Download Fundamentals of Thermodynamics: Laws and Concepts and more Study notes Physics in PDF only on Docsity! 24 jan 2010 L01–1 Review of Thermodynamics. 1: Basic Laws What is Thermodynamics? • Idea: The macroscopic counterpart to statistical mechanics, initially developed phenomenologically, grad- ually developed into a coherent framework, and now providing the observational context in which to verify many of the predictions of statistical mechanics. • Plan: Recall the main facts (definitions, laws and relationships) of thermodynamics; specific engines and processes will not be emphazied. We will not mention statistical mechanics yet; the connection between the two will be established later. Variables and State of a Thermodynamical System • Energy: Thermodynamics boils down to a theory of what happens to energy in various (mostly static) situations; in many of the most interesting ones energy is the only relevant macroscopic conservation law, as opposed to momentum and other quantities. • Extensive variables: The state of a thermodynamic system in equilibrium can be specified by assigning the values of a set of extensive variables (S, ~x), where S is the entropy (see below) and ~x are quantities that may include V , {Ni}, q, ~M , ~p, ~L, ... One equation of state expresses E as a function of (S, ~x). • Intensive variables: Each extensive variable has a conjugate intensive one. Examples are (S, T ) and, for the ~x, the pairs (V,−p), (Ni, µi), (q,Φ), ( ~M, ~B), etc. Intensive variables play an important role in equilibrium, and each one of them can replace its conjugate one to give a new complete set of state variables. • Other equations of state: Once values for a complete set of state variables are specified, values for all other variables can be obtained using equations of state (e.g., p = p(T, V )). • Typical task: For thermodynamics, finding the change in the value of any state variable after some transformation. For statistical mechanics, and in this course, one of the main tasks is to derive the equations of state from models of the microscopic interactions. Zeroth Law of Thermodynamics • Statement: If system A is in equilibrium (thermal, mechanical, chemical, ...) with systems B and C, then systems B and C are in equilibrium with each other. • Remark: This law amounts to the possibility of establishing universal scales for the corresponding intensive quantities. It has a statistical justification, as we will see soon. First Law of Thermodynamics • Statement: The law of conservation of energy, written as dE = δQ+ δW , where the work δW may have a contribution from each of the intensive-extensive pairs, δW = ~f · d~x . For example, −p dV , f dL, µdN ; the quantity ~f is called a generalized force. • Remark: For calculational purposes, this equation can be taken as a definition of the heat exchanged δQ, if we can account for all forms of work δW . However, from a conceptual point of view it is better to call heat any energy that flows spontaneously as a result of a difference in temperature, and work any other transfer of energy.
