Physics 114 – Lecture 43

1. Physics 114 – Lecture 43 • §15.4 The Second Law of Thermodynamics – Introduction • First Law of Thermodynamics – energy is conserved • BUT – some processes, which conserve energy, occur naturally, e.g., hot tea cooling, a glass breaking, an object falling and striking the ground, where its PE → KE → thermal energy,whereas others, which also conserve energy, do not occur naturally • The first law of thermodynamics holds in each case • Second Law of Thermodynamics addresses why some processes occur naturally, whereas others do not L43-s1,12

2. Physics 114 – Lecture 43 • The Second Law of Thermodynamics states which processes can occur in nature • This law can be and has been stated in several ways, all of which have been shown to be equivalent • Clausius’ statement: • Heat can flow spontaneously from a hot body to a cold body but heat will not flow spontaneously from a cold body to a hot body • A more general statement is needed • This more general statement was developed from studies of heat engines, which are devices that convert thermal energy into mechanical work L43-s2,12

3. Physics 114 – Lecture 43 • §15.5 Heat Engines • Examples of Heat Engines − Steam Engines • Schematic Diagram L43-s3,12

4. Physics 114 – Lecture 43 • Four Stroke Internal Combustion Engine L43-s4,12

5. Physics 114 – Lecture 43 • ΔT is needed to drive a heat engine • At higher T the pressure, P, is higher than at lower T, so W = P ΔV is higher on expansion than on compression, where T and hence P is lower • Efficiency • Efficiency, e = W/QH • From the First Law, QH = W + QL • → e = W/QH = (QH – QL)/ QH = 1 – (QL/QH) • Study example 15.9 L43-s5,12

6. Physics 114 – Lecture 43 • Carnot Engine • This is an ideal heat engine: it is reversible, as opposed to real processes which are irreversible L43-s6,12

7. Physics 114 – Lecture 43 • Carnot proved that, for this ideal heat engine, • eideal = (TH - TL)/TH = 1 – (TL/TH) • Study examples 15.10 and 15.11 • Kelvin-Planck statement of the 2nd Law of Thermodynamics • No device is possible the sole effect of which is to transform a given amount of heat completely into work L43-s7,12

8. Physics 114 – Lecture 43 • §15.6 Refrigerators, Air Conditioners and Heat Pumps • These devices operate in the opposite way to the heat engine L43-s8,12

9. Physics 114 – Lecture 43 • Clausius’ Statement of the 2nd Law of Thermodynamics • No device is possible, the sole effect of which is to transfer heat from a system at temperature, TL, to another system at temperature, TH • It can be shown that all statements of the 2nd Law of Thermodynamics are equivalent • Heat Pump L43-s9,12

10. Physics 114 – Lecture 43 • §15.7 Entropy and the Second Law of Thermodynamics • The concept of entropy was introduced by Clausius • The change in entropy in any process is the important parameter • When an amount of heat, Q, is added to a system by a reversible process at constant temperature, T, the change in entropy is defined to be • ΔS = Q/T L43-s10,12

11. Physics 114 – Lecture 43 • General Statement of the 2nd Law of Thermodynamics • The total entropy of any system plus that of its environment increases as a result of any natural process L43-s11,12

12. Physics 114 – Lecture 43 • Read • §15.8 Order to Disorder • §15.9 Unavailability of Energy; Heat Death • §15.10 Evolution and Growth; “Time’s Arrow” • §15.11 Statistical Interpretation of Entropy and the Second Law • §15.12 Thermal Pollution and Global Warming L43-s12,12