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Calorimetry 1. Heat: Every substance is made up of molecules. These molecules are in a state of constant random motion and also have some force of attraction between them. Due to their constant random motion they possess molecular kinetic energy and due to the force of attraction between them they maintain a certain configuration and possess molecular potential energy. We are explicitly using the word molecular because these energies are internal to molecules and have no relation with Mechanical Kinetic and Potential Energies that we have studied in the Chapter Work, Energy and Power. The Sum of Molecular Kinetic Energy and Molecular Potential Energy is known as Internal/Thermal Energy. A hotter body has more Internal Energy Compared to a Cooler Body. When a hotter body is placed in contact with a cooler body , then Internal Energy flows from the hotter body to the cooler body until both the bodies reach thermal equilibrium (i.e attain the same temperature). The Internal Energy in Transit is known as Heat Energy. The term heat is meaningful as long as there is flow of Internal Energy from a hotter body to a cooler body. When the flow stops , we can no longer use the term Heat energy . The phrases like Heat contained in a body are completely meaningless. Rather , we must say Thermal Energy contained in the body . The phrase like A body gets heated up should be used carefully because it can only be used when the temperature of a body rises due to flow of internal energy from hotter body to a cooler body. Eg: i) If we rub our palms against each other , them the temperature of our palm rises. But , can we say that Heat is produced in our palms . Unfortunately , we cant because in this case the rise in temperature is due to the work done by frictional forces acting between our palms and not because of flow of energy to our palms from a hotter body. But , if we stand in front of a bonfire in a Christmas Eve and after some time the temperature of our palm rises. Can we say that Heat has flown to our palms? . Well in this case , the answer is YES. , because for sure there has been a transfer of internal energy from the fire(hotter body) to our palms (cooler body). ii) When electrons move through a conductor under the action of a potential difference , then can we say that due to collisions between the atoms of the conductor and the electrons , the conductor gets heated up You will find many teachers/textbooks saying this. It has been so common that it is accepted from exam point of view. But from the point of view of a serious Physics Enthusiast , the above statement is blatantly a grammatical mistake in physics because as we know that the term heat is only used when there is a flow of internal energy from hotter to cooler body. In the above example , one should rather say The electrons collide/interact with the atoms of the conductor and transfer thermal energy to them So How should we Define Heat Energy? Whenever a hot body is brought in contact with a cold body then there a flow of Internal Energy from the hotter body to the cooler body. This Internal energy in transit is termed as Heat Energy. The Branch of physics that deals with the measurement of Heat Energy is known as Calorimetry Heat is measured by Principle of Calorimetry. SI unit: Joule(J) Other units: Calorie(cal) 1cal= 4.18J or 4.2J (approximately) One calorie is defined as the amount of heat energy required to raise the temperature of 1g of water from 14.5 0C to 15.5 0C. One Kilo calorie is the amount of heat energy required to raise the temperature of 1kg of water from 14.5 0C to 15.5 0C. 2. Temperature: It is one of the fundamental physical quantities which tells us about the thermal state of a body (i.e. degree of hotness and coldness). It depends upon the average kinetic energy per molecule and not on the total Internal Energy. (i.e temperature is related only with molecular kinetic energy and has no relation with molecular potential energy). It decides the direction of flow of heat energy when two bodies are kept in contact. SI unit: Kelvin(K) Other units: 0C , 0F Temperature is measured using thermometer. Let s understand few more facts about Heat and Temperature using examples: Heat energy depends upon the Total Internal Energy of a body whereas temperature only depends upon the Average Molecular Kinetic Energy of a molecule. ( We are saying Average kinetic energy because all molecules don t possess the same kinetic energy and thus we take a average value) Example 1: Suppose we have a body A .The molecules of the body A have kinetic energy=25J and Potential Energy= 50J. Therefore, Total Internal Energy = 50+25=75J Suppose we have another body B .The molecules of the body B have kinetic energy=50J and Potential Energy= 25J. Therefore, Total Internal Energy = 50+25=75J Both A and B have same Internal Energy i.e Heat Content is same in both. But since B has greater kinetic energy , so B is Hotter (at higher temperature) We could even