Chapter 2-thermodynamics 1

# Chapter 2-thermodynamics 1 - Chapter 2 WORK AND HEAT In the...

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Chapter 2 WORK AND HEAT In the previous chapter, the different thermodynamic systems and their characteristics were discussed. To undergo a change of state, the system has to interact with its surroundings. Work and heat transfers across the boundaries cause these changes. In this chapter various forms of work and modes of heat transfers are discussed. 2.1 Work as Defined in Mechanics w ork is done when the point of application of a force moves in the direction of the force. The product of the force and the distance moved in the direction of the force is equal to the amount of the work done. This simple definition of work confines only to the area of mechanics and can not be extended to the more complex problems in thermodynamics. Hence a new definition should be introduced to cover mechanical as well as the other forms of work. 2.2 The Thermodynamic Definition of Work Positive work is done by a system, during a given process, when sole effect external to the system could be reduced to the lifting of a mass. Consider a gas expanding in a piston cylinder arrangement as given in Figure 2.1. Here no mass is actually lifted against gravity. But if the existing surroundings is fitted with an arrangement as given in the Figure 2.2, there is a possibility of lifting the mass. Hence work is said to be done by the system. While exploring the possibility of lifting a mass the effects that are external to the system alone must be taken into account. For example, a lift with a person and a suitcase is considered as a system. If the person lifts the suitcase, it should not be taken into account, because this event occurs within the system. Thermodynamics 1 [MIME 3110] 10

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2.3 Units of Work and Power In the international system (SI), the unit of force is Newton (N) and that of distance is metre (m). Hence the unit of work is Nm which is also given a special name Joule. In most of the applications large quantity of work is involved. Therefore kJ is commonly used. Rate of doing work is known as power. Hence its unit is Nm/S or J/S which is again given a special name Watts(W). 2.4. Sign Convention of Work Work done by the system on the surroundings is considered as positive work. Work done on the system by the surroundings is taken as negative work. 2.5. Displacement Work Consider a piston cylinder arrangement as given in the Figure 2.4. If the pressure of the fluid is greater than that of the surroundings, there will be an unbalanced force on the face of the piston. Hence, the piston will move towards right. Thermodynamics 1 [MIME 3110] 11
Force acting on the piston = Pressure × Area = pA Work done = Force × distance = pA × dx = pdV where dV - change in volume. This work is known as displacement work or pdV work corresponding to the elemental

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## This note was uploaded on 09/16/2011 for the course MECHANICAL 101 taught by Professor Jskushawaha during the Spring '11 term at Indian Institute of Technology, Chennai.

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Chapter 2-thermodynamics 1 - Chapter 2 WORK AND HEAT In the...

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