Chapter 10 Section 1 Relationships Between Heat and
Chapter 10 Section 1 Relationships Between Heat and Work Preview • Objectives • Heat, Work, and Internal Energy • Thermodynamic Processes
Chapter 10 Section 1 Relationships Between Heat and Work Objectives • Recognize that a system can absorb or release energy as heat in order for work to be done on or by the system and that work done on or by a system can result in the transfer of energy as heat. • Compute the amount of work done during a thermodynamic process. • Distinguish between isovolumetric, isothermal, and adiabatic thermodynamic processes.
Chapter 10 Section 1 Relationships Between Heat and Work Heat, Work, and Internal Energy • Heat and work are energy transferred to or from a system. An object never has “heat” or “work” in it; it has only internal energy. • A system is a set of particles or interacting components considered to be a distinct physical entity for the purpose of study. • The environment the combination of conditions and influences outside a system that affect the behavior of the system.
Chapter 10 Section 1 Relationships Between Heat and Work Heat, Work, and Internal Energy, continued • In thermodynamic systems, work is defined in terms of pressure and volume change. • This definition assumes that P is constant.
Chapter 10 Section 1 Relationships Between Heat and Work Heat, Work, and Internal Energy, continued • If the gas expands, as shown in the figure, DV is positive, and the work done by the gas on the piston is positive. • If the gas is compressed, DV is negative, and the work done by the gas on the piston is negative. (In other words, the piston does work on the gas. )
Chapter 10 Section 1 Relationships Between Heat and Work Heat, Work, and Internal Energy, continued • When the gas volume remains constant, there is no displacement and no work is done on or by the system. • Although the pressure can change during a process, work is done only if the volume changes. • A situation in which pressure increases and volume remains constant is comparable to one in which a force does not displace a mass even as the force is increased. Work is not done in either situation.
Chapter 10 Section 1 Relationships Between Heat and Work Thermodynamic Processes • An isovolumetric process is a thermodynamic process that takes place at constant volume so that no work is done on or by the system. • An isothermal process is a thermodynamic process that takes place at constant temperature. • An adiabatic process is a thermodynamic process during which no energy is transferred to or from the system as heat.
Chapter 10 Section 2 The First Law of Thermodynamics Energy Conservation • If friction is taken into account, mechanical energy is not conserved. • Consider the example of a roller coaster: – A steady decrease in the car’s total mechanical energy occurs because of work being done against the friction between the car’s axles and its bearings and between the car’s wheels and the coaster track. – If the internal energy for the roller coaster (the system) and the energy dissipated to the surrounding air (the environment) are taken into account, then the total energy will be constant.
Chapter 10 Section 2 The First Law of Thermodynamics Energy Conservation
Chapter 10 Section 2 The First Law of Thermodynamics Energy Conservation, continued • The principle of energy conservation that takes into account a system’s internal energy as well as work and heat is called the first law of thermodynamics. • The first law of thermodynamics can be expressed mathematically as follows: DU = Q – W Change in system’s internal energy = energy transferred to or from system as heat – energy transferred to or from system as work
Chapter 10 Section 2 The First Law of Thermodynamics Signs of Q and W for a system
Chapter 10 Section 2 The First Law of Thermodynamics Sample Problem The First Law of Thermodynamics A total of 135 J of work is done on a gaseous refrigerant as it undergoes compression. If the internal energy of the gas increases by 114 J during the process, what is the total amount of energy transferred as heat? Has energy been added to or removed from the refrigerant as heat?
Chapter 10 Section 2 The First Law of Thermodynamics Sample Problem, continued 1. Define Given: W = – 135 J Tip: Work is done on the gas, so work DU = 114 J (W) has a negative Unknown: Q=? value. The internal energy increases during the process, so the change in internal energy (DU) has a positive value. Diagram:
Chapter 10 Section 2 The First Law of Thermodynamics Sample Problem, continued 2. Plan Choose an equation or situation: Apply the first law of thermodynamics using the values for DU and W in order to find the value for Q. DU = Q – W Rearrange the equation to isolate the unknown: Q = DU + W
Chapter 10 Section 2 The First Law of Thermodynamics Sample Problem, continued 3. Calculate Substitute the values into the equation and solve: Q = 114 J + (– 135 J) Q = – 21 J Tip: The sign for the value of Q is negative. This indicates that energy is transferred as heat from the refrigerant.
