Physics 114 – Lecture 42

1. Physics 114 – Lecture 42 • Chapter 15The Laws of Thermodynamics • Thermodynamics involves the study of processes where energy is converted into heat and work, c.f., the steam engine, the internal combustion engine, … • Heat and work: heat is defined as a transfer of energy arising from a temperature difference, whereas work is a transfer of energy that does not arise from a temperature difference • We need to define the system under consideration – this is any object or set of objects under consideration L42-s1,9

2. Physics 114 – Lecture 42 • §15.1 The First Law of Thermodynamics • Definitions: Q = net heat added to the system W = work done by system ΔU = increase in internal energy of system • Q = ΔU + W • This is known as the first law of thermodynamics and is a statement of energy conservation • Notice that if W is the work done by the system, then –W is the work done on the system – c.f., Newton’s third law L42-s2,9

3. Physics 114 – Lecture 42 • Notice that the state of a system is specified by P, V, T and U and are known collectively as state variables • Q and W are not state variables since they do not describe the state of a system • Study example 15.1 • §15.2 Thermodynamic Processes and the First Law • Isothermal Processes: ΔT = 0 L42-s3,9

4. Physics 114 – Lecture 42 • Example of an isothermal process, T = const, or ΔT = 0 → PV = n RT = const • This is accomplished by placing the cylinder on a heat reservoir, which exchanges heat with the cylinder L42-s4,9

5. Physics 114 – Lecture 42 • Adiabatic Process: Q = 0 – occurs if the system is isolated or if the process occurs very rapidly so that heat cannot enter or leave the system • From the 1st law: • Q = ΔU + W and with Q = 0 → ΔU = -W • For an expansion W > 0, giving ΔU < 0 , which, after using U = (3/2 ) n RT , leads to, ΔT < 0 • For a compression W < 0 , leading to ΔT > 0 L42-s5,9

6. Physics 114 – Lecture 42 • Isobaric Process: ΔP = 0 • Isochoric or Isovolumetric Process: ΔV = 0 L42-s6,9

7. Physics 114 – Lecture 42 • Work done by a gas in an expansion • W = Fgas d cos θ where θ = 0 • W = Fgas d = P A d = P ΔV • Notice that ΔV > 0 for an expansion but ΔV < 0 for a compression Fgas L42-s7,9

8. Physics 114 – Lecture 42 • Work done by gas equals area under PV curve and, in general, the area enclosed by the PV cycle • Study examples 15.3, 15.4, 15.5, 15.6 and 15.7 • Read §15.3 Human Metabolism and the First Law L42-s8,9

9. Physics 114 – Lecture 42 • Example 15.5 L42-s9,9