4. Heat

Heat Chapter-4 Heat is the transfer of energy from a hot body. The sense of touch can be used to understand the degree of hotness or coldness of something. But the sense of touch is not reliable and cannot be always used to say how much hot anything is. Moreover, using the sense of touch can be risky in case of something being very hot. Thus, hotness of anything is measured in terms of TEMPERATURE in reliable way. To measure temperature a device called THERMOMETER is used. Unit of heat: There are three units which are used to measure the temperature: Degree Celsius, Fahrenheit and Kelvin. Degree Celsius: Degree Celsius is written as °C and read as degree Celsius or simply Celsius. For example 20°C; it is read as twenty degree Celsius. Fahrenheit: Fahrenheit is written as °F and read as degree Fahrenheit. For example 25°F; it is read as twenty five degree Fahrenheit. Kelvin: Kelvin is written as K. For example 100K; it is read as hundred Kelvin. Thermometer: Thermometer is a device which is used to measure temperature. Thermometer is made of a long narrow glass tube; with a bulb at one end. The narrow tube appears as a continuous silver line; because it is filled with mercury. Mercury is a metal which is in liquid state at room temperature and it readily expands or contracts at the slightest change in temperature. Hence, mercury is used in thermometer. Types of thermometer: Laboratory Thermometer: Laboratory thermometer is used to measure the temperature. The scale of temperature is graduated generally from –10°C to 110°C over the glass tube. Each division of temperature scale is further divided into 10 parts to read fraction of temperature. Clinical Thermometer: Clinical thermometer is used to measure the body temperature. The scale of temperature is graduated from 35°C to 42°C and or from 94°F to 108°F. The temperature of human body always remains within this range and this is the range on the clinical thermometer. There is a kink near the bulb of clinical thermometer which prevents the automatic fall of mercury level. Digital Thermometer: In digital thermometer, reading of temperature is displayed digitally as in digital watches. This is safer because no mercury is used in this. It is important to note that mercury is a highly toxic substance. Maximum-Minimum Thermometer: Maximum – minimum thermometer is used to measure the daily temperature to prepare weather reports. Reading of thermometer and measuring of temperature: Take a clinical thermometer and hold it horizontally with reading scale towards your eye. Do not hold the thermometer from the bulb. Rotate the thermometer slightly clockwise and anticlockwise. By doing this you will see a shiny thin silvery thread. The end of the silvery thread shows the reading of temperature. If mercury lining ends at 37, the reading is 37°C. Wash the bulb end of thermometer with an antiseptic solution. Give two or three jerks slightly. By doing this the mercury level would fall. When it falls to 35°C or below, put it below the tongue and wait for one minute.

Take out the thermometer and read the temperature. Temperature would be near 37°C. The normal body temperature is 37°C. This can differ from person to person.

Use of Laboratory Thermometer: Take some water in a beaker. Take a laboratory thermometer and immerse its bulb end in water; holding it vertically. Ensure to dip whole portion of bulb end. The bulb end should not touch the bottom or side of the beaker. Observe the movement of rise of mercury. When it becomes stable, take the reading of the thermometer. Repeat this with hot water and take the reading. Difference between clinical and laboratory thermometer: Clinical Thermometer Laboratory Thermometer Clinical thermometer is scaled from 35°C to 42°C Laboratory thermometer is generally scaled from or from 94°F to 108°F. 10°C to 110°C. Mercury level does not fall on its own, as there is a kink near the bulb to prevent the fall of mercury Mercury level falls on its own as no kink is present. level.

Temperature can be read after removing the thermometer from armpit or mouth.

Temperature is read while keeping the thermometer in the source of temperature, e.g. a liquid or any other thing.

To lower the mercury level jerks are given.

No need to give jerk to lower the mercury level.

It is used for taking the body temperature.

It is used to take temperature in laboratory.