have taken values , such that both had same molecular kinetic energy but different total Internal Energy. Conclusion: i) Two bodies having same quantity of heat may differ in their temperature ( because they might have different molecular kinetic energies) ii) Two bodies having same temperature may different in the quantities of heat in them ( because molecular potential energy in both bodies may be different) Example 2: A cup of water contains water at 300C. Another cup contains water at 160C. If water is poured from both the cups into a third cup , what do you think will the temperature of water in the third cup be? Will it be 30+16= 460C? Well that ridiculous ,right? If this was true , it would mean that mixing two cups of water each at 500C will amount to 1000C. Actually , the temperature of water in the third cup will be between 16 0C and 30 0C. But what about the total heat content? Well the total heat content of the water in the 3rd cup will be the sum of individual heat contained in the 1st and 2nd cup. (Since heat is a form of energy) Conclusion: i) When two bodies are connected/ mixed then the total amount of heat contained is equal to sum of the heats of the individual bodies. Thus , we say Heat is additive in nature ii) When two bodies at different temperature are connected/ mixed then the final temperature attained by the mixture lies between the temperature of the individual bodies . The final temperature is greater than the temperature of cooler body ad lesser than the temperature of hotter body . # Heat is supplied to a substance. Explain how is this heat utilized? i) For Temperature Change: If a substance is kept at a temperature other than its melting or boiling point , then the molecules of the substance will absorb heat energy and start vibrating with large amplitude. Due to increased amplitude of vibrations , their average kinetic energy will increase and hence temperature will increase. It is worth mentioning that during temperature change the force of attraction between the molecules ( or their average distance of separation) doesn t change and hence the molecular potential energy doesn t change. ii) For Phase/State Change: If a substance is at its melting point or boiling point , then the molecules of the substance are already vibrating with maximum amplitude and their kinetic energy is also maximum for the physical state they are in. Therefore , the supplied heat energy will no longer increase their kinetic energy ( no temperature change) and will only be utilized to do work against the force of attraction between molecules and increase their average distance of separation. This heat energy will be stored in the final physical state as its potential energy. 3. Specific Heat Capacity: (S) It is defined as the amount of heat energy required to raise the temperature of unit mass of a substance by 1K. It depends upon the nature of substance (i.e different substances have different specific heat capacity) SI unit : J kg-1 K-1 If specific heat capacity of a substance is low , then it means that less amount of heat energy is required to bring a greater change in temperature. This is possible if the substance readily absorbs heat i.e. is a good conductor of heat. If specific heat capacity of a substance is high , then it means that greater amount of heat energy is required to bring a small change in temperature. This is possible if the substance is reluctant to absorb heat i.e is a bad conductor of heat. We conclude that Good conductors of heat have low specific heat capacity and Bad Conductors of heat have high specific heat capacity . If specific heat capacity of a substance is infinite , it means that it requires infinite amount of heat energy to raise the temperature of unit mass of substance by 1K. In other words , the temperature of the substance will never change. 4. Heat Capacity(C) It is defined as the amount of heat energy required to raise the temperature of the entire mass of body by 1K. It depends on the mass and nature of substance. Greater the mass of the body , larger is its heat capacity. If Heat Capacity of a body is 50 J K-1 , then it means that 50J of Heat energy is required to raise the temperature of entire body by 1K . SI unit: J K-1 Note: The fundamental difference between Specific Heat Capacity and Heat Capacity is that Specific Heat Capacity is defined for unit mass of substance ( hence it doesn t depend on mass) whereas heat capacity is defined for the entire substance( hence it depends on mass) 5. Calorimeter: A calorimeter is a cylindrical vessel that measures the amount of heat exchanged between two bodies ( i.e the amount of heat lost by hot body or the amount of heat gained by cold body) It is made up of a thin sheet of copper because of two reasons i) Copper is a good conductor of heat so the vessel readily acquires the temperature of its contents. ii) Copper has a low specific heat capacity , so the amount of heat absorbed by copper to acquire the temperature of its contents is negligible. Since