaction between two or more bodies.
  • The strength of a force is expressed by its magnitude and is a vector quantity.
  • Net force on an object can be zero if two (or more) forces acting on it in opposite directions are equal in magnitude.
  • Frictional force is responsible for change of speed of an object. It tends to stop/slow down a moving object in the absence of external forces. Frictional force can be due to a surface, wind (drag) or water (ex. a moving boat stops in water if you stop rowing).
  • Force acting on a unit area of a surface is called pressure. Pressure = Force/Area on which it acts.
Note: S.I. unit of force = newton S.I. unit of pressure = newton \[{{(metre)}^{-2}}\] or pascal  
  • Pressure depends upon the area of contact.
  • : A sharp needle causes more pain in the body as compared to a blunt object.
  • Pressure exerted by water at the bottom of a container depends upon the height of the water column.
  • Both liquids and gases exert pressure on the walls of a container in which they are stored.
  • Liquids exert equal pressure at the same depth.
  • Pressure exerted by the air surrounding the earth is called the atmospheric pressure. The reason we do not feel this pressure is due to the fact that the pressure inside our bodies (Our blood contains dissolved oxygen at a pressure, equal to or slightly more than the atmospheric pressure) is equal to the atmospheric pressure and hence, cancels the pressure from outside. Atmospheric pressure is measured in Pascals or kilopascals.
1 atmosphere (1 atm) = 101325 Pa or 101.325 k Pa.   Atmospheric pressure decreases with height (high altitude/mountains) and increases with depth (deep sea).

Reaching the Age of Adolescence   Synopsis  
  • Humans become capable of reproduction after puberty sets in. Between the ages of 11 years and 19 years children are called adolescents.
  • The period of time when a person changes from a child into an adult is called puberty.
  • A boy reaches puberty when he is 13-14 years old. Girls reach puberty earlier than boys at the age of 11 -12 years.
  • During puberty, physical and emotional changes take place. These are called secondary sexual characters. In girls, puberty is marked by the onset of menstruation and the development of secondary sexual characters, namely, enlargement of breasts, widening of pelvic girdle and growth of hair in the armpits and pubic region.
  • In boys, puberty begins with the production of sperms and is marked by secondary sexual characters, namely, deepening of the voice, growth of hair on the face, armpits and pubic region and enlargement of the scrotum and penis.
  • Menstruation is the periodic discharge of blood from the uterus through the vagina. The discharged blood usually contains a dead egg cell and some tissues from the lining of the uterus.
  • If the egg cell is fertilized, it will form a zygote which will implant itself on the wall of the uterus. It will eventually develop into an embryo. Menstruation does not take place during pregnancy.
  • The onset of puberty and maturity of reproductive parts are controlled by hormones.
  • Hormones are secretions of endocrine glands which are poured directly into the blood stream.
  • Pituitary gland secretes hormones which include growth hormone and hormones that make other glands such as those of the testes, ovaries, thyroids and adrenals secrete hormones. Pancreas secretes insulin/thyroid produces thyroxine and adrenals produce adrenaline.
  • Testosterone is the male hormone and estrogen is the female hormone. The uterine wall in females prepares itself to receive the developing fertilised egg. In case there is no fertilisation, the thickened lining of the uterine wall breaks down and goes out of the body along with blood. This is called menstruation.
  • The sex of the unborn child depends on whether the zygote has XX or XY chromosomes.
  • It is important to eat balanced food and maintain personal hygiene during adolescence.
  • Personal hygiene must be maintained during menstruation to avoid itch, unpleasant odour, bacterial and fungal infection.

Reproduction in Animals   Synopsis  
  • Reproduction is the production of new individuals more or less similar to the parent organisms. This may be achieved by a number of means and serves to perpetuate increase of species.
  • There are two main methods in which organisms give rise to new individuals - Asexual reproduction and sexual reproduction.
  • Asexual reproduction is the process of producing new organism(s) from a single parent without the involvement of sex cells or gametes.
  e.g., Binary fission in amoeba, regeneration in planaria, budding in hydra.  
  • Sexual reproduction is the process of producing new organism(s) from two parents with the involvement of sex cells or gametes. Male sexual unit is known as male gamete or sperm while female sexual unit is termed as female gamete or ova.
  • The fusion of sperm and ovum is known as fertilisation. Thus, the two major processes, i.e., formation of gametes and fusion of gametes constitute sexual reproduction.
  • The reproductive organs of human beings, i.e., testes is in male and ovaries in female produce gametes and also secrete hormones like testosterone (male hormone), estrogen and progesterone (female hormones).
  • Fertilisation takes place in the fallopian tube. The embryo develops in the uterus, and receives oxygen and nutrients through the placenta.
  • Animals such as human beings, cows, dogs which give birth to young ones are called viviparous animals. Animals such as hen, frog, lizard which lay eggs are oviparous animals.
  • The transformation from the larval stage to the adult stage in the life cycles of frog and insects is called metamorphosis.
  • The cloning of animals produces offspring with genetic materials which are identical to the parent. The most famous animal clone is Dolly, the sheep.

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