Transfer of heat: Heat is transferred by conduction, convection and radiation. Conduction: Transfer of heat from one particle to the adjacent particle is known as conduction of heat. In solids, heat is transferred by the process of conduction. In this process, the transfer of heat takes place through adjacent molecules. Example: When one end of an iron rod is put over flame then part which is nearer to the flame is heated first and heat is gradually transferred to the other end of the rod. This happens because particles of iron rod which are nearer to the flame receive the heat and transfer this to the adjacent particles. Subsequently, the adjacent particles transfer the heat to the next adjacent particles. This process continues and heat reaches to the other end of the rod. Thus, heat transfer in solid takes place through conduction. Conductor and Insulator: Materials which allow heat to pass through it are called conductor or good conductor of heat, such as iron, copper, aluminium, etc. All metals are good conductors of heat. Since, mercury is a metal and found in liquid state at room temperature, that’s why it is used in thermometer. Since metals are the good conductor of heat that’s why kitchen utensils are made of metals or alloys of metals. Materials which do not allow heat to pass through them are called bad conductor or poor conductor of heat. They are also called insulators. Example: rubber, wood, plastic, etc. This is the cause that handles of frying pan or other kitchen utensils are made of plastic. Convection: The transfer of heat because of movement of the molecules of the medium; via mass transfer; is called convection or convection of heat. Water and air are bad conductors of heat. But they do become hot, in spite of being bad conductors. Heat transfer in fluids takes place through convection.

Convection in water: When water is heated in a pan, the particles of water which are near the source of heat; get heated first. Because of heating, they become light; and rise in water. The gap which is created because of rise of hot particles is filled by cold particles of water from the surrounding area. Thus a cyclical movement of particles begins and ends up heating the whole water of the pan. The cyclical movement in fluids because of heating is called convection current. Convection in air: Air gets heated because of convection; the way water gets heated. Air near the source of heat gets heated and rises above. This leaves a gap; which is filled by the colder air from the surrounding. The convection current thus starts in air which results in heating up of air.

When you place your palm above a flame you will feel the hotness of the flame. But when you will place your palm below the flame the area will be colder. This shows how the colder air from below moves up; due to convection current. Land and Sea Breeze: Sea Breeze: In coastal areas, the breeze that moves from sea surface to the land is called sea breeze. This happens because, during daytime, land gets heated more quickly than water. As a result, warm air from land rises up; leaving a gap. To fill that gap, colder air from the ocean surface rushes towards the land. This phenomenon continues and a continuous flow of cold air keeps coming towards the land. This gives rise to the phenomenon which is called the sea breeze. Because of this, people living in coastal areas prefer to live in a sea facing house.

Land Breeze: In coastal areas, the breeze which moves from land towards the sea is called land breeze. In the night, the land cools down more quickly than the ocean surface. This makes the air over the water surface warmer than air over the land surface. Warmer air over the water surface rises in the air and air from the land rushes towards the water surface to fill the gap. This phenomenon continues which creates a flow of air from land to the sea. This phenomenon is called land breeze. Radiation: All hot bodies emit heat by the process of radiation. Radiation of heat does not require a medium. Sunlight comes to the earth because of radiation as there is no medium present between the atmosphere of the earth and the sun. One can feel the heat of bonfire by standing around it. We get warmth from the room heater because of radiation. Reflection and absorption of heat: When heat falls over an object some of the heat is absorbed by the body and some of the heat is reflected. The temperature of an object increases because of absorption of heat. In conventional room heater you can see the reflector attached with it. The reflector of the room heater reflects the heat towards the person sitting or standing near the room heater. Reflection is the cause that umbrella is used to protect from heat of the sun in summer. Dark color absorbs more heat while light color reflects most of the heat. That’s why wearing light colored clothes is preferred in summer, dark colored clothes are preferred in winter. Dark clothes absorb more heat and keep one comfortable in winter. On the other hand, light clothes reflect most of the heat and keep one comfortable in summer. Now-a-days many kitchen utensils come in black color, since utensils of black color absorb more heat and thus cooking becomes faster. Woolen Cloth: Woolen clothes are used in winter season. Wool is a poor conductor of heat. In addition to it; air gets trapped in woolen fiber to further increase the poor conductivity of wool. This prevents the

