Unit-2 Heat and thermodynamics 1. Heat and temperature HSEB syllabus: Heat and temperature- Concept of temperature; Thermal equilibrium, Thermal expansion: linear expansion, cubical expansions and their relation: Measurement of linear expansivity, Liquid Expansion: Absolute and apparent expansion of liquid, Measurement of absolute expansivity by Dulong and Petit method Short Question –Answers 1. During high fever a wet cloth is kept on the forehead of a person. Why? [HSEB 2051] Ans: During high fever a wet cloth is kept on the forehead of a person to decrease the temperature of body. We know that water has highest value of specific heat capacity. When the wet cloth is placed on the forehead, water in wet cloth starts to evaporate and it starts to absorb a lot of heat from our body. Hence it is used to decrease the temperature of body. 2. Water pipes burst in winter why? (HSEB 2052) Ans: In winter temperature of surrounding is low. Due to the low temperature of surrounding, temperature of water in pipes also starts to decrease. As we know that water has highest density and minimum volume at 40C, as temperature is decreased from 40C, its volume is increased. So water pipe burst. 3. Why does the level of mercury in a thermometer fall initially and rises when it is dipped in hot bath? (HSEB 2054) Ans: Hint: Because of the thermal expansion of the bulb in the thermometer, the level of mercury initially falls. As the mercury inside the bulb gets heated, it starts to expand, so level of mercury rises. 4. Differentiate between heat and temperature. [HSEB 2058] Ans: S.N. Heat Temperature 1. It is the form of energy which is produced due to the kinetic energy of molecules. It is the degree of hotness and coldness of a body. 2. Heat is the cause of temperature. Temperature is the effect of heat. 3. Its unit in SI is Joule. Its unit in SI is Kelvin. 4. It is measured by using calorimeter. It is measured by using thermometer. 6. What is thermal equilibrium? Ans: If two bodies at different temperatures are brought in contact, the flow of heat energy will continue until both bodies acquire same temperature. The state of equality of temperatures of two bodies is known as thermal equilibrium. If the two bodies are in thermal equilibrium, there is no net exchange of heat between them but heat flowing to the body is equal to heat flowing away from it. 7. Why is heat energy produced when we rub our hands? Ans: Heat is the form of energy which is produced due to the kinetic energy of molecules. When we rub our hands, the kinetic energy of moving hands has been converted into heat energy. So heat energy is produced when we rub our hands. 8. Why is mercury commonly used as the thermometric liquid? Ans: Mercury is commonly used as thermometric substance because of the following reasons: a. Mercury has a wide range of melting and boiling points i.e. from - 390C to 3570C. b. It is a very good conductor of heat and expands uniformly. c. It is opaque and can be seen easily through glass. d. It doesn’t stick to the glass wall because it is cohesive. e. It absorbs very less amount of heat from the body as its specific heat capacity is low. 9. What is the function of constriction in clinical thermometer? Ans: The main function of constriction in clinical thermometer is to keep the level of mercury constant in the thermometer. When the thermometer is taken out from the body, the level of mercury remains constant such that there is no error in temperature reading. 10. Why is gas thermometer more sensitive than liquid thermometer? Ans: When gas is heated extreme expansion takes place. Gases are more sensitive with temperature compared to solids and liquid. Therefore gas thermometers are more sensitive than liquid thermometer. 11. Why are the telephones cables taut (tight) in winter and slacken (loose) in summer? Ans: In winter the temperature of the surroundings is low so the telephone cables are compressed and become taut. But in summer the temperature of surroundings is increased so due to linear expansion the cables become little bit slackened. 12. Why should measuring scale is made up of insulators or metals whose coefficient of linear expansion is small? Ans: Measuring scale is an instrument which is used to measure the length of different bodies. It should give correct reading from summer to winter i.e. it should not expand or compress when the temperature of the surroundings is increased or decreased. Hence it should be made up of insulators like polythene or wood or metals whose coefficient of linear expansion is small. 13. Bridges are provided with small gaps why? Ans: Bridges are made up of metals like iron. When the temperature of the surrounding is increased iron blocks used in bridge start to expand. If bridges are not provided with gaps, they may be spoilt and cracked. Hence gaps are provided to expand the iron blocks. 14. In railway track small gaps are provided between rails. Why? Ans: Hint: Gaps are provided for the iron rails to expand. 15. Why a jammed lid of pen can be opened when the pen is dipped in hot water? Ans: A jammed lid of pen can be opened when the lid of the pen is dipped in hot water because of the thermal expansion of the lid. When the pen is dipped in hot water, first the outer lid gets heat so it starts to expand. Hence a jammed lid of pen can be opened when the pen is dipped in hot water. 16. A cup made of glass cracks when very hot water is poured into it, why? Ans: Glass is a poor conductor of heat. When very hot water is poured into a glass cup, it will suddenly crack because of irregular expansion of glass. When very hot water is poured the inner part of the cup, it starts to expand but the outer part of the cup is in equilibrium. Due to the unequal expansion between inner and outer part a stress is produced on the glass and the cup cracks.
Posted on: Sun, 04 Aug 2013 06:59:23 +0000
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