Current Affairs 7th Class

       Radiation of Heat   Heat from the Sun comes down on the surface of the Earth through atmosphere. A major part of solar radiation is reflected into space by atmosphere. Only some part of radiation comes on the surface of the earth. In the figure below, 25% of solar radiation is reflected by wind and 3% of solar radiation is reflected by the earth surface.                        Look at the following picture of solar radiation                                             Solar Radiation   In the figure below, a bundle of wood stick burns and radiates heat energy.                                   Radiation     Air around the fire gets heated first and moves upward. To fill the air gap, cold air comes around the fire and gets heated too. Our hands near the fire feel heat due to the radiation of heat.                                       Radiation of heat energy is useful for the human being. The temperature of the human body is 98.4°F and should be constant. Air, around the fire, is heated due to convection of heat. Other end of the iron rod, which is away from fire, is heated by the conduction of heat. In the following figure, a solar plate converts solar radiation into electrical energy.    Look at the following picture of radiation of heat                       Which one of the following reflects more heat energy, coming from Sun, into space? (a) Surface of the earth (b) Atmosphere of the earth (c) Core of the earth (d) All of these (e) None of these   Answer: (b)           Whole part of the iron rod is heated by heating just its one end. Which one of the following features of iron is responsible for the same? (a) Iron is the poor conductor of heat (b) Iron is the good conductor of heat (c) Iron is the insulator of heat (d) All of these (e) None of these   Answer: (b)

     Conductors of Heat   A device, which can conduct heat is called heat conductor. A rod of iron conducts heat, hence it is heat conductor. A piece of wood does not conduct heat. Therefore is called insulator or bad conductor or poor conductor of heat. The food preparation is possible due to conduction of heat. Mostly, aluminum or iron jar is used for the preparation of food, as they are good conductor of heat. Look at the following pictures of conductor of heat                                               An iron rod is heated                                      Copper wire                                                                                                Steel                                                                      Aluminum        Knife     In the above figure (1), an iron rod is heated by a burning candle. Iron rod is a good conductor of heat because heat is transferred from one end to other end. The property of the conductor is called conduction. In the figure (2), copper wire is the conductor of heat because, it transfers heat from one end to other. In the figure (3), alloy steel is the conductor of heat, therefore, utensils made of steel are widely used for the preparation of food. Aluminum is also a good conductor of heat and therefore, used as utensils for the food preparation.              Copper and Iron are the good conductors of heat. Which one of the following is not a good conductor of heat? (a) Soil (b) Aluminum (c) Tungsten (d) Steel (e) None of these   Answer: (a)           Why aluminum metal is used for making utensils for food preparation? (a) Because it is a good insulator of heat                (b) Because it is a good conductor of heat (c) Because it is good conductor of electricity (d) All of these (e) None of these   Answer: (b)

     Transfer of Heat   Heat is transferred from one medium to another by three ways. These are conduction, convection and radiation. In a solid, heat is transferred from one part to another by the conduction method. In liquid and gases, heat is transferred from one molecule to another by convection method. And in vacuum, where medium is not present, heat is transferred by radiation. Look at the following picture of heat transfer in solid   In the above picture an iron rod has been heated and heat is transferred from its rounded part to whole part. The rate of transfer of heat depends on the length of the rod and the amount of heat received by the rod from heat producing elements. The other parts of the rod gets heated by conduction. And the rods around the heated rod get heat by radiation.   Look at the following picture of the method of heat transfer from one object to another   In the above picture a pan filled with water is heated by the radiation of heat from a stove. Water, at the lower level of the pan gets heated first because of its closeness with the heat radiating stove. Heated water goes upward and cold water comes down ward. Water inside the pan is heated by convection method. The whole part of the pan is heated by conduction of heat.                Which one of the following process is involved for heating the liquid inside the pan? (a) Conduction (b) Convection (c) Radiation (d) All of these (e) None of these   Answer (b)         Heating of the Earth by the Sun is an example of: (a) Conduction method of heat (b) Convection method of heat (c) Radiation method of heat (d) All of these (e) None of these   Answer: (c)