radiation of heat of our body to the surrounding and prevents the cold from surrounding to affect our body. Thus, wearing woolen cloth makes one comfortable in winter season. Important Terms: Temperature: The reliable measure of hotness or coldness of anything. Thermometer: A device to measure the temperature. Degree Celsius: Unit of measurement of temperature. Clinical Thermometer: A device to measure the temperature of human body. Laboratory Thermometer: Used to measure the temperature in laboratory. Maximum-Minimum Thermometer: Use to measure the temperature of climate. Conduction: Mode of transfer of heat in solids. Convection: Mode of transfer of heat in liquids and air. Radiation: Mode of transfer of heat without medium. Conductor: Materials that allow the flow of heat through them. Insulator: Materials that do not allow the flow of heat through them. Land breeze: Breeze that flows from land surface to sea surface in summer nights. Sea breeze: Breeze that flows from sea surface to water surface in summer days. Q.1. What is heat? Ans: Heat is form of energy that produces feeling of hotness. It is measured in Joule . Calorie is used to measure energy value of food. 1 cal. =4.2 joule Q. 2.What is energy? Ans: Energy is capacity to do work. Energy is neither created nor destroyed it can only changed one form to another. Q. 3.What are the effects of heat? Ans: Heat cause many change in living and non living thing. They are 1. Heat increase in temperature 2. Heat expand a substance 3. Heat changes the state 4. Heat bring chemical change 5.Heat effect living thing Q.4. What is temperature? Ans: The degree of hotness and coldness of a body is called temperature of body. Thermometer is an instrument used to measure temperature. It is work on principle of expansion in liquid on heating. It is measured in Celsius and Fahrenheit scale but for scientific work Kelvin scale is used. 0 F = (9/5 x 0C) +32 and oC = (9/5)(F- 32) and K= C + 273o Q. 5.What is thermometry? Ans: The science of measurement of temperature is known as thermometry Q.6. What are different types of thermometer? Ans: Thermometer: It consist of long narrow glass tube having fine bore.There is a glass bulb filled with liquid mostly mercury at the one end and sealed at other end. There is small bend just above bulb called Kink. it does not allow the mercury to fall as soon as taken out from our mouth. (i). Laboratory Thermometer: It ranges from -10oC to 110oC (ii) Clinical thermometer: It is used to measure body temperature. It ranges from 35oC to 42oC and 94oF to 108oF. Mercury thermometer can measure temperature from 35oC to 357oC.

(iii) Alcohol thermometer used to measure temp. from -125oC to 50oC. Q. 7.What is the use of the m a x i m u m - m i n i m u m thermometer? Ans: The maximum and minimum temperatures atmosphere reported in weather reports which is measured by a thermometer called the m a x i m u m - m i n i m u m thermometer. Q.8. What is the normal body temperature? Ans: 37oC or 98.6oF. Q.9. Why mercury preferred in thermometers? Ans: Mercury used due to following reasons (i) It expand uniformly (ii) It does not stick to wall (iii) It is shiny and easy to see (iv) It remain liquid at room temperature. Q.10. Find at what temperature measurement on Celsius equal to Fahrenheit. [Ans: - 400] Q.11. Find at what temperature measurement on Fahrenheit double of Celsius [Ans: 1600] Q. 12.What is thermal expansion (enlargement)? Ans: The expansion produced in substance on heating is called thermal expansion. When we heat a substance its molecules gain energy and start vibrating rapidly and spread out . As a result a substance expands. Q.13. Why does gas expand more than solid and liquid? Ans: Gas expands more than solid and liquid because its molecules are bound with very weak force of attraction. Q.14. A bimetallic strip made of brass and iron welded together. When it is heated, the strip bend why? Ans: Brass expand and contract more than iron on heating and cooling. This unequal expansion and contraction bent the strip. Q. 15.The same quantity of heat supplied to same amount of different substances does not necessarily produce the same increase in temperature. Why? Ans: Different substances absorb differently that depends on (a) Mass of body (b) change in temperature (c) Nature of substance. For this reason the same quantity of heat supplied to same amount of different substances does not necessarily produce the same increase in temperature Q.16. If difference in temperature between A and B is 50C , What is the difference in Kelvin? Ans: 5K Q.17. Give some examples expansion causes problems? Ans (i). Gaps are left between two sections of railway tracks that allow expansion in summer and protect from bending causing accident. (ii). During summer, iron expands and cause serious accidents. To allow this expansion Space left between two sections of rail tracks. (ii). When we pour hot liquid, sometime glass tumbler crack because inner wall get heated quickly and expand than outer wall. A Pyrex glass resist from this unequal expansion. Q. 18.Explain one good use of expansion:

1. In fire alarm expansion put in good use. The alarm contains two strips of iron and brass bolted together. When it is heated because of fire, brass expands more than iron and bends towards iron. Bell starts ringing. As the fire put out strip gets cooled and straighten again. Q.19. What is heat capacity? Ans: The amount of heat required to raise temperature of a substance by 10C . It is different for different substance. It depends on (a) Mass of body (b) change in temperature (c) Nature of substance. Q µ m and Q µ t Þ Q = S m t Here, S is a constant and named Specific heat of body. Q. 20.What is specific heat capacity? Ans. The amount of heat required to raise the temperature of 1 kg by 10C . Water has highest heat capacity. SI unit is j/kg0C Q.21. Define melting and boiling point Ans .The temperature at which solid melts is called Melting point. Melting point of ice is 00C and wax is 630 C. The temperature at which liquid changes into vapour is called boiling point. Q. 22.What is latent heat of fusion? Ans: Latent heat or hidden is amount of heat that only change state not temperature. The amount of heat required to melt 1 kg substance from solid to liquid without change of temperature is called latent heat of fusion. Q. 23.What is latent heat of vaporization? Ans: The amount of heat required to change 1 kg substance from liquid to vapour state without change of temperature is called latent heat of vaporization. Q. 24.Why stem burn more than boiled water? Ans: Because stem has more latent heat than boiled water[ about 2260j/g] Q.25. why does water used as coolant in car? Ans: Water has high specific capacity this makes water to take long time to heat up by absorbing very large quantity of heat. Therefore water used as coolant in car and factories. Q.26. What are different mode of transfer of heat? Ans: Heat flows from higher to lower temperature till both body attend same temperature. There are three mode of transfer of heat (a) Conduction: The mode of transfer of heat from molecules to molecules without movement of particles.. Conduction takes place in solid as its molecules ate closely packed. Solids, metals and alloy are good conductor. Non metals, plastic glass are bad conductor of heat. (b)Convection: The mode of transfer of heat from molecules to molecules with movement of particles. In liquids and gasses heat transferred by convection as molecules are far apart from each other . (c) Radiation: The mode of transfer of heat that does not require any material medium. Heat of sun reach the earth by radiation. Q.27. At what factor heat absorbed on radiation by body depends on? Ans: (a) Distance between body and source of heat (b) its colour(Black coloured surface absorb more than white surfaces) Q. 29. A desert is very hot in the day and very cold at night? Ans: Due to low specific heat capacity air above land heated up quickly in the day and cooled quickly at night. For this reason a desert is very hot in the day and very

cold at night. Q.30. Describe construction and working of thermos flask. Ans: thermos flask is doubled walled glass vessel. Air inside glass wall prevents conduction and convection. Glass walls are silvered on the inside. Silver is good reflector of heat this prevent heat loss due to radiation. If we remove the silvering thermos flask not keep liquids hot or cold for quite a long time. (a) The hotness of an object is determined by its temperature. (b) Temperature of boiling water cannot be measured by a clinical thermometer. (c) Temperature is measured in degree Celsius. (d) No medium is required for transfer of heat by the process of Radiation. (e) A cold steel spoon is dipped in a cup of hot milk. It transfers heat to its other end by the process of conduction. (f ) Clothes of dark colours absorb heat better than clothes of light colours. Questions: Match the following : (i) Land breeze blows during à night (ii) Sea breeze blows during-àday (iii) Dark coloured clothes are preferred during à winter (iv) Light coloured clothes are preferred during-àsummer Questions: What is heat? Heat is a form of energy, entry or exit of which correspondingly increases or decreases internal energy of a body when no work is done on the body or by the body. Questions: What is temperature? A reliable measure of the hotness of an object is its temperature. Temperature is measured by a device called thermometer. Questions:What is clinical thermometer? The thermometer that measures our body temperature is called a clinical thermometer A clinical thermometer consists of a long, narrow, uniform glass tube. It has a bulb at one end. This bulb contains mercury. Questions: Why clinical thermometer ranging 35oC to 42oC.? The normal temperature of human body is 37°C.The temperature of human body normally does not go below 35oC or above 42oC. That is the reason that this a clinical thermometer has the range 35oC to 42oC. Questions: What is the use of the m a x i m u m - m i n i m u m 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 m a x i m u m - m i n i m u m thermometer. Questions: What is the range of a laboratory thermometer? The range of a laboratory thermometer is generally from –10°C to 110°C

