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Chapter 15

Chapter 15. Thermodynamics Thermodynamics The First Law of Thermodynamics Thermal Processes that Utilize an Ideal Gas The Second Law of Thermodynamics Heat Engines Carnot’s Principle and the Carnot Engine. Thermodynamics. Thermodynamics: study of heat flow and work done on or by a system

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Chapter 15

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  1. Chapter 15 Thermodynamics Thermodynamics The First Law of Thermodynamics Thermal Processes that Utilize an Ideal Gas The Second Law of Thermodynamics Heat Engines Carnot’s Principle and the Carnot Engine

  2. Thermodynamics • Thermodynamics: study of heat flow and work done on or by a system • Four laws of Thermodynamics • 0- if two systems are in equilibrium (thus have the same temperature) there will be no net flow between them. Further, if systems A and B are in thermal equilibrium with system C, they must be in equilibrium with each other. • 1- will be addressed further in this powerpoint • 2- will be addressed further in this powerpoint • 3- it is impossible to reach a temperature of absolute zero • The 1st and 2nd laws appear on the AP-B exam • Ex:

  3. The First Law of Thermodynamics • Energy can be transformed but total energy must remain constant (provided the system is isolated and energy can neither go in or out) • A system can exchange energy with it’s surroundings in two ways: heat or work • 1st Law- the change in the internal energy ΔU of a system is equal to the heat Q added to the system plus the work done on the system: • ΔU = Q + W **On the AP-B exam, if work is done ON a system, the system gains energy and W is positive. If work is done BY the system, the system loses energy and W is negative.

  4. First Law cont. • For ideal gas systems, expansion against some external pressure means that work is done BY the system while compression implies work being done ON the system. • Ex. If a system has 60J of heat added to it resulting in 20 J of work being done BY the system, the change in internal energy Q-W = 40J. If 60 J of heat was added to a system AND 20 J of work is done ON the system, the internal energy would increase even more to Q+W = 80 J. • If heat is added to a system and no work is done, then the heat lost by one element in the system is equal to the heat gained by another element. (lab) • Ex:

  5. Thermal Processes that utilize an Ideal Gas • Changes in pressure, volume and temp of a gas can be studied by plotting pressure vs. volume (PV diagram) • These relationships can be described with the following terms: • Isobaric- when the pressure remains constant as the volume changes; constant pressure • Isochoric/Isovolumetric- when pressure varies but the volume is kept constant; constant volume • Isotherm- if we want the gas to return to its original state w/o changing temp, must trace a curve from original temp to original volume; constant temp • Adiabatic- any process which is done without the transfer of heat, no heat lost or gained ΔU = W • Ex:

  6. Second Law of Thermodynamics • Entropy S is the measure of the disorder, or randomness of a system. Entropy is important because it determines whether a process will occur spontaneously. • 2nd law- all spontaneous processes proceeding in an isolated system lead to an increase in entropy • An isolated system will naturally pursue a state of higher disorder

  7. Heat Engines • Heat engine is any device that uses heat to perform work. Three essential features: • Heat is supplied to the engine at a high temp from a hot reservoir • Part of the input heat is used to perform work • The remainder of the input heat which did not do work is exhausted into a cold reservoir, which is at a lower temp than the hot reservoir. • The percent efficiency %e of the heat engine is equal to the ratio of the work done to the amount of input heat: • %e = (Work/Qhot )x 100 • Ex:

  8. Carnot’s Principle and the Carnot Engine • Sadi Carnot suggested that a heat engine has a maximum efficiency when the processes within the engine are reversible; the system and its environment can be returned exactly to the states they were in before the process occurred • There can be no dissipative forces (ie: friction) in the Carnot cycle of an engine for it to operate at max efficiency. A reversible engine is called a Carnot engine. • Non-examples are those with spontaneous processes (heat flow from hot to cold reservoir) since work would have to be done to restore heat to the hot reservoir thus changing the environment by using its energy to do work

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