       Thermal Expansion and application   Thermal expansion is defined as the physical change in unit length, at per degree temperature. Coefficient of thermal expansion of the material is defined as, the degree of expansion divided by the change in temperature at constant pressure. Thermal expansion of a body depends on the applied temperature. If applied temperature is increased then it causes the thermal expansion of the body. Decreased temperature causes the contraction. In the general case of volumetric thermal coefficient of solid, liquid and gas is expressed as, Volumetric change  Change in temperature  Actual volume of the material  At constant pressure  Volumetric coefficient of thermal expansion     Pressure does not have affect on solid and liquid. But in the case of gas, pressure plays a very important role and coefficient of thermal expansion will vary, if pressure varies. Coefficient of thermal expansion (Linear expansion) in a solid depends on the applied temperature. Pressure on a solid object does not cause change inside the object. Therefore in the case of solid, applied pressure is assumed to be negligible.  Pressure is negligible   Thermal coefficient in the area of a solid Coefficient of thermal expansion of a liquid is calculated by its coefficient of thermal expansion in solid state and in gaseous state where, p»3a. Coefficient of thermal expansion in gases is volumetric expansion. Volume of the gases at constant pressure will increase if temperature increases and vice versa. Volumetric thermal V = Actual volume P = At constant pressure The object whose shape and size changes due to applied temperature can be used for some applications, like preparing thermometer. Thermal expansion can be seen in thermometer. The principle at which the application of thermal expansion works is defined as the expansion of materials at increasing temperature.                      Gas expands with the application of heat. Which kind of expansion is noticed during the thermal expansion of the gases? (a) Linear expansion (b) Volumetric expansion (c) Isotopic expansion (d) All of these (e) None of these   Answer: (b)              The thermal expansion of a solid is the linear expansion. Which one of the following should be increased for increasing the thermal expansion? (a) Light (b) Sound (c) Heat (d) All of these (e) None of these   Answer: (c)

       Temperature and Measuring Device   The degree of hotness and coolness of an object is called its temperature.  Temperature of an object can be measured with devices. Earlier thermo scope was used for measuring temperature. Now an improved instrument, "Thermometer" is used for the measurement of temperature.   Mercury inside the thermometer receives temperature and expands or contracts according to variation in the temperature. However, mercury based thermometer cannot be used for the measurement of temperature below 39°C. In that case, alcohol based thermometer is used .which freezes at - 117°C. Reading, shown by the clinical thermometer is in the form of degree Celsius and written as °C. Water converts into vapor at' 100°C. The temperature at which water converts into vapor is called upper fixed point. Water converts into ice at 0°C and this is called lower fixed point. Rate of vaporization of water will increase with an increase in temperature. Another scale for the measurement of the temperature is Fahrenheit and it is denoted by "F and read as degree Fahrenheit. In this scale, the freezing point of water is defined at 32°, while the boiling point of water is defined to be 212°. There are 100 small divisions between the lower fixed point and upper fixed point in a Celsius scale. In the Fahrenheit scale, 180 small divisions are marked. Therefore the ratio of both the scale, °C : °F is 100 : 180 =5:9. Another scale for the measurement of temperature is kelvin scale (also called absolute scale). In the kelvin scale water freezes at 273.16k and water boils at 373.16k. Therefore the ratio of the interval in the kelvin scale is same as the interval in the celsius scale. The temperature of 0°K is called absolute zero temperature.                Water is converted into vapour at a certain temperature. The temperature at which water converts into vapor is called? (a) Lower fixed point (b) Upper fixed point (c) Melting point (d) All of these  (e) None of these   Answer: (b)           0°C is equal to: (a) 243°K (b) 234°K (c) 273°K (d) All of these (e) None of these   Answer: (c)                   Thermometers Two types of thermometers are there in use. These are, laboratory thermometer and clinical thermometer. The temperature between, - 10°C to 110°C is measured by laboratory thermometer. Clinical thermometer is used for the measurement of body temperature. The body temperature varies slightly. Therefore the range of clinical thermometer is between 35°C to 42°C. Clinical thermometer has graduated scale, on which scale is mentioned in degree Celsius and in Fahrenheit. Small division on the body of the clinical thermometer represents the temperate of  degree C. The range in terms of Fahrenheit on the more...