Questions: Why does the mercury not fall or rise in a clinical thermometer when taken out of the mouth? Kink prevents mercury level from falling on its own. Questions: What is conduction? 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. Questions: Why conduction is only possible in solids In solids, generally, the heat is transferred by the process of conduction because particles of solids are closely packed and heat is transferred from the hotter end to the colder end of an object. Questions: What are conductor and insulators? 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 known as insulators such as plastic and wood. Questions: Explain how water heated by convection? The water is poor conductors of heat so do not heated by conduction. When water is heated, the water became lighter. Hot water rises up. The cold water from the sides moves down towards the source of heat This water also gets hot and rises and water from the sides moves down. This process continues till the whole water gets heated. This mode of heat transfer is known as convection Questions: What is land and sea breeze explain? During the day, the land gets heated faster than the water. The air over theland 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. At night , 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 Questions: 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. Radiation can take place whether a medium is present or not. Questions: In summer we prefer light-coloured clothes and in winter we usually wear dark-coloured clothes. Why 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. Questions: How Woolen clothes keep us warm in winter Woollen clothes keep us warm during winter. It is so because wool is a poor conductor of heat and it has air trapped in between the fibres

Questions: Why one thick blanket is less warm up than two thin blankets joined together? There ia a layer of air in between the blankets..Since air is bad conductor of heat prevent body heat to escape out. Questions: How is heat transferred in solids, liquids and gases? The heat flows from a body at a higher temperature to a body at a lower temperature In solids, generally, the heat is transferred by conduction. In liquids and gases the heat is transferred by convection. No medium is required for transfer of heat by radiation Questions: Why we wear light cloths in summer? Dark-coloured objects absorb radiation better than the light-coloured objects. That is the reason we feel more comfortable in light-coloured clothes in the summer Questions: What are the different thermometer scales? Different thermometer scales (a) The centigrade or Celsius scale: It is introduced by Celsius, is usually used in scientific laboratories. In this scale the lower fixed point or the ice-point is 0°C and the steam-point 100°C. The fundamental interval is divided in 100 equal parts; each part is called 1°C (one degree Celsius). (b) The Fahrenheit scale: It is suggested by Gabriel Fahrenheit, is usually used in clinical and meteorological purposes. Here, the ice-point and the steam-point are correspondingly marked as 32°F and 212°F, and the fundamental interval is divided into 180 equal divisions. Each division is called one degree Fahrenheit. (1°F). (c) Absolute scale or Kelvin scale: It is designed by Lord Kelvin, is used internationally in modern scientific world. In this scale the ice-point is marked 273K and the steam-point 373K, the fundamental interval is divided into100 equal divisions, like that in the Celsius scale. Each division is read as one degree absolute or one Kelvin (IK). In fact, Kelvin scale of temperature starts from the temperature corresponding to -273°C, taken as zero Kelvin Questions: What is the Relation between Celsius, Fahrenheit and Kelvin scales? Since the range of temperature from ice-point to steam-point is equal in all the three scales, 100 centigrade degrees = (212 - 32) or 180 Fahrenheit degrees = (373 - 273) or 100 absolute degrees. We consider three thermometers in the above three scales are dipped simultaneously in a liquid of certain temperature. Let the temperatures recorded in the Celsius, Fahrenheit and Kelvin thermometers respectively be C, F and K. Now it can be proved that C / 5 = F- 32 / 9 = K- 273 /5 Questions: What is the unit of heat Units of Heat (a) C.G.S. unit of heat is Calorie. (b) The M.K.S. or S.I. unit of heat is Joule 1 calorie equals 4.18 or 4.2 joules approximately. Questions: The quantity of heat absorbed or given out by a substance during a thermal change depends on what factors? The quantity of heat absorbed or given out by a substance during a thermal change depends on (a) mass,

