First and Second Laws of Thermodynamics

1. First and Second Laws of Thermodynamics

2. RAT 11b

3. Class Objectives • Understand and apply: • work, energy, reversibility, heat capacity • First and Second Laws of Thermodynamics

4. Reversibility • Reversibility is the ability to run a process backwards and forwards infinitely without losses.

5. Money analogy Reversible Irreversible (no service fee) (5% service fee) Day Dollars Pounds Dollars Pounds Monday 100.00 40.00 100.00 38.00 Tuesday 100.00 40.00 90.25 34.30 Wednesday 100.00 40.00 81.45 30.95 Thursday 100.00 40.00 73.51 27.93 Friday 100.00 40.00 66.34 25.20 Each morning, dollars are converted to pounds. Each evening, pounds are converted to dollars.

6. Pair Exercise 1 Using Excel, reproduce the previous table, except use a service charge of 10%.

7. Pressure Voltage GeneratorMotor Pump Turbine Electric Current Fluid Flow Reversibility and Energy • If irreversibilities were eliminated, these systems would run forever. • Perpetual motion machines

8. X Not reversible without expending energy Not reversible unless energy is expended Example: Popping a Balloon • A “reversible process” can go in either direction, but these processes are rare. • Generally, the irreversibility shows up as waste heat

9. Sources of Irreversibilities • Friction • Voltage drops • Pressure drops • Temperature drops • Concentration drops

10. Basic Laws of Thermodynamics • First Law of Thermodynamics • energy can neither be created nor destroyed • Second Law of Thermodynamics • naturally occurring processes are directional

11. First Law of Thermodynamics • One form of work may be converted into another, • or, work may be converted to heat, • or, heat may be converted to work, • but, final energy = initial energy

12. 2nd Law of Thermodynamics • We intuitively know that heat flows from higher to lower temperatures and NOT the other direction. • i.e., heat flows “downhill” just like water • You cannot raise the temperature in this room by adding ice cubes. • Thus processes that employ heat are inherently irreversible.

13. Heat/Work Conversions • Heat transfer is inherently irreversible. This places limits on the amount of work that can be produced from heat. • Heat can be converted to work using heat engines • Jet engines (planes), steam engines (trains), internal combustion engines (automobiles)

14. W High-temperature Source, Thot Heat Engine Low-temperature Sink, Tcold Qcold Qhot (e.g., cooling pond) (e.g., flame) Heat into Work • A heat engine takes in an amount of heat, Qhot, and produces work, W, and waste heat Qcold. • Nicolas Carnot (kar nō) derived the limits of converting heat into work.

15. Carnot Equation: Efficiency • Given the heat engine on the previous slide, the maximum work that can be produced is governed by: where the temperatures are absolute temperatures. • Thus, as ThotTcold, Wmax  0. • This ratio is also called the efficiency, h.

16. Pairs Exercise 2 • Use Excel to create a graph showing the amount of work per unit heat for a heat engine in which the source temperature increases from 300 K to 3000 K and the waste heat is rejected to an ambient temperature of 300 K.

17. Work into Heat • Although there are limits on the amount of heat converted to work, work may be converted to heat with 100% efficiency. • This is shown by Joule’s experiment…

18. Joule’s Experiment Joule’s Mechanical Equivalent of Heat This proved 1 kcal = 4,184 J DT = 1oC m F Dx 1 kg H2O E = FDx = 4,184 J

19. Where did the energy go? • By the First Law of Thermodynamics, the energy we put into the water (either work or heat) cannot be destroyed. • The heat or work added increased the internal energy of the water.

20. Translation Rotation Vibration Molecular Interactions Internal Energy

21. Heat Capacity • An increase in internal energy increases the temperature of the medium. • Different media require different amounts of energy to produce a given temperature change.

22. Heat Capacity Defined • Heat capacity: the ratio of heat, Q, needed to change the temperature of a mass, m, by an amount DT: • Sometimes called specific heat

23. Heat Capacity for Constant Volume Processes (Cv) • Heat is added to a substance of mass m in a fixed volume enclosure, which causes a change in internal energy, U. Thus, Q = U2 - U1 = DU = m CvDT The v subscript implies constant volume insulation DT Heat, Q added m m

24. Dx DT Heat, Q added m m Heat Capacity for Constant Pressure Processes (Cp) • Heat is added to a substance of mass m held at a fixed pressure, which causes a change in internal energy, U, AND some PV work.

25. Cp Defined • Thus, Q = DU + PDV = DH = m CpDT The p subscript implies constant pressure • Note: H, enthalpy. is defined as U + PV, so dH = d(U+PV) = dU + VdP + PdV At constant pressure, dP = 0, so dH= dU + PdV For large changes at constant pressure DH = DU + PDV

26. Experimental Heat Capacity Experimentally, it is easier to add heat at constant pressure than constant volume, thus you will typically see tables reporting Cp for various materials (Table 21.2 in Foundations of Engineering).

27. Pair Exercise 3 • Calculate the change in enthalpy per lbm of nitrogen gas as its temperature decreases from 500 oF to 200 oF. • Two kg of water (Cv=4.2 kJ/kg K) are heated using 200 Btu of energy. What is the change in temperature in K? In oF?