RI Chap 13 Thermodynamic Systems Lecture Notes

13 THERMODYNAMIC SYSTEMS H2 Physics 9478 Content Page 13.1 Internal energy 3 13.2 Heating and work done 9 13.3 Laws of thermodynamics 10 13.4 Specific heat capacity and specific latent heat 25 13.5 Appendix 39 Learning Outcomes Candidates 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 pV= ∆ . (e) recall and use 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, UQW∆=+ , that the increase in internal energy of a system is equal to the sum of the heat supplied to the system and the work done on the system. (g) define and use the concepts of specific heat capacity and specific latent heat.

Page | 2 Thermodynamic Systems – An Overview The first law of thermodynamics is central to understanding thermodynamic processes which involve heat transfer and mechanical work. This law is an extension of the conservation of energy used in mechanics as it considers energy exchange in a system by means of both heat transfer and mechanical work. Through the ways which energy can be transferred between a system and its surroundings, changes in the internal energy of the system can result. The concept of internal energy is introduced to make the link between heat and mechanical work to measurable macroscopic properties like pressure, volume and temperature. This internal energy is also related to microscopic kinetic and potential energies associated with the particles of the system. Linking the ideas about heat and temperature is the zeroth law of thermodynamics, which states that when two objects at different temperature are placed in thermal contact, there will be energy exchange between them until thermal equilibrium is reached. The zeroth law allows the use of thermometers to measure temperature. When thermal equilibrium is achieved, the thermometer reflects its own temperature which is of the same value as the other body that it is in thermal contact with. Energy