Current Affairs 8th Class

  Combustion and Flame   ·                     Combustion is a process in which a substance reacts with oxygen to give off heat and light. ·                     The substances which burn in air are called combustible substances while those that do not burn easily are called non-combustible substances. ·                     The conditions required for combustion to take place are:
(i) the presence of a combustible substance
(ii) the presence of oxygen and
(iii) heat to raise the temperature of the fuel beyond the ignition temperature.
·                     The lowest temperature at which a combustible substance catches fire is called its ignition temperature. ·                     Inflammable substances have very low ignition temperature. ·                     Slow combustion, rapid combustion, spontaneous combustion and explosion are the different types of combustion.   §  Slow combustion: This type of combustion takes place at low temperatures. Respiration is an example of slow combustion. §  Rapid combustion: in this type of combustion, the gas burns rapidly and produces heat and light. §  Spontaneous combustion: in this type of combustion a material suddenly bursts into flames, without the application of any apparent cause. §  Explosion: In this type of combustion; a sudden reaction takes place with the evolution of heat/ light and sound. A large amount of gas is also liberated in this reaction.   ·                     A fire can be controlled either by cutting off the supply of oxygen or by removing the combustible substance. ·                     Water is commonly used to control fires but it cannot be used to control fires caused by chemicals that react violently with water, electrical equipment?s or oils. ·                     Carbon dioxide is used to extinguish the fire caused by electrical equipment or chemicals.                                                                     ·                     A flame has three different zones - dark zone, luminous zone and non-luminous zone.                                                                    ·                     A fuel that is cheap, readily available, readily combustible and easy to transport is called an ideal fuel. Such fuel is expected to have high calorific value. It does not produce gases or residues that pollute the environment.                            ·                     Fuel efficiency is expressed by calorific value. The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value. The calorific value of a fuel is expressed in kilojoule per kg (kJ/kg). Fuels more...

  Coal and Petroleum   ·                     Natural resources can be classified into two types:
(i) inexhaustible or renewable resources and
(ii) exhaustible or non-renewable resources.
·                     Inexhaustible resources are the resources that are present in unlimited quantity in nature and are not likely to be exhausted by human activities. ·                     Exhaustible resources are the resources that are present in limited amount in nature. They can be exhausted by human activities. Forests, wildlife, minerals, coal, petroleum, natural gas etc. are the examples of exhaustible natural resources. ·                     Coal, petroleum and natural gas are fossil fuels that are formed under the earth crust by the decomposition of dead plant and animal remains. ·                     The slow process of conversion of dead vegetation into coal is called carbonisation. ·                     Coke, coal tar and coal gas are the products of distillation of coal. ·                     Petroleum gas, petrol, diesel, kerosene, paraffin wax, lubricating oil are obtained by refining petroleum. ·                     Coal and petroleum are primarily used as the fuel all over the world. These are exhaustible natural resources and thus, present in limited amount in nature. So, we should use them judiciously. ·                     One should follow the following advises to save petrol/diesel while driving. These are: §  Drive at a constant and moderate speed as far as possible. §  Switch off the engine at traffic lights or at a place where you have to wait. §  Ensure correct tyre pressure, and take care of the regular maintenance of the vehicle.      

  Materials - Metals and Non-Metals   ·                     Materials can be classified into metals and non-metals. ·                     Metals can be distinguished from non-metals on the basis of their physical and chemical properties.   ·                     Physical properties of metals §  Metals are hard, lustrous, malleable, ductile, sonorous and good conductors of heat and electricity. Iron, copper, aluminum, calcium and magnesium are some examples of the metals.   ·                     Metals are generally hard but the metals like sodium and potassium are soft and can be cut with a knife. Similarly, mercury is the only metal which is found in liquid state at room temperature.   ·                     Chemical properties of metals §  Metals on burning in the presence of oxygen, produces metal oxides which are basic in nature. §  Metals on reaction with water produce metal hydroxides and hydrogen gas. §  Metals on reaction with acids produce metal salts and hydrogen gas. §  Some metals react with bases to produce hydrogen gas. §  More reactive metals displace less reactive metals from their compounds in aqueous solutions. This type of reaction is called displacement reaction.   ·                     Physical properties of non-metals §  Non -metals are soft and dull in appearance, break down into powdery mass on tapping with hammer, are not sonorous and are poor conductors of heat and electricity.   ·                     Diamond which is the hardest known element on the earth is a non-metal.   ·                     Chemical properties of non-metals §  Non-metals react with oxygen to produce non- metallic oxides which are acidic in nature. §  Non-metals generally do not react with water. §  Non-metals do not react with acids. ·                     Metals and non-metals are used widely in everyday life.      