Chapter 10 Section 2 The First Law of Thermodynamics Sample Problem, continued 4. Evaluate Although the internal energy of the refrigerant increases under compression, more energy is added as work than can be accounted for by the increase in the internal energy. This energy is removed from the gas as heat, as indicated by the minus sign preceding the value for Q.
Chapter 10 Section 2 The First Law of Thermodynamics Cyclic Processes • A cyclic process is a thermodynamic process in which a system returns to the same conditions under which it started. • Examples include heat engines and refrigerators. • In a cyclic process, the final and initial values of internal energy are the same, and the change in internal energy is zero. DUnet = 0 and Qnet = Wnet
Chapter 10 Section 2 The First Law of Thermodynamics Cyclic Processes, continued • A heat engine uses heat to do mechanical work. • A heat engine is able to do work (b) by transferring energy from a high-temperature substance (the boiler) at Th (a) to a substance at a lower temperature (the air around the engine) at Tc (c). • The internal-combustion engine found in most vehicles is an example of a heat engine.
Chapter 10 Section 2 The First Law of Thermodynamics The Steps of a Gasoline Engine Cycle
Chapter 10 Section 2 The First Law of Thermodynamics The Steps of a Refrigeration Cycle
Chapter 10 Section 2 The First Law of Thermodynamics of a Refrigerator
Chapter 10 Section 3 The Second Law of Thermodynamics Objectives • Recognize why the second law of thermodynamics requires two bodies at different temperatures for work to be done. • Calculate the efficiency of a heat engine. • Relate the disorder of a system to its ability to do work or transfer energy as heat.
Chapter 10 Section 3 The Second Law of Thermodynamics Efficiency of Heat Engines • The second law of thermodynamics can be stated as follows: No cyclic process that converts heat entirely into work is possible. • As seen in the last section, Wnet = Qh – Qc. – According to the second law of thermodynamics, W can never be equal to Qh in a cyclic process. – In other words, some energy must always be transferred as heat to the system’s surroundings (Qc > 0).
Chapter 10 Section 3 The Second Law of Thermodynamics Efficiency of Heat Engines, continued • A measure of how well an engine operates is given by the engine’s efficiency (eff ). • In general, efficiency is a measure of the useful energy taken out of a process relative to the total energy that is put into the process. • Note that efficiency is a unitless quantity. • Because of the second law of thermodynamics, the efficiency of a real engine is always less than 1.
Chapter 10 Section 3 The Second Law of Thermodynamics Sample Problem Heat-Engine Efficiency Find the efficiency of a gasoline engine that, during one cycle, receives 204 J of energy from combustion and loses 153 J as heat to the exhaust. 1. Define Given: Diagram: Qh = 204 J Qc = 153 J Unknown eff = ?
Chapter 10 Section 3 The Second Law of Thermodynamics Sample Problem, continued 2. Plan Choose an equation or situation: The efficiency of a heat engine is the ratio of the work done by the engine to the energy transferred to it as heat.
Chapter 10 Section 3 The Second Law of Thermodynamics Sample Problem, continued 3. Calculate Substitute the values into the equation and solve: 4. Evaluate Only 25 percent of the energy added as heat is used by the engine to do work. As expected, the efficiency is less than 1. 0.
Chapter 10 Section 3 The Second Law of Thermodynamics Entropy • In thermodynamics, a system left to itself tends to go from a state with a very ordered set of energies to one in which there is less order. • The measure of a system’s disorder or randomness is called the entropy of the system. The greater the entropy of a system is, the greater the system’s disorder. • The greater probability of a disordered arrangement indicates that an ordered system is likely to become disordered. Put another way, the entropy of a system tends to increase.
Chapter 10 Section 3 The Second Law of Thermodynamics Entropy, continued • Greater disorder means there is less energy to do work. • If all gas particles moved toward the piston, all of the internal energy could be used to do work. This extremely well ordered system is highly improbable.
Chapter 10 Section 3 The Second Law of Thermodynamics Entropy, continued • Because of the connection between a system’s entropy, its ability to do work, and the direction of energy transfer, the second law of thermodynamics can also be expressed in terms of entropy change: The entropy of the universe increases in all natural processes. • Entropy can decrease for parts of systems, provided this decrease is offset by a greater increase in entropy elsewhere in the universe.
Chapter 10 Section 3 The Second Law of Thermodynamics Energy Changes Produced by a Refrigerator Freezing Water Because of the refrigerator’s less-than-perfect efficiency, the entropy of the outside air molecules increases more than the entropy of the freezing water decreases.
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