Thermal 06

Lect No 06

1. Unit No:01 Thermodynamics Prof. Yash B. Parikh M.Tech (Computer Integrated Manufacturing) B.E.(Mechanical Engineering) Assistant Professor Department of Mechanical Engineering

2. First Law of Thermodynamics 2 This law of thermodynamics is a particular statement of the general principles of law of conservation of energy applied to heat & work transfer. The law states that, “Energy can neither be created nor destroyed but it can only be converted from one form to another”. Though the law can not be prove analytically, it has never been disproved & on the contrary many experiments have verified its statement.

3. Joule’s Experiment 3 During the year (1840 – 1849), James Joule analyzed the statement of conservation of energy by arrangement shown in the fig.

4. Joule’s Experiment 4 It consists of a paddle wheel arrangement with the liquid filled in an insulated container . When the weight falls, it supplies the work energy equals to its potential energy to paddle wheel & causes to rotate it. This causes the fluid to heat up due to friction between the paddle wheel & the fluid. As a result of work transfer the temperature of the liquid rises, which is measured with the help of the thermometer .

5. Joule’s Experiment 5 The system has undergone a process (1-A-2) as shown.

6. Joule’s Experiment 6 Now the heat is transferred from liquid to surrounding till the system returns to its original state of pressure & temperature . The heat transferred is shown by process (2-B-1 ).

7. Joule’s Experiment 7 With such experiment Joule concluded that in every case the work input W was always proportional to heat transfer Q at the end of the cycle. Mathematically we can write, ʘʃ d’W = J ʘʃ d’Q Where , J is a constant known as mechanical equivalent of the system.

8. Joule’s Experiment 8 In M.K.S unit J = 4.187 kJ/k cal In S.I unit J = 1 J/Nm Since W and Q are both measured in J or Nm we can write, ʘʃ d’W = ʘʃ d’Q

9. Corollaries of First Law 9 The first law of thermodynamics has the number of important consequences, which forms corollaries. ( A  corollary  is a statement that follows readily from a previous statement. ) There are three main corollaries of first law, Corollary 1 (Perpetual Motion Machine 1) A machine operating on a cycle and producing work continuously or periodically without absorption of energy at the same time is known as perpetual motion machine. Therefore, the first law can also be stated in following terms,

10. Corollaries of First Law 10 “It is impossible to construct a perpetual motion machine of first kind”. i.e. no machine working over a cycle can produce energy without corresponding expenditure of energy. Converse of PMM 1 is also true. i.e. there can not be any machine which would consume work energy without some other form of energy appearing simultaneously.

11. Corollary 2 11 While studying thermodynamics we come across with processes rather than cycles. When a system executes a process, the net heat transfer equals to sum of net work transfer and change in stored energy, dE . d’ Q = d’ W + dE This is known as First law for a process.

12. Corollary 3 12 For an isolated system, there are no mass & energy transfer, i.e. dQ = 0 & dW = 0. Therefore, ( dE ) isolated = 0 Total energy of an isolated system remains constant.