(b) difference of temperature and (c) Specific heat of the material of the substance. (a) Mass: A larger mass of a substance absorbs or gives out more heat than a smaller mass of it for a certain change of temperature. So, if 'H' be the quantity of heat absorbed or given out by a substance of mass 'm’ for a given change of temperature, H = m. (b) Temperature: The heat absorbed or released by a certain quantity of a given substance increases or decreases accordingly as the difference between the initial and final temperatures is large and small. Thus, H= (T-t) where, (T-t) is the difference between the initial and the final temperatures of the body. (c) Specific heat: This is a fundamental property of matter. An equal mass of different materials absorb or give out different quantities of heat, even if they are heated or cooled through the same range of temperature.This is also called specific heat capacity (abbreviated as SHC) in S.I. system. Heat (H) absorbed or given out by body of mass (m) for rise or fall of temperature through t is given by H = mst. Questions: State similarities between the laboratory thermometer and the clinical thermometer 1. Laboratory thermometer and the clinical thermometer consist of a long, narrow, uniform glass tube. 2. Laboratory thermometer and the clinical thermometer have a bulb containing mercury at the end of the tube. 3. Laboratory thermometer and the clinical thermometer are marked with Celsius scale on the glass tube. Questions: Discuss why wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing. In between the layers of cloths there is trapped air. As air is bad conductor of heat so the out side low temperature do not get transferred to body as well as prevent our body heat to escape out side. Hence more layers of cloths keep us warmer during cold winter. Questions: In places of hot climate it is advised that the outer walls of houses be painted white. Explain. In places of hot climate it is advised that the outer walls of houses be painted white because a light colour absorb very less radiant heat and we feel comfortable inside such houses due to lower temperature inside house. Questions: Why stainless steel cooking utensils are usually provided with copper bottoms? The reason for this could be that copper is the best conductor of heat than the stainless steel. Heat Q1. Fill in the blanks.  The hotness of an object is determined by its temperature.  Temperature is measured in degree celsius.  A cold steel spoon is dipped in a cup of hot milk. It transfers heat to its other end by the process of conduction.  Land breeze blows during night.  Sea breeze blows during day.  Dark coloured clothes are preferred during winter. Poor conductors are known as insulators.

Q2. True/False  Temperature of boiling water cannot be measured by a clinical thermometer. True  Medium is required for transfer of heat by the process of radiation. False  Clothes of dark colours absorb heat better than clothes of light colours. True  Light coloured clothes are preferred during summer. True  We should hold the thermometer by the bulb while holding it. False  The bulb of laboratory thermometer should not touch the bottom or the sides of the container. True Q3. In which direction does the smoke go? Ans. Smoke will go upward. Q4. What do you mean by temperature? Ans. A reliable measure of the hotness of an object is its temperature. Q5. Which device is used to measure temperature? Ans. Thermometer is used to measure temperature. Q6. What is that silver stuff in a thermometer? Ans. Silver stuff in a thermometer is mercury. Q7. What is the unit of temperature as adopted by India? Ans. India has adopted the celsius scale. Q8. What is the normal temperature of a human being? Ans. The normal temperature of human body is 37°C. Q9. What is the range of a laboratory thermometer? Ans. The range of a laboratory thermometer is generally from –10°C to 110°C. Q10. What is the use of the kink in clinical thermometer? Ans. It prevents mercury level from falling on its own. Q11. How does heat flow from one object to another? Ans. Heat flows from a hotter object to a colder object. Q12. Does transfer of heat by radiation require any medium? Ans. The transfer of heat by radiation does not require any medium. Q13. Do all hot bodies radiate heat? Ans. Yes, all hot bodies radiate heat. Q14. How does heat transfer in liquids and gases? Ans. In liquids and gases the heat is transferred by convection. Q15. Will heat transfer if the temperature of two objects is the same? Ans. Heat will not be transferred if the temperature of two objects is the same.

Q16. What should be the level of mercury in the clinical thermometer before use? Ans. Mercury level should be below 35°C. Q17. How does heat transfer in solids? Ans. In solids, generally, the heat is transferred by the process of conduction. Q18. What is the best way to clean a thermometer? Ans. Thermometer should be washed before and after use, preferably with an antiseptic solution. Q19. What is Conduction? Ans. The process by which heat is transferred from the hotter end to the colder end of an object is known as conduction. Q20. What are insulators of heat? Ans. The materials which do not allow heat to pass through them easily are insulators of heat. Example: plastic and wood. Q21. What are conductors of heat? Ans. The materials which allow heat to pass through them easily are conductors of heat. Example: aluminum, iron and copper. Q22. What is clinical thermometer? Ans. The thermometer that measures our body temperature is called a clinical thermometer. Q23. Give two examples each of conductors and insulators of heat. Ans. Conductors – aluminum and copper Insulators - water and air Q24. How does the heat travel in air? Ans. 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. Q25. What do you understand by heat? Ans. Heat is a form of energy that can be transferred from one object to another or even created at the expense of the loss of other forms of energy. Q26. What is the use of maximum-minimum thermometer? Ans. The maximum and minimum temperatures of the previous day, reported in weather reports, are measured by maximum-minimum thermometer. Q27. One litre of water at 30°C is mixed with one litre of water at 50°C. The temperature of the mixture will be (a) 80°C (b) more than 50°C but less than 80°C (c) 20°C (d) between 30°C and 50°C. Ans. (d) between 30°C and 50°C. Q28. Why does the mercury not fall or rise in a clinical thermometer when taken out of the mouth? Ans. Clinical thermometer has kink near the bulb prevents mercury level from falling on its own.

