# Current Affairs 8th Class

#### 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...

#### Notes - Force and Pressure

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.

#### Notes - Reaching the Age of Adolesence

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...

#### Notes - Reproduction in Animals

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.

#### Notes - Cell-Structure and FUnctions

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.

#### Notes - Conservation of Plants and Animals

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.

#### Notes - Microorganism-Friend and Foe

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.

#### Notes - Pollution of Air and Water

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.

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