Current Affairs 8th Class

  Force and Pressure   ·                     A push or a pull on a body is called force.   ·                     A force can change the shape of an object, move the object, stop a moving object and change the direction of motion.   ·                     Force has magnitude as well as direction.   ·                     Some forces come into play when at least two objects interact. These forces are called contact forces. Muscular force and friction force are the examples of two such forces. Two important points about contact forces are as follows: §  Forces applied on an object in the same direction add to one another. §  If the two forces act in the opposite directions on an object, the net force acting on it is the difference between the two forces.   ·                     Some forces can act on an object without being in contact with it. These forces are called non-contact forces. Magnetic force, electrostatic force and gravitational force of the earth are the examples of non-contact forces.   ·                     Every object in the universe, whether it is small or large, exerts a force on every other object. This force is known as the gravitational force.   ·                     The force acting on the unit area of a surface is called pressure.   ·                     A column of air of the height of the atmosphere and area \[\text{10 cm }\!\!~\!\!\text{ }\,\text{ }\!\!\times\!\!\text{ }\,\text{10 cm}\] exerts a weight as large as 1000 kg on us, but still we are not crushed under this weight because the pressure inside our bodies is equal to the atmospheric pressure and cancels the pressure from outside.   ·                     Liquids and gases exert pressure on the wails of their containers which Is same in all directions.   ·                     The pressure exerted by air on us is known as atmospheric pressure.      

  Reaching the Age of Adolescence   ·                     Humans can reproduce only after growing up to a certain age called adolescence. It is a transitional stage of physical and psychological human development that generally occurs during the period from puberty to legal adulthood (age of majority). It is generally the age period of 11 years to 19 years. ·                     At the age of adolescence, puberty sets in. The onset of puberty brings about growth of the reproductive organs. ·                     The growth of hair at various places on the body, increase in height, development of breasts in girls, appearance of facial hair (moustache and beard) in boys are some of the changes that can be seen in adolescents. ·                     The onset of puberty and maturity of reproductive parts are controlled by hormones. ·                     Hormones are chemical messengers that are secreted by the endocrine glands directly into the blood stream. ·                     A few glands such as sweat glands, oil glands and salivary glands release their secretions through ducts. ·                     The hormones secreted by the Pituitary gland stimulate testes and ovaries to release testosterone in male and estrogen in female. It also stimulates the pancreas, thyroids and adrenal glands to make them secrete insulin, thyroxine and adrenalin. ·                     When a sperm fuses with the egg inside the body of a human female, 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 first menstrual flow begins at puberty and is termed menarche. At 45 to 50 years of age, the menstrual cycle stops. Stoppage of menstruation is termed menopause. Initially, menstrual cycle may be irregular. It takes some time to become regular. ·                     All human beings have 23 pairs of chromosomes in the nuclei of their cells. Two chromosomes out of these are the sex chromosomes, named X and Y. ·                     A sperm carries X and Y chromosomes while a female egg carries both X and X chromosomes. Sex of the unborn child depends on whether the zygote has XX or XY chromosomes. The XY combination results in male child while the XX combination results in a female child. ·                     Metamorphosis in frogs is controlled by thyroxine produced by thyroid gland. Thyroxine production requires the presence of iodine in water. If the water in which the tadpoles are growing does not contain sufficient iodine then tadpoles cannot become adults. ·                     The physical and mental well-being of an individual is regarded more...

  Reproduction in Animals   ·                     Reproduction is the biological process by which an individual organism gives birth to its young one. Animals can reproduce by two modes ?

(i) Sexual reproduction,

(ii) Asexual Reproduction.

·                     The mode of reproduction in which an individual can reproduce without involvement of another individual of that species is called asexual reproduction. ·                     Sexual reproduction is the type of reproduction in which two individual, called parents, are needed to reproduce the young ones. ·                     Human male reproductive system consists of a pair of testes (singular testis), two sperm ducts and a penis. ·                     Human female reproductive system consists of a pair of ovaries, oviducts (fallopian tubes) and the uterus. ·                     The testes produce male gametes called sperms and the ovary produces female gametes called ova. ·                     In sexual reproduction, male gamete fuses with female gamete. The fusion of ovum and sperm is called fertilization and the fertilized egg is called a zygote.       ·                     The zygote divides repeatedly to give rise to an embryo which gets embedded in the wall of the uterus for further development. ·                     The stage of the embryo in which all the body parts are identifiable is called foetus. ·                     Fertilization that takes place inside the female body is called internal fertilization. This type of fertilization is observed in human beings and other animals such as hens, cows and dogs. ·                     Fertilization that takes place outside the female body is called external fertilization. This type of fertilization is observed in animals like frogs, fish, starfish, etc. ·                     Animals which give birth to their young ones are called viviparous animals. Human beings, cows and dogs are some examples or the viviparous animals. ·                     Animals which lay eggs are called oviparous animals. For example, hen, frog, lizard and butterfly etc. ·                     Some animals physically develop after birth or hatching, involving a conspicuous and relatively abrupt change in their body structure through cell growth and differentiation. This transformation of the larva into adult through drastic changes is called metamorphosis. ·                     Binary fission and budding are some of the methods of asexual reproduction, Hydra reproduces by budding and amoeba reproduces by binary fission.      

