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Class 7 Science NCERT-Physics Lesson 1
Chapter 4: Heat
What is heat? Woollen clothes keep us warm in winters. We prefer to wear light coloured cotton clothes when it is hot. These give us a feeling of coolness. One might wonder why particular types of clothes are suitable for a particular season. In winter you feel cold inside the house. If you come out in the sun, you feel warm. In summer, you feel hot even inside the house. These facts indicate the concept of heat. We often compare the degree of hotness or coolness by touching the objects. But our sense of touch is not reliable as it may confuse and deceive us sometimes. A reliable measure of the hotness of an object is its temperature. Temperature is measured by a device called thermometer. The thermometer that measures our body temperature is called a clinical thermometer Clinical thermometer consists of a long, narrow, uniform glass tube. It has a bulb at one end. This bulb contains mercury. Outside the bulb, a small shining thread of mercury can be seen. The scale we use is the celsius scale, indicated by C. A clinical thermometer reads temperature from 35 C to 42 C
a. b. c. d. e. f. Precautions to be observed while reading a clinical thermometer: Thermometer should be washed before and after use, preferably with an antiseptic solution. Ensure that before use the mercury level is below 35 C Read the thermometer keeping the level of mercury along the line of sight. Handle the thermometer with care. If it hits against some hard object, it can break. Don't hold the thermometer by the bulb while reading it. Also avoid keeping the thermometer in the sun or near a flame. It may break. The normal temperature of human body is 37 C, The temperature of every person may not be 37 C. It could be slightly higher or slightly lower. Actually, what we call normal temperature is the average body temperature of a large number of healthy persons. The clinical thermometer is designed to measure the temperature of human body only. The temperature of human body normally does not go below 35 C or above 42 C. That is the reason that this thermometer has the range 35 C to 42 C. We can not use a clinical thermometer for measuring the temperature of any object other than the human body. How do we measure the temperature of other objects? For this purpose, there are other thermometers. One such thermometer is known as the laboratory thermometer.
Different types of thermometers are used for different purposes.The maximum and minimum temperatures of the previous day, reported in weather reports, are measured by a thermometer called the maximum-minimum thermometer. The range of a laboratory thermometer is generally from-10 C to 1 10 C. In addition to the precautions needed while reading a clinical thermometer, the laboratory thermometer should be kept upright not tilted. The bulb should be surrounded from all sides by the substance of which the temperature is to be measured. (Therefore it cannot be used to measure human body temperature, as the mercury level rapidly falls once we take it out of our mouth). The bulb should not touch the surface of the container There is a lot of concern over the use of mercury in thermometers. Mercury is a toxic substance and is very difficult to dispose of if a thermometer breaks. These days, digital thermometers are available which do not use mercury. TRANSFER OF HEAT: A frying pan becomes hot when kept on a flame. It is because the heat passes from the flame to the utensil. When the pan is removed from the fire, it slowly cools down. It happens because the heat is transferred from the pan to the surroundings. So in both cases, the heat flows from a hotter object to a colder object. In fact, in all cases heat flows from a hotter object to a colder object.
The process by which heat is transferred from the hotter end to the colder end of an object is known as conduction. In solids, generally, the heat is transferred by the process of conduction. The materials which allow heat to pass through them easily are conductors of heat. For examples,aluminum, iron and copper. The materials which do not allow heat to pass through them easily are poor conductors of heat such as plastic and wood. Poor conductors are known as insulators The water and air are poor conductors of heat. The process by which heat transfer take place in these substances is called as convection. The air near the heat source gets hot and rises. The air from the sides comes in to take its place. In this way the air gets heated. The people living in the coastal areas experience an interesting phenomenon. During the day, the land gets heated faster than the water. The air over the land becomes hotter and rises up. The cooler air from the sea rushes in towards the land to take its place. The warm air from the land moves towards the sea to complete the cycle. The air from the sea is called the sea breeze. To receive the cooler sea breeze, the windows of the houses in coastal areas are made to face the sea.
At night it is exactly the reverse The water cools down more slowly than the land. So, the cool air from the land moves towards the sea. This is called the land breeze When we come out in the sun, we feel warm. How does the heat from the sun reach us? It cannot reach us by conduction or convection as there is no medium such as air in most part of the space between the earth and the sun. From the sun the heat comes to us by another process known as radiation. The transfer of heat by radiation does not require any medium. It can take place whether a medium is present or not. When we sit in front of a room heater, we get heat by this process. A hot utensil kept away from the flame cools down as it transfers heat to the surroundings by radiation Our body too, gives heat to the surroundings and receives heat from it by radiation. All hot bodies radiate heat. When this heat falls on some object, a part of it is reflected, a part is absorbed and a part may be transmitted. The temperature of the object increases due to the absorbed part of the heat.
We often use electricity and fuels like coal and wood to keep our houses cool or warm. Is it possible to construct buildings, that are not affected much by heat and cold outside? This can be done by constructing outer walls of buildings so that they have trapped layers of air. One way of doing this is to use hollow bricks, which are available these days. What is the reason that it is more comfortable to wear white or light-Coloured clothes in the summer and dark-coloured clothes in the winter? Dark surfaces absorb more heat and, therefore, we feel comfortable with dark coloured clothes in the winter. Light coloured clothes reflect most of the heat that falls on them and, therefore, we feel more comfortable wearing them in the summer. Why do Woollen clothes keep us warm in the winter? Wool is a poor conductor of heat. Moreover, there is air trapped in between the wool fibres. This air prevents the flow of heat from our body to the cold surroundings. So, we feel warm. Similarly if given the choice in winter of using either one thick blanket or two thin blankets joined together what would you choose and why? Remember that there would be a layer of air in between the blankets. Now you should know the answer.