Part II First Law of Thermodynamics

# Part II First Law of Thermodynamics - Part II First Law of...

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2 - 1 Part II First Law of Thermodynamics Introduction The first law deals with macroscopic properties, work , energy , enthalpy , etc. One of the most fundamental laws of nature is the conservation of energy principle . It simply states that during an interaction, energy can change from one form to another but the total amount of energy remains constant. That is, energy cannot be created or destroyed. Or, during an interaction between a system and its surroundings, the amount of energy gained by the system must be exactly equal to the amount of energy lost by the surroundings. A rock falling off a cliff, for example, picks up speed as a result of its potential energy being converted to kinetic energy. The first law of thermodynamics is simply an expression of the conservation of energy principle, and it asserts that energy is a thermodynamic property. Energy can cross the boundary of a closed system in two distinct forms: heat and work . It is important to distinguish between these two forms of energy. Therefore, they will be discussed first, to form a sound basis for the development of the first law of thermodynamics. We can use the principle of conservation of energy to define a function U called the internal energy. When a closed system undergoes a process by which it passes from state A to state B, if the only interaction with its surroundings is in the form of transfer of heat Q to the system, or performance of work W on

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2 - 2 the system, the change in U will be U = U B – U A = Q + W 2-1 Note: In Equation 2-1 we have defined W as the work done on the system and Q is added to the system. If we had defined W as work done by the system, Equation 2-1 would become U = Q- W. For an isolated system there is no heat or work transferred with the surroundings, thus, by definition W = Q = 0 and therefore U = 0. The first law of thermodynamics states that this energy difference U depends only on the initial and final states, and not on the path followed between them. Both Q and W have many possible values, depending on exactly how the system passes from A to B, but Q + W = U is invariable and independent of the path. If this were not true, it would be possible, by passing from A to B along one path and then returning from B to A along another, to obtain a net change in the energy of the closed system in contradiction to the principle of conservation of energy. For a differential change, Equation 2-1 becomes dU = dQ +dW 2-2 For a cyclic process, A B A, when the system returns to state A, it has the same U , thus = 0 dU 2-3 Next we will take a look separately at the heat transferred ( dQ ) and the work ( dW ) exchanged between the system and the surroundings. 2-1 Heat Transfer Heat is defined as the form of energy that is transferred between two systems (or a system and its surroundings) by virtue of a temperature difference. That is, an energy interaction is heat only if it takes place because of a temperature difference. Then it follows that there cannot be any heat transfer between two systems that are at the same temperature
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Part II First Law of Thermodynamics - Part II First Law of...

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