  Synthetic Fibres and Plastics   ·                     Fibres that are obtained by chemical processing of petrochemicals are called synthetic fibres. Like natural fibres, these fibres can also be woven into fabrics. ·                     Synthetic fibres and plastics are made of very large units called polymers. ·                     Polymers are made up of many smaller units called monomers. ·                     Synthetic fibres have various uses in our day to day life. They are used in household articles like ropes, buckets, furniture, containers, etc. Apart from items of household uses, they are also used in aircrafts, ships, spacecraft?s, healthcare, etc. ·                     Rayon, nylon, polyester and acrylic are some of the main types of synthetic fibres. ·                     The properties of rayon are similar to that of silk. Due to this rayon is also called as artificial silk. ·                     The different types of fibres differ from one another in their strength, water absorbing capacity, nature of burning, cost, durability, etc. ·                     The material which gets decomposed through natural processes, such as action by bacteria, is called biodegradable. ·                     The material which is not easily decomposed by natural processes is termed as non-biodegradable. ·                     Plastic becomes one of the most important things of our life. We can see lots of things made up of plastics at home, or outside, everywhere. ·                     Plastics can be classified mainly into two types ?
(i) Thermoplastics and
(ii) Thermosetting plastics.
·                     Thermoplastics are the plastics that do not undergo chemical change in their composition when heated and can be molded again and again. Polyethylene, polypropylene, polystyrene and polyvinyl chloride are some of the examples of thermoplastics. ·                     Thermosetting plastics are the plastics that can melt and take shape once; after they have solidified, they stay solid. They undergo chemical change in their composition when heated and cannot be molded again and again. Polystyrene, polyisoprene, rubber are examples of thermosetting polymers. ·                     Plastics release poisonous gases on burning. On dumping in the ground they may take years to get decomposed. They are non-biodegradable in nature. ·                     We need to use synthetic fibres and plastics in such a manner that we can enjoy their good qualities and at the same time minimise their environmental hazards. ·                     Be a responsible citizen and always remember the 4 R principle - Reduce, Reuse, Recycle and Recover to minimise the pollution which more...

  Crop Production and Management   ·                     Crop production and management plays a key role in our country's economy. By adopting advanced and latest technologies in agriculture, enough yields of crops can be grown to provide food to our growing population. ·                     The plants of same kind grown and cultivated at one place on a large scale are called a crop. ·                     There are mainly two types of crops grown in India - rabi and kharif crops. ·                     Rabi crops are the agricultural crops that are sown in winter and harvested in the spring. Wheat, Gram, Pea, Mustard, Linseed, Barley, Peas are the examples of rabi crop. ·                     Kharif crops are referred to those crops that are sown in the rainy (monsoon) season. These crops are planted for autumn harvest and may also be called the summer or monsoon crop. Paddy, cotton, maize, sugarcane are some examples of the kharif crops. ·                     For growing a good crop, it is necessary to prepare soil by tilling and livelong before sowing the seeds. Ploughs and levellers are used for this purpose. ·                     Selection of good quality seeds is another important step of growing a good crop. The seeds should be sown at appropriate depth and at enough distance from one another to avoid competition for nutrients. ·                     The use of organic manure and fertilizers helps in replenishing the quality of soil. Use of chemical fertilizers is commonly used in present time. The use of chemical fertilizers in excessive amount is also harmful for environment. ·                     Water is important for proper growth and development of plants. The supply of water to crops at different intervals is called irrigation. Plant roots absorb water and mineral from the soil. Along with water, minerals and fertilisers are also absorbed. The time and frequency of irrigation varies from crop to crop, soil to soil and season to season. ·                     Moat, Chain pump, Rahat and Dhekli are the traditional methods of irrigation. At present irrigation is done by pumps and tube wells run on electricity, by sprinkler system and by drip system. ·                     Sprinkler system and drip irrigation methods also minimise the loss of water. ·                     Some unwanted plants called weeds also need to be removed from the field from time to time as they compete with the main crops for nutrients. The practice of removing of weeds is also called weeding. It can be done by using weedicides like 2, 4 -D. ·                     The cutting of the mature crop manually or by machines is called harvesting. ·                     The process of separating out grains more...