  Cell - Structure and Functions   ·                     Cell is the basic structural unit of life. ·                     Cells were first observed by Robert Hooke in 1665. He observed slices of cork (a part of the bark of a tree) under a simple magnifying device. He noticed partitioned boxes or compartments in the cork slice. ·                     Cells are found in wide variety of shapes and sizes. Some cells are big enough to be seen with the unaided eye like egg of a hen while some are too small to be seen by the naked eyes like an amoeba. ·                     Number of cells also varies from organism to organism. The organisms made of a single cell are called unicellular organisms while the organisms made of more than one cell are called multicellular organisms. ·                     In unicellular organisms, the single cell performs all the basic functions performed by a variety of cells in multicellular organisms. ·                     In multicellular organisms, a group of specialised cells forms a tissue which in turn forms an organ to perform various functions. ·                     The three main parts of a cell are

(i) the cell membrane

(ii) cytoplasm and

(iii) the nucleus.

·                     The cytoplasm and nucleus are enclosed within the cell membrane which is also called the plasma membrane. ·                     Nucleus is separated from cytoplasm by a nuclear membrane. ·                     The entire content of a living cell is known as protoplasm. ·                     Nucleus contains thread-like structures called chromosomes. They carry genes and help in inheritance or transfers of characters from one generation to another. The chromosomes can be seen only when the cell divides. ·                      The cells without well organised nucleus are called prokaryotic cells. These cells do not have nuclear membrane. Some bacteria and blue green algae are prokaryotes. ·                     The cells having well organised nucleus with a nuclear membrane are called as eukaryotic cells. All organisms other than bacteria and blue-green algae are called eukaryotes. ·                     Plant cells have an additional layer around the cell membrane known as cell wall which is not present in animal cells. ·                     Plant cells also contain plastids. Plants cells have a big central vacuole unlike a number of small vacuoles I animal cells.         

  Conservation of Plants and Animals   ·                     Deforestation is one of the major environmental problems of the Present time. Deforestation leads to ail major environmental problems like desertification and global warming which consequently leads to floods or droughts. ·                     Biosphere is the part of the earth in which living organisms exist or which supports life. ·                     Biological diversity or biodiversity refers to the variety of organisms existing on the earth, their interrelationships and their relationship with the environment. ·                     Species is a group of population which is capable of Interbreeding. ·                     Plants and animals of a particular area are known as the flora and fauna of that area. ·                     An ecosystem is made of all the plants, animals and microorganisms in an area along with non- living components such as climate, soil, river deltas etc. ·                     Wildlife sanctuary, national park and biosphere reserve are the specific areas meant for conservation and preservation of forest and wild animals. ·                     The species that are found only in a particular area are called endemic species. ·                     The species, which are facing the danger of extinction, are called endangered species. ·                     Red Data Book is the source book which keeps a record of all the endangered animals and plants. There are different Red Data Books for plants, animals and other species. ·                     The phenomenon of mass movement of a species from its own habitat to some other habitat for a particular time period every year to escape from harsh weather conditions, to breed or in search of food is called migration. ·                     According to a study, 17 full grown trees are cut down to make one tonne of paper. Thus, we should save, reuse and recycle paper to save trees. ·                     Reforestation is the natural or intentional restocking of existing forests and woodlands that have been depleted, usually through deforestation.      

  Microorganisms - Friend and Foe             ·                     Some organisms are too small to be visible by the naked eyes. These are called microorganisms. They may be unicellular or multicellular. ·                     Microorganisms are found everywhere and in almost all types of environment, ranging from ice cold climate to hot springs and deserts to marshy lands. ·                     Microorganisms are also found inside human beings and other animals. ·                     Microorganisms are classified mainly into five categories ?

(i) bacteria,

(ii) fungi

(iii) protozoa and

(iv) algae

(v) virus.

Viruses, though different from the living microorganisms, are considered microbes. They can reproduce only inside the host organism like bacterium, plant or animal cell. Because of this viruses are also called at the borderline of living and non-living organisms. ·                     Some microorganisms are harmful for human beings while some others are useful in various aspects. ·                     Some microorganisms are used in making medicines. Some microorganisms decompose the organic waste and dead plants and animals into simple substances and help in cleaning up of the environment. ·                     The diseases like typhoid, cholera, tuberculosis are caused by the microorganisms. Protozoa also cause diseases like dysentery and malaria. ·                     Some microorganisms can also cause food poisoning. ·                     Some microorganisms like rhizobium reside in the root nodules of leguminous plants. They help in fixing nitrogen from air into the soil and increase the soil fertility. ·                     Blue green algae present in the soil also helps in fixing nitrogen from the atmosphere and convert into nitrogenous compounds.      

  Pollution of Air and Water   ·                     Air pollution refers to the contamination of air by impurities like solid particles and gases in the air which may have a harmful impact on the living organisms and the non-living components. ·                     An air pollutant is a substance that contaminates air and water. Pollutants can have adverse effects on humans and the ecosystem. Pollutants can be solid particles, liquid droplets, or gases. ·                     Carbon monoxide, nitrogen oxides, carbon dioxide, methane and Sulphur dioxide are the major pollutants of air. ·                     The greenhouse effect is the natural process by which the atmosphere traps some of the Sun's energy, warming the Earth enough to support life. Increasing levels of greenhouse gases like \[C{{O}_{2}}\] are leading to global warming. ·                     The contamination of water by the substances that are harmful for human beings and other life forms is called water pollution. These contaminants may be in the form of solid particles, liquids or in the form of gases. ·                     Some of the major contaminants of water are sewage, agricultural chemicals and industrial waste. ·                     The water safe enough to be consumed by humans or used with low risk of immediate or long term harm is called potable water. ·                     Water is a precious natural resource. Life on the earth cannot be possible without water. It is necessary to conserve water to continue the life processes on earth.      

  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.