Dr. Jie ZouPHY Chapter 20 Heat and the First Law of Thermodynamics (cont.)

Dr. Jie ZouPHY Outline The first law of thermodynamics (20.5) Some special cases: an isolated system and a cyclic process Definitions for some idealized thermodynamic processes (20.6) Adiabatic process Adiabatic free expansion Isobaric process Isovolumetric process Isothermal process Example: Isothermal expansion of an ideal gas (20.6) Energy transfer mechanisms (conceptual) (20.7)

Dr. Jie ZouPHY The first law of thermodynamics The first law of thermodynamics:  E int = Q + W  E int : Change in the internal energy of the system Q: Energy transferred by heat to the system W: Work done on the system The first law of thermodynamics is a special case of the law of conservation of energy that relates the change in internal energy of a system to the net transfer of energy by heat and work. Internal energy is a state variable (like P, V, and T): its value is determined by the state of the system, independent of the path. Some special cases: Isolated system: one that does not interact with its surroundings. Since Q = W = 0,  E int = 0 and E int, i = E int, f. Cyclic process: a process that starts and ends at the same state. Since  E int = 0, Q = -W.

Dr. Jie ZouPHY Definitions for some idealized thermodynamic processes Reading assignment: P. 619 – 623 Adiabatic process: one during which no energy enters or leaves the system by heat, i.e. Q = 0. Adiabatic free expansion: both Q = 0 and W = 0. Isobaric process: one that occurs at constant pressure. Isovolumetric process: one that takes place at constant volume. Isothermal process: one that occurs at constant temperature. Quick Quiz 20.5 Characterize the paths below as isobaric, isovolumetric, isothermal, or adiabatic. Note that Q = 0 for path B.

Dr. Jie ZouPHY Example 20.6 An isothermal expansion A 1.0-mol sample of an ideal gas is kept at 0.0°C during an expansion from 3.0 L to 10.0 L. (A) How much work is done on the gas during the expansion? (B) How much energy transfer by heat occurs with the surroundings in this process? (C) If the gas is returned to the original volume by means of an isobaric process, how much work is done on the gas? What can we learn from this example? (1) Isothermal expansion of an ideal gas (proof): (2) Isothermal process involving an ideal gas:  E int = 0 since the internal energy of an ideal gas is a function of temperature only.

Dr. Jie ZouPHY Energy transfer mechanisms (conceptual) Reading assignment: P. 623 – 629 Thermal conduction (on an atomic scale): Exchange of kinetic energy between microscopic particles, in which less-energetic particles gain energy in collisions with more energetic particles. The law of thermal conduction: P = kA|dT/dx|; P: the rate of energy transfer by heat; k: thermal conductivity of the material; |dT/dx|: temperature gradient; A: cross-sectional area. Convection (a form of matter transfer): Energy transferred by the movement of a warm substance is said to have been transferred by convection. Electromagnetic radiation: All objects radiate energy continuously in the form of electromagnetic waves (see Ch. 34). When an object is hotter than its surroundings, it radiates more energy than it absorbs, and its temperature decreases.

Dr. Jie ZouPHY Homework Ch. 20, P. 634, Problems: #28, 30, 32, 34, 39.