       Heat   Heat is a form of energy, which is transferred from one body to another of different temperatures. Heat is measured in degrees and units of heat are Celsius, Fahrenheit and Kelvin. Thermometer is an instrument, which is used for the measurement of heat.             Effect of Heat Every object absorbs more or less heat from the atmosphere. A peace of iron heats up quickly, because it absorbs heat fast. Water boils at 100° centigrade. Heat plays an important role in our daily life. Plastic is converted into little solid form of liquid at certain temperature. These properties of plastic are used for the making of various kinds of plastic based products.   Look at the following pictures                                                              Cold water                                                              Water evaporates     Cold water evaporates slowly. Evaporation of cold water depends on the temperature of the atmosphere, when external heat is not applied. Water evaporates fast when heat is applied. Rate of evaporation of water will increase if applied heat is increased. Some solid converted into liquid and then gas, when heat is applied. For example ice (solid) converts to water (liquid) and then into water vapor (gas) with the application of heat.                At what temperature water starts to boil? (a) 0°C (b) 10°C (c) 37°C (d) 100°C (e) None of these   Answer: (d)          The temperature at which a liquid starts to convert into gas is known as: (a) Melting point (b) Freezing point (c) Boiling point (d) Point (e) None of these   Answer: (c)

     Use of Fibres   Fibres are used for various purposes. Some natural Fibres from plants are eatable. Animal's fibres are not eatable and flesh of animal contains very less amount of Fibres. Fibres from the leaves of coconut trees are used for making coils, mats, baskets etc. Plants of sugarcane are also good source of natural Fibres. They are used for making paper cartoons and various other paper products. Fibres from animals are used for making various kinds of wearable dresses. Optical Fibres are widely used in the telecommunication sectors for the transmission of electrical signals.   Look at the following picture of optical fibres:                    Which one of the following contains large amount of fibres? (a) Fruits (b) Egg (c) Fishes (d) All of these (e) None of these   Answer: (a)           Rayon is known as synthetic fiber. The raw materials for making of rayon is obtained from which one of the following sources? (a) Cotton plants (b) Tree pulp (c) Sheep (d) Sisal (e) None of these   Answer: (b)     Fibres obtained from plants and animals are called natural fibres. Wool is mostly obtained from sheep for knitting warm dresses. Silk is obtained from silkworm. Synthetic fibres are made of chemicals.

     Manmade Fibres or Synthetic Fibres   Manmade fibres are the composition of chemicals. Nylon, Acetate, Polyester, and Rayon are the example of manmade or artificial fibres. The chemicals required for the making of nylon are dichloride and diaminohexane. These two chemicals are required in the ratio of 2:5. In other words, for every 5ml of hexanedioyl dichloride, 2ml of diaminohexane is required. The resulting solution will make about 1/2 to 1 meter of nylon. Rayon is made from cellulose. Acetate Fibres are used for the making of wearable dresses. It can be dyed in different colors. Cloths, made up of acetate Fibres, are smooth like silk. Acetate Fibres do not absorb moisture and dry fast. Polyesters are hard Fibres. Polyester also refers to the various polymers in which the backbones are formed by the esterification, condensation of polyfunctional alcohols and acids.               Look at the following flow chart                                        Which one of the following fiber does not absorb moisture and dry fast? (a) Cotton (b) Wool (c) Acetate (d) All of these (e) None of these   Answer; (c)             Which one of the following is a synthetic fiber? (a) Rayon (b) Cotton (c) Wool (d) All of these (e) None of these   Answer: (a)