Calorimeter will measure the heat exchanged between substances mixed , so there shouldn t be any loss of energy to the surroundings. To ensure that we have to prevent the loss of heat energy due to conduction, convection and radiation. The outer and inner surfaces of the calorimeter is polished in order to reduce loss of heat due to radiation. The calorimeter is placed in a wooden case/jacket and the space between the calorimeter and the jacket is filled with a poor conductor of heat such as wool or cotton in order to prevent loss of heat due to conduction. The mouth of the vessel is covered with a wooden lid to prevent loss of heat due to convection. The wooden lid has two holes , one for the stirrer( which is used to mix the contents properly) and another for the thermometer ( which is used to record the temperature of the contents). 6. Principle Of Calorimetry: When two or more bodies at different temperature are brought in contact or mixed then the heat lost by hotter bodies is equal to the heat gained by cooler bodies , considering no loss of energy to the surroundings. When we apply principle of calorimetry , we consider the system to be perfectly insulated ( i.e there is no loss of energy to the surroundings) It is applicable for mixing of substances in any physical state. ( Water-Water , Ice-Water , WaterSteam) This principle is also known as Principle of Method of mixtures It is based on the law of conservation of energy. 7. Natural Consequences of High Specific Heat capacity of Water: Water has a very high specific heat capacity due to which it requires a large amount of heat to warm up and releases a large amount of heat when it cools down i) Explain the formation of Land Breeze and Sea Breeze / Why is the climate near the sea shore moderate? Important Terminology: Before I explain this , I want you to get this clear that in physics Heat flows from hotter body to cooler body but here we are going to state the direction of movement of breeze. Breeze is nothing but cold air. If heat flows from body A to B , then we can restate the statement as breeze flows from B to A The Specific Heat Capacity of water is much larger than that of sand. So for same mass of water and sand , water requires greater amount of heat to warm up and releases greater amount of heat when it cools down. During the day: The sun rays fall on both land and sea , but land gets heated up much faster than the sea ( because specific heat capacity of water is more than sand). Due to difference is temperature between land and sea , convection currents are set up and we can say that cold air moves from sea to land. This is known as sea breeze. During Night: In absence of sun , both land and sea start cooling down. But the land(cooler) gets cooled down much faster than the sea(hotter). Due to difference in temperature between land and sea , the cold air moves from land to sea. This is known as land breeze. The sea breeze during the day makes the hotter land relatively cool and the land breeze during the night makes the cooler land relatively warm. Thus , they influence the climate near the shore. ii) Water is used in hot water bottles for fomentation? Water has a very high specific heat capacity due to which it releases a large amount of heat when it cools down. Due to this property hot water provides heat energy for fomentation for longer period of time. iii) Water is used as an effective coolant? (Used in radiators of car and in generators for cooling) Water has a very high specific heat capacity due to which it requires a large amount of heat to warm up. Water when made to flow around the heated parts of a machine absorbs all the heat without considerable rise in temperature and hence is used as an effective coolant. iv) In cold countries water is used as a heat reservoir for wine and juice bottles , why? Water has a high specific heat capacity due to which it releases a large amount of heat when in cools down. In cold countries as the surrounding temperature decreases , water cools down and releases a large amount of heat which is absorbed by the bottles immersed in water and hence they don t freeze. v) Why do farmers fill their fields with water on a cold winter night? Water has a high specific heat capacity due to which it releases a large amount of heat when in cools down. In a cold winter night as the surrounding temperature decreases , water cools down and releases a large amount of heat and doesn t allow the surrounding temperature of plants to fall below 00C as otherwise the water in the fine capillaries of plants will freeze and they will die. vi) All living creatures have a high percentage of water in their body? Water has a high specific heat capacity hence when the surrounding temperature falls the heat released helps the internal temperature to remain constant i.e helps in maintaining homeostasis. 