Heat, Work, and the First Law of Thermodynamics Chapter 18 of Essential University Physics, Richard Wolfson, 3 rd Edition 1
Different ways to increase the internal energy of system: 2
Joule s apparatus to determine the conversion of mechanical work into changes of internal energy: 3
It s all about the system!! A thermodynamic system is any collection of objects that may exchange energy (work and/or heat) with its surroundings. In a thermodynamic process, changes occur in the state of the system. 4
First Law of Thermodynamics: QQ: heat transferred to the system WW: work done on the system EE iiiiii = QQ + WW Be careful with the signs: Q is positive when heat flows into the system. W is positive when work is done on the system. 5
Thermodynamic Equilibrium: A state in which the macroscopic properties (p, V, and T) no longer change with time, if the system is thermally and mechanically isolated. 6
If the System is in Thermodynamic Equilibrium: There is a precise relation between p, V, and T (phase diagram). For example, given p and V, T can be determined exactly and uniquely. Then just two physical properties (e.g., p and V) are sufficient to characterize the state of the system in thermodynamic equilibrium. 7
Thermodynamic Processes WW: work done on the system The system is no longer thermally and mechanically isolated. QQ: heat transferred to the system How can we describe the system as it changes? 8
The Quasi-Static Process: A process in which the system is always in thermodynamic equilibrium. Its evolution from one state to another is described by a continuous sequence of points in its pv diagram. Quasi-static processes are reversible!! 9
Under Those Conditions: 10
Work Done on the System: dddd = FF dddd = pp AA dddd = pp dddd VV 2pp WW = dddd VV 1 11
As an Example of Reversible Thermodynamic Processes, we will use the Ideal Gas. Why? Because we have a simple relation between p, V, and T. pp VV = nn RR TT 12
The Isothermal Process VV 2pp WW = dddd VV 1 VV 2 nn RR TT = VV 1 VV dddd = nn RR TT llll VV 2 VV 1 13
Internal Energy of the Ideal Gas KK = 3 2 kk TT EE iiiiii = NN KK = NN 3 2 kk TT = nn 3 RR TT 2 EE iiiiii = nn 3 2 RR TT 14
Back to the Isothermal Process TT = 0 EE iiiiii = 0 EE iiiiii = QQ + WW = 0 QQ = WW = nn RR TT llll VV 2 VV 1 15
Reversible Thermodynamic Processes of the Ideal Gas QQ nn CC vv TT = EE iiiiii QQ nn CC pp TT EE iiiiii = nn CC vv TT γγ CC pp CC vv = nn 3 RR TT 2 WW = pp VV = nn RR TT 16
Specific Heat of the Ideal Gas EE iiiiii = nn 3 2 RR TT = nn CC vv TT CC vv = 3 2 RR γγ CC pp CC pp = CC vv + RR = 5 2 RR = 5 CC vv 3 17
Kinetic Theory of the Ideal Gas Gas pressure arises from the average force the particles exert when they collide with the container walls. The ideal-gas law follows by assuming that a gas consists of particles that obey Newton's laws. For analysis we assume: N identical particles of mass m and no internal structure No intermolecular forces and molecules only have kinetic energy Molecular motion is random Collisions with the wall of the container are elastic 18
Monatomic Molecule: He, Ne, Ar, etc KK = 3 2 kk TT Translational motion in 3D along x, y, z 3 degrees of freedom Each degree of freedom contributes with 1 2 internal energy: EE iiiiii = nn 3 RR TT 2 kk TT to the CC vv = 3 2 RR CC γγ = 5 pp = 5 2 RR 3 19
Diatomic Molecules: H 2, O 2, N 2, etc. Translational motion in 3D along x, y, z Rotational motion along two axis 5 degrees of freedom Each degree of freedom contributes with 1 2 internal energy: EE iiiiii = nn 5 RR TT 2 kk TT to the CC vv = 5 2 RR CC γγ = 7 pp = 7 2 RR 5 20