       Fiber   Fibres are the natural substances, slender, elongated and of threadlike structures. They are obtained from both plants and animals, and are also manufactured in factories. Fibres can be classified into two main groups, that is, natural Fibres and synthetic fibres. Natural fibres are obtained from natural sources, such as animals and plants. Fibres, which are not obtained from natural sources, are called synthetic fibres. Let us study about them in detail            Natural Fibres Fibres obtained from plants and animals are called natural fibres. Cotton, flax and jute are obtained from plants, therefore these are called plant fibres. Wool and silk are obtained from animals, and called animal fibres.              Animal Fibers There are mainly two types of animal fibers, that is, Wool and Silk. Wools are obtained from animals like sheep, yak and goat. They are actually thin hairs of the animals, which give protection to them from cold. Woolen cloths are mostly used in winter season. Silks are mainly used for making sarees and other dresses.   Look at the following pictures of fiber producing animals                                                                                 Yak                                                                              Sheep                                                                                  Goat                                                                                  Alpaca     During the winter season, thin hairs on animal’s body keep them warm. After winter, these hairs are shaved off, which again start growing during non-winter season.              Production of Wool from Sheep Wool is obtained from sheep through the following process: Shearing, Scouring,   Dyeing, Combing and Spinning.                                        The process by which the hair of a sheep is removed along with a thin layer of N skin in one piece is called shearing. Removed hair along with thin layer of skin of a sheep is called fleece.                                                  Look at the Following Picture of Shearing of Sheep   Dust particles are present on haired skin of sheep, therefore, fleece of sheep must be scoured by soap or detergent.   Look at the following picture of process of making Wool from fleece of sheep   more...

       Nutrition in Plants   We have come to know about the nutrition in plants and animals in the previous classes. In this chapter, we will learn in detail about the nutrition, both in plants and animals. All kind of plants need food to survive, and the process by which plants obtain food and utilize them for their nutritional needs, is called nutrition in plants. Plants obtain raw materials for making food from sun light and soil. The process, by which roots of a plant absorb necessary minerals (dissolved in water) from soil and sent them to whole part of the plant, is called transportation. The leaves of the plants are made up of thousands of tiny holes, called stomata, and chlorophyll. Sunlight and carbon dioxide enters into the leaves through stomata. Look at the Following Picture of Stomata       Open and closed stomata are shown in the picture above. Carbon dioxide and water enters and oxygen is released, when stomata is open. Photosynthesis Photosynthesis is a process by which all the green plants prepare their own food . In this process, complex organic material is synthesized using carbon dioxide, water, and inorganic salts, in the presence of sunlight. Oxygen is produced during photosynthesis as by-product.                                                                              Let's know about the different types of nutrition, by which plants obtain their food.               Autotrophic Nutrition The mode of nutrition, in which living organisms (green plants and algae obtain their food by the process of photosynthesis is known as autotrophic nutrition. In this process, plants get energy form Sun and minerals from soil, thereby make glucose or sugar to fulfill the nutritional need, necessary for their survival. This food is then supplied to all the parts of plant. The extra food is stored as starch by the plant. Look at the following picture of nutrition in plants:                   Transportation and Transpiration in Plants Roots of plants absorb minerals from soil and transport to whole part of the plants. Water acts as reactant and solvent in the cell metabolism. Minerals from soil travel throughout the plants through vascular tissues. Vascular tissue is consisting of xylem and phloem. Xylem of vascular tissues are responsible for transporting water and mineral ions from root to whole part of the plants. Phloem is responsible for transporting food to all the parts of plant. In other word phloem is involved in transporting substances of organic food prepared by the leaves during photosynthesis.   Look at the following picture of food transporting tissue   more...