Q29. Why clinical thermometer cannot be used to measure high temperatures? Ans. Clinical thermometer cannot be used to measure high temperatures because the range of this thermometer is from 35°C to 42°C only. Q30. Explain land breeze. Ans. At night, 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. Q31. What is the concern associated with the use of mercury thermometer? Ans. 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. Q32. Explain the construction of clinical thermometer. Ans. A 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. Q33. In places of hot climate it is advised that the outer walls of houses be painted white. Explain. Ans. In places of hot climate it is advised that the outer walls of houses be painted white because light color reflects most of the heat that falls on them and thus keep the house cool. Q34. Is it possible to construct buildings that are not affected much by heat and cold outside? Ans. 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. Q35. What do you mean by maximum-minimum thermometer? Ans. 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. Q36. Why clinical thermometers range from 35°C to 42°C? Ans. 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. Q37. Discuss why wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing. Ans. Wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing because air get trapped in between the layers of clothing and being a bad conductor of heat, prevents the flow of heat from our body to the cold surroundings. Q38. When we come out in the sun, we feel warm. How does the heat from the sun reach us? Ans. 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.

Q39. In summer we prefer light-coloured clothes and in winter we usually wear dark-coloured clothes. Why is it so? Ans. 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. Q40. What is sea breeze? Ans. 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. Q41. Two thin blankets joined together are usually warmer than one thick blanket. Give reason. Ans. Two thin blankets joined together are usually warmer than one thick blanket because air gets trapped between the layers of blankets and being a bad conductor of heat, prevents the flow of heat from our body to the cold surroundings. Q42. How water gets heated when kept on flame? Ans. When water is heated, the water near the flame gets hot. Hot water rises up. The cold water from the sides moves down towards the source of heat. This water also gets hot and rises and water from the sides moves down. This process continues till the whole water gets heated. This mode of heat transfer is known as convection. Q43. An iron ball at 40°C is dropped in a mug containing water at 40°C. The heat will (a) flow from iron ball to water. (b) not flow from iron ball to water or from water to iron ball. (c) flow from water to iron ball. (d) increase the temperature of both. Ans. (b) not flow from iron ball to water or from water to iron ball. Q44. A wooden spoon is dipped in a cup of ice cream. Its other end (a) becomes cold by the process of conduction. (b) becomes cold by the process of convection. (c) becomes cold by the process of radiation. (d) does not become cold. Ans. (d) does not become cold. Q45. Stainless steel pans are usually provided with copper bottoms. The reason for this could be that (a) copper bottom makes the pan more durable. (b) such pans appear colourful. (c) copper is a better conductor of heat than the stainless steel. (d) copper is easier to clean than the stainless steel. Ans. (c) copper is a better conductor of heat than the stainless steel. Q46. State the similarities and differences between the laboratory thermometer and the clinical thermometer. Ans. Similarities Both thermometers consist of a long, narrow, uniform glass tube. Both have a bulb at one end. This bulb contains mercury. Both have celsius scale.

Differences A clinical thermometer reads temperature from 35°C to 42°C whereas the range of a laboratory thermometer is generally from –10°C to 110°C. A clinical thermometer has a kink in it whereas there is no kink in laboratory thermometer. Q47. What are the precautions that need to be observed while reading a clinical thermometer? Ans. 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. Q48. State the precaution to be observed while using a laboratory thermometer. Ans. Precaution to be observed while using a laboratory thermometer Handle the thermometer with care. If it hits against some hard object, it can break. Thermometer should be kept upright not tilted. Bulb should be surrounded from all sides by the substance of which the temperature is to be measured. The bulb should not touch the surface of the container. Do not move the thermometer while measuring the temperature of the substance. Read the temperature of the object when the thermometer is in the substance. Q49. Why can't we use a laboratory thermometer to measure human body temperature? Ans. We can’t use a laboratory thermometer to measure human body temperature because the range of a laboratory thermometer is high generally from –10°C to 110°C while the normal body temperature of human body is 37°C. Moreover, a laboratory thermometer does not have a kink, so the mercury falls on its own upon removing it from the body orifice. Thus, it does not give accurate temperature of the human body.