Chapter 2: 1St Law,KE,PE, and Polytropic Work

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Chapter 2: 1St Law,KE,PE, and Polytropic Work

Chapter 2: 1st law,KE,PE, and Leader: Will Polytropic Work Course: ME 231 Supplemental Instruction Instructor: Professor Gomes Iowa State University Date: 1/24/18 Introduction: Last session: KE and PE introduction and CHANGE = FINAL – INITIAL Key concepts for today: 1. KE = (½)mV^2 2. PE = mgh 3. CHANGE =FINAL – INITIAL 4. Energy is neither created nor destroyed, it is conserved. 1st law of thermodynamics 5. ∆E=∆KE+∆PE+∆U = Q – W Closed System Energy Balance 5. pV^n= constant Polytopic Process 6. Boundary work (W_b) = Integral of (pdv) = (p_2*V_2-p_1*V_1)/(1-n) for a polytropic process with n not equal to 1. Body: Kinetic and potential energy 1. Determine the gravitational potential energy, in kJ, of 2 m^3 of liquid water at an elevation of 30 m above the surface of Earth. The acceleration of gravity is constant at 9.7 m/s^2 and the density of the water is uniform at 1000 kg/m^3. Determine the change in gravitational potential energy as the elevation decreases by 15 m. (Problem 2.2) 1060 Hixson-Lied Student Success Center 515-294-6624 [email protected] http://www.si.iastate.edu 2. An object whose mass is 100 lb falls freely under the influence of gravity from an initial elevation of 600 ft above the surface of the Earth. The initial velocity is downward with a magnitude of 50 ft/s. The effect of air resistance is negligible. Determine the velocity, in ft/s, of the object just before it strikes Earth. Assume g = 31.5 ft/s^2. (Problem 2.14) Calculating Boundary Work During a Polytropic Process 3. Four-tenths kilogram of a certain gas is contained within a piston-cylinder assembly. The gas undergoes a process for which the pressure-volume relationship is: pV^1.5 = constant The initial pressure is 3 bar, the initial volume is 0.1 m^3, and the final volume is 0.2 m^3. The change in specific internal energy of the gas in the process is u_2 – u_1 = -55 kJ/kg. There are no significant changes in kinetic or potential energy. Determine the net heat transfer for the process, in kJ. (Example 2.2) a) Write out the variables you are given b) Write out your energy balance equation, how many variables do you know? How many variables are unknown? c) Think about what other equations you know for this problem and how they can help you solve for the net heat transfer. Hint: Look in the question and in the key concepts portion in the introduction of this worksheet Closing: Next Session: Thursday @ 4pm in Pearson 1106 .

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