Chapter 18 The Laws of Thermodynamics

1. Chapter 18 The Laws of Thermodynamics • The zeroth law of thermodynamics • The first law of thermodynamics • The second law of thermodynamics • The third law of thermodynamics Dr. Jie Zou PHY 1151G Department of Physics

2. The zeroth law of thermodynamics • Zeroth law of thermodynamics: If object A is in thermal equilibrium with object C, and object B is separately in thermal equilibrium with object C, then objects A and B will be in thermal equilibrium if they are placed in thermal contact. • The physical quantity that is equal when two objects are in thermal equilibrium is the temperature. • If two objects have the same temperature, we can be assured that no heat will flow when they are placed in thermal contact. • If heat does flow between two objects, it follows that they are not in thermal equilibrium, and they do not have the same temperature. Dr. Jie Zou PHY 1151G Department of Physics

3. The first law of thermodynamics The internal energy of a system • First law of thermodynamics: The change in a system’s internal energy, U, is related to the heat Q and the work W as follows: U = Q – W. • Sign conventions for the heat Q and work W: • Q positive System gains heat • Q negative System loses heat • W positive Work done by system • W negative Work done on system • Adding heat to a system, and/or doing work on it, increases the internal energy of the system. • If the system does work on the external world, and/or heat is removed, its internal energy deceases. Work and internal energy Dr. Jie Zou PHY 1151G Department of Physics

4. Example: numerical application of the 1st law of thermodynamics • (a) Jogging along the beach one day you do 4.3  105 J of work and give off 3.8  105 J of heat. What is the change in your internal energy? • (b) Switching over to walking, you give off 1.2  105 J of heat and your internal energy decreases by 2.6  105 J. How much work have you done while walking? Dr. Jie Zou PHY 1151G Department of Physics

5. The second law of thermodynamics • Second law of thermodynamics: Heat flow – When objects of different temperatures are brought into thermal contact, the spontaneous flow of heat that results is always from the high temperature object to the low temperature object. Spontaneous heat flow never proceeds in the reverse direction. • The second law of thermodynamics is more restrictive than the first law; it says that of all the processes that conserve energy, only those that proceed in a certain direction actually occur. • In a sense, the second law implies a definite “directionality” to the behavior of nature. The second law is sometimes referred to as the “arrow of time”. Dr. Jie Zou PHY 1151G Department of Physics

6. The third law of thermodynamics • The third law of thermodynamics: It is impossible to lower the temperature of an object to absolute zero in a finite number of steps. • The essential idea of the third law: Absolute zero is the limiting temperature and, though it can be approached arbitrarily closely, it can never be attained. Dr. Jie Zou PHY 1151G Department of Physics