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 more...

     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     more...

     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 more...

       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. more...

       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 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                                                         more...

     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 more...

     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.   more...

       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 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     more...