TJC 13 Thermodynamic Systems

Temasek Junior College 1 13 Thermodynamic Systems Learning Outcomes Students should be able to: (a) show an understanding that the macroscopic state of a system determines the internal energy of the system, and that internal energy can be expressed as the sum of a random distribution of microscopic kinetic and potential energies associated with the particles of the system. (b) show an understanding that the thermodynamic temperature of a system is (directly) proportional to the mean microscopic kinetic energy of particles. (c) show an understanding that when two systems are placed in thermal contact, energy is transferred (by heating) from the system at higher temperature to the system at lower temperature, until they reach the same temperature and achieve thermal equilibrium (i.e. no net energy transfer). (d) show an understanding of the difference between the work done by a gas and the work done on a gas, and calculate the work done by a gas in expanding against a constant external pressure: 𝑊 = 𝑝∆𝑉. (e) recall and apply the zeroth law of thermodynamics that if two systems are both in thermal equilibrium with a third system, then they are also in thermal equilibrium with each other. (f) recall and apply the first law of thermodynamics, ∆𝑈 = 𝑄 + 𝑊, that the increase in internal energy of a system is equal to the sum of the energy transferred to the system by heating and the work done on the system. (g) define and use the concepts of specific heat capacity and specific latent heat. student copy

Temasek Junior College 2 1 Internal Energy LO (a), (b) We have seen from the previous topic that molecules of a gas possess kinetic energies due to their random motion. Not all molecules have the same kinetic energy because they are moving with different speeds, but the sum of all the kinetic energies will be a constant at that particular temperature. For a real gas, because the molecules exert intermolecular forces on each other, there will be a certain potential energy due to the forces between the molecules and their positions relative to each other. Thus for a real gas, the internal energy is given by the sum of the potential energies and the kinetic energies of all the molecules. The internal energy U, of a system, is the sum of a random distribution of kinetic and potential energies associated with the molecules of a system.  += cmicroscopicmicroscopi EPEKU .... Microscopic kinetic energy • The microscopic KE arises from the continuous random motion of the particles. • For monatomic gases, the microscopic KE refers to translational motion of atoms. • For diatomic and polyatomic gases, the kinetic energies include other forms such as rotational and vibrational kinetic energies of the molecules. The temperature T of a gas is a measure of the mean kinetic energy of particles in the gas.