  Number System   Learning Objectives
  • Properties of Rational Numbers
  • Square and Square Roots
  • Cube and Cube Roots
  • Playing with Numbers
  • Divisibility Test
  Properties of Rational Numbers Rational numbers are the numbers that can be expressed in the form \[\frac{p}{q}\], where p and q are integers and \[q\ne ~0\] . The collection of rational numbers is denoted by Q. These rational numbers satisfy following laws or properties
  •  Rational numbers are closed under addition, subtraction and multiplication. If a, b are any two rational numbers, then the sum, difference and product of these rational numbers is also a rational number, thus we can say that rational numbers satisfy the closure law.
  • Rational numbers are commutative under addition and multiplication. If a, b are rational numbers, then:
  • Commutative law under addition: \[\mathbf{a}+\mathbf{b}=\mathbf{b}+\mathbf{a}\]
  • Commutative law under multiplication: \[\mathbf{a}\times \mathbf{b}=\mathbf{b}\times \mathbf{a}\]
 
  • Rational numbers are associative under addition and multiplication. If a, b, c are rational numbers, then:
  • Associative law under addition: \[\mathbf{a}+\left( \mathbf{b}+\mathbf{c} \right)=\left( \mathbf{a}+\mathbf{b} \right)+\mathbf{c}\]
  • Associative law under multiplication: \[\mathbf{a}\left( \mathbf{bc} \right)=\left( \mathbf{ab} \right)\mathbf{c}\]
  • 0 is the additive identity for rational numbers.
  • 1 is the multiplicative identity for rational numbers.
 
  • The additive inverse of a rational number \[\frac{p}{q}\] is \[\left( -\frac{p}{q} \right)\], and the additive inverse of \[\left( -\frac{p}{q} \right)\] is\[\frac{p}{q}\].
  • If \[\frac{a}{b}\times \frac{p}{q}=1\], then \[\frac{a}{b}\] is the reciprocal or multiplicative inverse of \[\frac{p}{q}\] and vice versa.
  • For all rational numbers p, q and r; p (q + r) = pq + pr and p (q - r) = pq - pr is known as the distributive property.
  Example 1. Find the multiplicative identity of the rational number \[\frac{\mathbf{455}}{\mathbf{1024}}\] (a) \[\frac{1024}{455}\]                (b) \[\frac{1}{455}\] (c) \[\frac{1}{1024}\]                (d) 1 (e) None of these Answer: (a)   2. Which one of the following rational numbers lies between \[\frac{\mathbf{45}}{\mathbf{78}}\] and \[\frac{\mathbf{26}}{\mathbf{52}}\]? (a) \[\frac{75}{156}\]               (b) \[\frac{84}{156}\] (c) \[\frac{95}{156}\]               (d) \[\frac{105}{156}\] (e) None of these Answer: (b) Explanation: On equating the denominator, the given rational number reduce to \[\frac{90}{156}\] and \[\frac{78}{156}\] and the rational number lying between these two is \[\frac{84}{156}\].   Square and Square Roots If a natural number m can be expressed as \[{{n}^{2}}\], where n is also a natural number, then m is called the square root of a square number, \[{{n}^{2}}\]. For example, 1, 4, 9, 16, 25 are the square numbers. Some interesting properties of square numbers are as follows:
  • All square numbers end with 0, 1, 4, 5, 6 or 9 at unit's place.
  • No square number ends with 2, 3, 7 or 8 at unit's place.
  • When a square number ends with 6 at unit's place, the number whose square it is, will have either 4 or 6 in unit's place.
  • There are 2n non perfect square numbers between the squares of the numbers n and (n + 1).
  • If a natural number cannot be expressed as a sum of successive odd natural numbers starting with 1, then it is not a more...

  Geometry   Learning Objectives  
  • Understanding Polygons
  • Parallelogram
  • Rhombus
  • Trapezium
  • Kite
  • Rectangle
  • Square
  • Practical Geometry
  Understanding Polygons  
  • A polygon is a simple closed curve made up of only line segments.
  • A line segment that connects the two non-consecutive vertices of a polygon is called a diagonal.
  • A convex polygon is defined as a polygon with all its interior angles less than \[180{}^\circ \]. This means that all the vertices of the polygon will point outwards, away from the interior of the shape.
 
  • A concave polygon is defined as a polygon with one or more interior angles greater than \[180{}^\circ \]. It looks like a vertex has been 'pushed in' towards the inside of the polygon.
 
