Current Affairs 8th Class

  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.

  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.  

  Chemical Effects of Electric Current   Synopsis  
  • Materials which allow electric current to pass through them are good conductors of electricity, e.g., Metals.
  • Materials which do not allow electric current to pass through them are bad conductors of electricity, e.g., Wood, Plastic, etc.
  • Human body is a good conductor of electricity. Thus, we should take precautions while handling electrical appliances.
  • A bulb glows due to the passage of electric current since the filament of the bulb is heated to a very high temperature. However, if the current through a circuit is too weak/the filament does not get heated sufficiently and it does not glow.
  • An LED (Light Emitting Diode) glows even when a weak (small) electric current flows through it.
  • Electric current produces a magnetic effect. It causes a compass needle to deflect.
  • Pure water (distilled water) is a poor conductor of electricity.
  • Most liquids that conduct electricity are solutions of acids, bases and salts.
  • The passage of an electric current through a conducting solution causes chemical reactions. As a result any of the following activities can be observed.
(a) Formation of gas bubbles. (b) Deposit of metals on electrodes. (c) Change in colour of the solution.  
  • When electricity is passed through some substances, they decompose. Such reactions are called electrolytic reactions. This chemical effect is used to extract elements in metallurgy and for electroplating.
  • The process of depositing a layer of any desired metal on another metal by passing electricity is called electroplating.
  • Chromium plating is done to make the object scratch proof and shiny.
  • Tin cans used to preserve food items/soft drinks are made by electroplating tin onto iron.
  • A coating of zinc is deposited on iron to protect it from corrosion and rust.
  • CFLs (Compact Flourescent Light) consumes less electricity as compared to electric bulb and LED's, but contains mercury which is toxic and poses a disposal problem.

  Some Natural Phenomena   Synopsis  
  • High speed winds are accompanied by reduced air pressure.
  • Air moves from a region of high pressure to low pressure.
  • A thunderstorm occurs due to strong, upward rising winds along with swift movement of the falling water droplets carried with lightning and sound.
  • A thunderstorm may develop into a cyclone or a hurricane with wind speed being as high as \[\text{200}\,\text{km}\,{{\text{h}}^{-1}}.\]
  • A tornado is a dark, funnel shaped cloud that reaches the ground from the sky. A tornado may form within cyclones and can reach a speed up to \[\text{300}\,\text{km}\,{{\text{h}}^{-1}}.\]
  • Electroscope is an instrument used to detect the presence of electric charge on a body.
  • An electrical charge can be transferred from a charged object to another through a metal conductor.
  • The process of transferring charge from a charged body to the earth is called earthing.
  • Lightning takes place when large amount of accumulated charges in the clouds passes onto the earth through atmosphere.
  • Lightning conductors are used to protect tall buildings from the effect of lightning.
  • An earthquake takes place due to disturbance inside the earth's crust.
  • The weak zones on the earth's crust, prone to earthquakes, are known as seismic or fault zones.
  • The power of an earthquake is expressed in terms of its magnitude on a scale called the Richter scale. Higher the magnitude, greater is the damage.
  • Seismographs are instruments which record tremors produced by the earth.
  • Lightning, cyclones and earthquakes can cause extensive damage to mankind and properties. We should take necessary steps to protect ourselves during such natural calamities.

  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.

Synthetic Fibres and Plastics   Synopsis  
  • Natural fibres like cotton are obtained from plants, while wool and silk fibres are obtained from animals.
  • The rearing of silk worms for obtaining silk is called sericulture.
  • Silk fibres are made up of a protein.
  • Synthetic fibres are made by human beings.
  • A synthetic fibre is also a chain of small units joined together. Each small unit is called a monomer and actually a chemical substance. Many such small units combine to form a large single unit called a polymer.
  • Synthetic fibres are durable, less expensive and dry up fast.
  • Rayon (artificial silk) is made from wood pulp while nylon is made from coal, air and water.
  • Nylon by and far is the most used synthetic fibre.
  • Many articles like socks, ropes, tents, parachutes, etc., are made from nylon. A nylon thread is stronger than a steel wire.
  • Polyester is made up of esters which is a mixture of acid and alcohol. Polycot, poly wool, terry cot, etc., are the products prepared by mixing two kinds of fibres, both artificial and natural.
  • Plastic because of its mouldability finds versatile use. Thermoplastics like PVC and polythene can be remoulded, while thermosetting plastics (like bake lite and melamine) cannot be remoulded.
  • Thermosetting plastics are used as kitchenware and also for electrical switches and handles.
  • Plastic is non-reactive and does not corrode easily, hence it is suitable as containers of food, but are non-biodegradable. We should reduce the use of plastic.
  • Waste created by plastics is not eco-friendly. The burning of plastic releases poisonous gases. Hence, these should not be disposed by burning.
  • Polybags carelessly thrown are responsible for clogging the drains and also cause health problems for animals since cows and other stray animals sometimes swallow plastic bags and choke their respiratory systems.
  • Use the 4R principle for the use of plastic-reduce, reuse, recycle and recover.

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.

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.                                       

Combustion and Flame   Synopsis  
  • Fuels
Substances which are burnt to produce heat energy are known as fuels. Fuels can be in the form of solid, liquid or gas. Gaseous fuels are better than liquid fuels due to the highest calorific value. Liquids in turn have more calorific value than solid fuels. The main gaseous fuels are natural gas, producer gas, water gas, LP.G and biogas. L.P.G is a mixture of two hydrocarbons - butane and isobutane. The advantages of L.P.G are (i) high calorific value. (ii) burns with a smokeless flame. (iii) does not produce any poisonous gases on burning.  
  • Calorific value of fuel
The amount of heat produced by burning 1 g of fuel completely is known as its calorific value.  
  • Characteristics of an ideal fuel
An ideal fuel should be cheap, readily available, combustible, easy to transport and store, safe and should have a high calorific value.  
  • Combustion
It is an oxidation reaction which is accompanied by the evolution of heat and light. Paper, kerosene, petrol, straw, etc. are combustible substances. Iron, glass and diamond are non- combustible. The lowest temperature at which a substance starts burning is called its ignition temperature.  
  • Conditions necessary for combustion are
(i) the presence of combustible substance, (ii) the presence of supporter of combustion (oxygen), (iii) the attainment of ignition temperature.  
  • Types of combustion
(i) If combustion of a substance takes place with high speed, it is known as rapid combustion, e.g., candle starts burning when a burning matchstick is brought near its wick. (ii) The combustion in which no external heat is supplied to the substance is known as spontaneous combustion, e.g., burning of white phosphorus in air. (iii) The combustion in which large amounts of gases are evolved with the production of large amounts of heat, light and sound by a substance is called explosion, e.g., burning of crackers.

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