8. Some examples of High and Low Heat Capacity: i) Why is the base of a cooking pan made thick and heavy? Ans. Heat capacity of a body is directly proportional to its mass , so by making the pan thick and heavy its heat capacity becomes large due to which the food gets heated slowly and uniformly for proper cooking and after cooking even remains warm for a long time. ii) The base of an electric iron/press is made thick and heavy? Ans. Heat capacity of a body is directly proportional to its mass , so by making the base of electric press thick and heavy its heat capacity becomes large due to which its gets heated slowly and uniformly and even remains hot for longer period of time after the current is switched off. 9. Change of Physical State/Phase: The process in which a substance undergoes a change of physical state at constant temperature is known as phase change. It is brought about by exchange of heat. (Either heat is supplied to the substance or extracted from it) 10. Concept of Boiling and Condensation. The process in which liquid gets converted to vapour at a constant temperature on heating is known as Boiling/Vaporisation. The process in which gas gets converted into liquid at a constant temperature on cooling is known as condensation. The Boiling Point of liquid is that temperature at which if it is heated then it generates sufficient vapours( or vapour pressure) so as to counter balance the atmospheric pressure. Note: i) For pure substance , the boiling point and condensation points are equal. ii) Amount of heat absorbed by a given mass of substance during boiling is same as the amount of heat released by that mass during condensation. Factors affecting boiling point: i) Pressure: If the atmospheric pressure increases , then the liquid needs to be heated to a greater temperature so as to generate more vapour pressure to counterbalance the atmospheric pressure and vice-versa. Thus , Boiling point of liquid increases with increase in pressure and vice versa ii) Addition of non-volatile impurities: When non-volatile impurities like salt is added to a liquid, then it obstructs the formation of vapours. Thus , liquid with added salt needs to be heated at a higher temperature to form required amount of vapours as compared to pure liquid. Hence , Boiling Point Increases. Boiling Point of a liquid increases with addition of non-volatile impurities 11. Concept of Melting and Freezing: The process in which a solid gets converted into liquid at a constant temperature on heating is known as melting. The constant temperature at which melting takes place is known as melting point. The process in which a liquid gets converted into soild at a constant temperature on cooling is known as freezing. The constant temperature at which freezing takes place is known as freezing point. For a pure substance , melting and freezing points are same. 12. Latent Heat: When ice at 00C is heated ,then its average kinetic energy ( and hence temperature) doesn t change. So the question is Where does the supplied heat go? . Well , the supplied heat is utilised in doing work against the intermolecular forces in ice and thus increasing the average distance of separation between the molecules of ice causing it to get converted to water at 00C. Since the supplied heat has done work , so it is stored in water at 00C as its molecular potential energy. Since the effect of this supplied heat doesn t get reflected as change in temperature so it is known as latent heat. When water at 1000C is heated , then its average kinetic energy (and hence temperature doesn t change). The supplied heat is used up in doing work against the intermolecular forces (Vander wall s forces) between water molecules and is ultimately stored in steam at 1000C as its molecular potential energy. Thus, the overall internal energy content of steam at 1000C is more than that if water at 1000C , and for this reason steam burns are more painful. When water at 00C is cooled ,then its average kinetic energy ( and hence temperature) doesn t change. The Heat released by water causes the intermolecular distance of separation between the molecules to decrease and ultimately it gets converted to ice at 00C. Thus we conclude , Heat energy is absorbed by a solid on melting and an equal amount of heat energy is released by the liquid during freezing , without any rise in temperature . Similarly , Heat energy is absorbed by a liquid during vaporisation and an equal amount of heat energy is released by the vapour during condensation , without any rise in temperature . The total amount of heat energy absorbed/released during state change at a constant temperature is known as latent heat. The latent heat is an intrinsic/characteristic property of a substance and is different for different substances depending on their composition/physical state. 13. Specific Latent Heat: If Latent heat is defined for a unit mass of substance then it is known as Specific Latent Heat. Thus , Specific Latent Heat is the amount of heat energy absorbed or released during state change for a unit mass of substance at a constant temperature. SI unit: J kg-1
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