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PHYS1003 ENERGY AND RELATED ENVIRONMENTAL ISSUES LECTURE 4 LAWS of THERMODYNAMICS I Energy Conversion Map Chemical Energy Mechanical Energy Nuclear Energy Thermal Energy Radiation Electrical Energy Chemical Energy Heat Engine Mechanical Energy Nuclear Energy Thermal Energy Radiation Electrical Energy Basic Concepts about Thermal Energy We know that Thermal energy is produced by burning fuel chemical energy Heat thermal energy flows from high temperature region to low temperature region which is a spontaneous process This spontaneous process is an energy degradation process because energy in cold object cannot flow back to hot objects Thermal energy at high temperature is of higher quality and is thus more useful We want to 1 steal energy from the heat degradation process 2 reverse heat flow from low temperature region to high temperature region 4 What can we do We can indeed obtain useful mechanical energy by using a heat engine We can also reverse the heat flow by using a heat pump Fuel Oxygen Chemical Energies 5 Examples of such heat engine heat pump in your daily life 6 Review on Mechanical Work Work is the amount of energy transfer from one object to another through a force Work done on the box 25 N 2 m 50 Nm 50 J Work Zero even forces are applied 7 HEATING UP AN IDEAL GAS WHAT IS HAPPENING TO INTERNAL ENERGY p1 p1 p1 T0 p1 V1 Q Heat plate T1 T1 T0 p1 V1 Heat plate T1 T1 p1 V2 IDEAL GAS LAW pV nRT NkBT PARTICLE MOTION exerts pressure 2 2 Internal energy increases 8 HEATING UP AN IDEAL GAS AN IDEAL GAS CAN PERFORM WORK p1 1 Pa p1 p1 ds Area A Example d 2 3 2 2 0 T0 p1 V1 Q Heat plate T1 T1 T0 p1 V1 Heat plate T1 T1 p1 V2 IDEAL GAS performs work Fds on the environment Work done on the gas Work done by the gas Won by WORK is ENERGY 1 kg m 2 m 1 kg m2 2 1 J 9 COMPRESSING IDEAL GAS p1 1 Pa p1 ds Work done on the gas Work done by the gas p1 V1 p2 V2 by 1Pa 2 4 3 2 0 In compression 0 Net work is done on the gas system 10 WE CAN CALCULATE THE WORK p V diagrams CONSIDER THERMODYNAMIC PROCESS Ta pa Va Tb pb Vb p V diagrams 11 WE CAN CALCULATE THE WORK IT S THE AREA UNDER THE CURVE IN p V DIAGRAMS CONSIDER THERMODYNAMIC PROCESS Ta pa Va Tb pb Vb WORK DONE BY THE GAS corresponds to the area under the curve in a p V diagram Vb Va expansion Wby 0 Won 0 Va Vb compression Wby 0 Won 0 Won d 12 Mechanical Equivalent of Heat Heat as a form of energy was established by the recognition of the mechanical equivalent of heat by Joule in 1847 One can put work and heat in the same footing as energies 13 13 National Museum of Nature and Science Tokyo July 2023 14 15 FIRST LAW OF THERMODYNAMICS INTERNAL ENERGY of a system changes by heat added to and work performed on it p2 Heat plate T1 Work done on the gas system Fds Net Heat Transfer to the gas system Q Q Gas absorbs heat 0 Gas releases heat 0 16 Calculating Work in isobaric process constant p isochoric process constant V ISOBARIC EXPANSION pa Va pb Vb Vb Va THE GAS PERFORMS WORK W 0 W d ISOCHORIC PROCESS pa Va pb Vb Vb Va THE GAS PERFORMS NO WORK W 0 17 Calculating Work in isobaric process constant p isochoric process constant V ISOBARIC EXPANSION pa Va pb Vb Vb Va THE GAS PERFORMS WORK W 0 W d ISOCHORIC PROCESS pa Va pb Vb Vb Va HOW MUCH THERMAL ENERGY IS NEEDED IN THESE PROCESSES Answer in the next few pages 18 HEAT CAPACITY OF WORKING GASES OR HOW MUCH THERMAL ENERGY IS NEEDED 1 n Q HEAT CAPACITIES OF AN IDEAL GAS heat required to 1 K in 1 mole of gas at constant pressure CP CV at constant volume Cp CV CONSTANT VOLUME PROCESSES 0 for ideal gas 2 CONSTANT PRESSURE PROCESSES V can increase W 0 More heat is needed for thermal expansion 19 RATIO OF HEAT CAPACITY Ratio of heat capacity Examples Monatomic gas j 3 translations CV 3 2 R CP 5 2 R and 5 3 1 67 Diatomic gas j 3 translations 2 rotations 5 CV 5 2 R CP 7 2 R and 7 5 1 40 Polyatomic gases j 3 translations 3 rotations 6 CV 6 2 R CP 8 2 R and 8 6 1 33 Solids and liquids small thermal expansion CP CV 20 20 Check point THERMODYNAMIC PROCESSES An ideal gas changes its state from a to c in below diagram The work done by the gas is largest in path p a d 1 The largest along the path a b c 2 The smallest along a c 3 The largest along the path a d c 4 The same along all paths b c V 21 Check Point THERMODYNAMIC PROCESSES An ideal gas changes its state from a to c in below diagram The work done by the gas is p a d 1 The largest along the path a b c 2 The smallest along a c 3 The largest along the path a d c 4 The same along all paths b c WORK DONE BY THE GAS corresponds to the area under the curve in a p V diagram V Which one has the largest heat absorb Q Answer Path 1 Wby Won d Largest area under path Path 1 22 QoD THERMODYNAMIC PROCESSES An monoatomic ideal gas changes its state from c directly to a in below diagram p 2 0 atm 27 C a 1 0 atm d b c Question Find the following quantities for process c a a work done on the gas b Change in internal energy c heat absorbed by the gas 1 m3 3 m3 V 1 atm 1 013 105Pa Hints How many mole of gas do we have What is the temperature at c 23 Feedback Anonymous Poll Tell me what you think about this lecture Thank you 25


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hkust PHYS 1003 - Thermodynamics I

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