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 Reversible Irreversible (no service fee) (5% service fee) Day Dollars Pounds Dollars Pounds Monday Tuesday Wednesday Thursday Friday Each morning, dollars are converted to pounds. Each evening, pounds are converted to dollars. Money analogy

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

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

8 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 Not reversible unless energy is expended X Not reversible without expending energy

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 2 nd 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 Heat into Work  A heat engine takes in an amount of heat, Q hot, and produces work, W, and waste heat Q cold.  Nicolas Carnot (kar nō) derived the limits of converting heat into work. High-temperature Source, T hot Low-temperature Sink, T cold Heat Engine W Q hot Q cold (e.g., flame) (e.g., cooling pond)

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 T hot  T cold, W max  0.  This ratio is also called the efficiency, .

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 F m xx This proved 1 kcal = 4,184 J 1 kg H 2 O  T = 1 o C E = F  x = 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 Internal Energy Translation Rotation Vibration Molecular Interactions

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  T:  Sometimes called specific heat

23 Heat Capacity for Constant Volume Processes (C v )  Heat is added to a substance of mass m in a fixed volume enclosure, which causes a change in internal energy, U. Thus, Q = U 2 - U 1 =  U = m C v  T The v subscript implies constant volume Heat, Q added m m TT insulation

24 Heat Capacity for Constant Pressure Processes (C p )  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. Heat, Q added TT m m xx

25 C p Defined  Thus, Q =  U + P  V =  H = m C p  T The p subscript implies constant pressure  Note: H, enthalpy. is defined as U + PV, so d H = d (U+PV) = d U + V d P + P d V At constant pressure, dP = 0, so dH= dU + PdV For large changes at constant pressure  H =  U + P  V

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

27 Pair Exercise 3 1.Calculate the change in enthalpy per lb m of nitrogen gas as its temperature decreases from 500 o F to 200 o F. 2.Two kg of water (C v =4.2 kJ/kg K) are heated using 200 Btu of energy. What is the change in temperature in K? In o F?