  • A regular polygon is both 'equiangular' and ?equilateral?. For example, a square has sides of equal length and angles of equal measure and thus it is a regular polygon. A rectangle is equiangular but not equilateral, so it is a irregular polygon.
  Properties of Polygon  
  • The sum of the measures of the external angles of any polygon is \[360{}^\circ \].
  • For a regular polygon,
  • Number of sides \[\left( n \right)=360{}^\circ \]/ (measure of an exterior angle)
Or  
  • Number of sides \[\left( n \right)=360{}^\circ \]/ (\[180{}^\circ \]- measure of an interior angle)
  • For a regular polygon,
  • Measure of interior angle \[(\theta )=180{}^\circ -\left( 360{}^\circ /n \right)\]
Or
  • Measure of interior angle \[(\theta )=180{}^\circ \left( \left( n-2 \right)/n \right)\]
  • A polygon having three sides is called a triangle. Similarly a polygon having four sides is called quadrilateral, a polygon having five sides is called pentagon, a polygon having six sides is called hexagon.
  • A triangle can never be concave.
  • The sum of all interior angles of a triangle is \[180{}^\circ \].
  • The sum of all interior angles of a quadrilateral is \[360{}^\circ \].
  Parallelogram A parallelogram is a quadrilateral whose opposite sides are parallel. Properties of a parallelogram
  • Opposite angles of a parallelogram are equal.
  • Adjacent angles of a parallelogram are supplementary i.e. their sum is equal to \[180{}^\circ \].
  •  The sum of all interior angles of a parallelogram is \[360{}^\circ \].
  • Opposite sides of a parallelogram are parallel (by definition) and so will never intersect.
  • Diagonals of a parallelogram bisect each other.
  Rhombus A rhombus is a quadrilateral whose all sides are equal and opposite sides are parallel.   Properties of a Rhombus  
  •  Opposite angles of a rhombus are equal.
  •  Adjacent angles of a rhombus are supplementary i.e. their sum is more...

  Mensuration   Learning Objectives  
  • Mensuration
  • Visualizing Solid Shapes
  Mensuration   Mensuration is the branch of mathematics which deal with the study of geometric shapes, their area, volume and related parameters. Some important formulae of area and volume are listed below.   Area of some plane figures:   more...
  Algebra             Learning Objectives  
  • Liner equation in one variable
  • Algebraic expression and identities
  • Factorisation 
  • Exponents and Powers
  • Direct and Indirect Proportion
  • Comparing Quantities
  Linear Equations in one Variable  
  • An algebraic equation is an equality involving variables and an equality sign. The expression on the left of the equality sign is the Left Hand Side (LHS). The expression on the right of the equality sign is the Right Hand Side (RHS).
  • An algebraic equation involving only one variable with its highest power 1 is called a linear equation.
  • The values of the expressions on the LHS and RHS are equal. This happens to be true only for certain values of the variable. These values are the solutions of the equation.
  Example 1. Solve the equation: \[\frac{\mathbf{0}\mathbf{.5}\left( \mathbf{z-0}\mathbf{.4} \right)}{\mathbf{3}\mathbf{.5}}\mathbf{-}\frac{\mathbf{0}\mathbf{.6}\left( \mathbf{z-2}\mathbf{.7} \right)}{\mathbf{4}\mathbf{.2}}\mathbf{=z+6}\mathbf{.1}\] (a) \[-\frac{202}{35}\]    (b) \[\frac{202}{35}\] (c) \[\frac{35}{202}\]      (d) \[-\frac{35}{202}\] (e) None of these Answer: (a) Explanation: \[\frac{5\left( z-0.4 \right)}{35}-\frac{6\left( z-2.7 \right)}{42}=z+6.1\Rightarrow \frac{30z-12-30z+81}{210}=z+6.1\Rightarrow \frac{69}{210}=z+6.1\Rightarrow z=-\frac{202}{35}\]   2. David cuts a bread into two equal pieces and cuts one half into smaller pieces of equal size. Each of the small pieces is twenty gram in weight. If he has seven pieces of the bread all with him, how heavy is the original cake. (a) 120 gm                     (b) 180 gm (c) 300 gm                     (d) 240 gm (e) None of these Answer: (d) Explanation: There are total of seven pieces, so number of smaller pieces is six. Weight of each smaller piece is 20 gm Therefore, weight of six such pieces is \[6\times ~20=120\text{ }gm\] Hence the total weight of original cake \[=2\times ~120=240\text{ }gm\]   Algebraic expressions and identities  
  • The expression that contains only one term is called a monomial.
          For example,  \[3{{x}^{2}},8xy,\,-6z,\,9x{{y}^{2}},\,2x,\,-3,\,22qrs,\,\,\] etc. are the monomials.
  • The expression that contains two terms is called a binomial.
          For example, \[4a+5b,\,3l-8m,\,2m+7,3-7{{x}^{2}}y,\,4{{x}^{2}}-{{z}^{2}},\] etc. are the binomials.
  • The expression that contains two terms is called a trinomial.
          For example, \[a+b+c,\text{ }2x+3y-5z,{{x}^{2}}{{y}^{2}}z-{{x}^{3}}{{y}^{2}}z+1,\] etc. are the trinomials.
  • An expression containing, one or more terms with non-zero coefficient (with variables having non-negative exponents) is called a polynomial. A polynomial may contain any number of terms, one or more than one.
          For example, \[6xy,\,\,8{{x}^{2}}yz-7,\,\,5x+9y+8z,\] etc. are the polynomials.
  • The terms which contain similar variables having same powers are called like terms. Coefficients of like terms need not be the same.
  • The terms which contain different variables are called unlike terms.
  • Only like terms can be added or subtracted.
  • Some standard algebraic identities are as follows:
         \[{{\left( a+b \right)}^{2}}={{a}^{2}}+2ab+{{b}^{2}}\]          \[{{\left( a-b \right)}^{2}}={{a}^{2}}-2ab+{{b}^{2}}\]          \[{{a}^{2}}-{{b}^{2}}=\left( a+b \right)\left( a-b \right)\]          \[\left( x+a \right)\left( x+b \right)={{x}^{2}}+\left( a+b \right)x+ab\]   Example 1. The product of \[\left( \mathbf{-3}{{\mathbf{x}}^{\mathbf{2}}}\mathbf{-x+7} \right)\] and \[\left( \mathbf{3-2x+}{{\mathbf{x}}^{\mathbf{2}}} \right)\] is____. (a) \[-3{{x}^{4}}+5{{x}^{3}}-17x+21\]                     (b) \[{{x}^{5}}+24{{x}^{4}}+5{{x}^{2}}+x+21\] (c) \[8{{x}^{5}}+{{x}^{4}}-12+7x+1\]         (d) \[3{{x}^{5}}-4{{x}^{4}}+1{{x}^{3}}-5{{x}^{2}}-7x+2\] (e) None of these Answer: (a) Explanation: \[\left( more...

