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Unformatted text preview: Chapter 6 – The First Law of Thermodynamics The first law of thermodynamics is an extension of the principle of conservation of energy. It broadens this principle to include energy exchange by both heat transfer and mechanical work and introduces the concept of internal energy of a system. We introduced systems in the last chapter. The system might be a mechanical device, a biological organism, or a specified quantity of material such as the refrigerant in an air conditioner or steam expanding in a turbine. A thermodynamic system is a system that can interact (and exchange energy) with its surroundings, or environment, in at least two ways, one of which is heat transfer. A familiar example is a quantity of popcorn kernels in a pot with a lid. When the pot is placed on a stove, energy is added to the popcorn by conduction of heat; as the popcorn pops and expands, it does work as it exerts an upward force on the lid and moves it through a displacement. Fig. 6-1. The popcorn in the pot is a thermodynamic system. In the thermodynamic process shown here, heat is added to the system, and the system does work on its surroundings to lift the lid of the pot. The state of the popcorn changes in this process, since the volume, temperature, and pressure of the popcorn all change as it pops. A process as this one, in which there are changes in the state of a thermodynamic system, is called a thermodynamic process . With thermodynamic systems, it is essential to define clearly at the start exactly what is and is not included in the system. Only then can we describe unambiguously the energy transfers into and out of that system. For instance, in our popcorn example we defined the system to include the popcorn but not the pot, lid, or stove. Signs for Heat and Work in Thermodynamics We describe the energy relations in any thermodynamic process in terms of the quantity of heat Q added to the system and the work W done by the system. Both Q and W may be positive, negative, or zero (Fig. 6-2). 1 Fig. 6-2. A thermodynamic system may exchange energy with its surroundings (environment) by means of heat and work. (a) When heat is added to the system, Q is positive. (b) When heat is transferred out of the system, Q is negative. (c) When work is done by the system, W is negative. (d) When work is done on the system, W is negative. Energy transfer by both heat and work can occur simultaneously; in (e), heat is added to the system and work is done by the system, and in (f), heat is transferred out of the system and work is done on the system. A positive value of Q represents heat flow into the system, with a corresponding input of energy to it; negative Q represents heat flow out of the system. A positive value of W represents work done by the system against its surroundings, such as work done by an expanding gas, and hence corresponds to energy leaving the system. Negative W , such as work done during compression of a gas in which work is done on the gas by its surroundings, represents energy...
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This note was uploaded on 09/12/2011 for the course ASC 1640 taught by Professor Abbitt during the Spring '08 term at Santa Fe College.
- Spring '08