  Statistics   Learning Objectives  
  • Data Handing
  • Introduction to Graphs
  Data Handling  
  • Pictograph: A pictograph is a way of showing data using images.
  • The number of times that a particular entry occurs in a data is called its frequency.
  • In the class interval, 10-20, 10 is called the lower class limit and 20 is called the upper class limit.
  • The difference between the upper class limit and lower class limit is called the width or size of the class interval.
  Introduction to Graphs  
  • Bar Graph: It is a display of information using bars of uniform width, their heights being proportional to the respective values.
  • Double Bar Graph: It is a type of bar graph showing two sets of data simultaneously. It is useful for the comparison of the data.
  • Histogram: A histogram is a bar graph that shows data in intervals. It has adjacent bars over the continuous intervals.
  • Pie graph: The representation of data on a pie chart is called a pie graph. It is used to compare parts of a whole.
  • A line graph displays data that changes continuously over periods of time. It is a whole unbroken line.
  • For fixing a point on the graph sheet we need, x-coordinate and y-coordinate.
  • The relation between dependent variable and independent variable is shown through a graph.
  Commonly Asked Questions   1. Observe the following histogram and answer the questions.   2. What information is being given by the graph? (a) Height of the students of class 8           (b) Number of students in class 8 (c) Weight of the students of class 8           (d) All of these (e) None of these   3. The following pie-chart represents the expenditures of a family on different items. What percent of total expenditure is spent by the family on housing? (a) 61%                         (b) 16.7% (c) 13.9%                       (d) 12.7% (e) None of these Answer: (b) Explanation: Percentage expenditure on housing  \[=\frac{60{}^\circ \times 100}{360{}^\circ }=16.7%\]  


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Figure Shape Area
    Rectangle   \[a\times b\]
      Square     \[\begin{align}   & {{a}^{2}} \\  & \frac{1}{2}{{d}_{1}}{{d}_{2}} \\ \end{align}\] Where \[{{d}_{1}}={{d}_{2}}=\sqrt{2}a\]
      Triangle     \[\frac{1}{2}\times b\times h\]
    Parallelogram     \[b\times h\]