The Laws of Thermodynamics

1. The Laws of Thermodynamics Metro Community College 7/14/2010

2. Main Definitions • Thermodynamics – The study of processes in which heat is transferred as heat and as work. • Heat is the transfer of energy due to a difference in temperature. (Kinetic energy due to molecular motion moves from high temperature and internal energy to low.) • Work is a transfer of energy due to other means, not a temperature difference.

3. Other definitions • System – the set of objects or object we wish to consider. • Environment – Everything that surround the system we are studying. • Open system – a system where mass and energy can be exchanged with the environment • Closed system – a system where no mass leaves the system, but energy can. • Isolated system – a system where no energy in any form (or mass) leaves or enters the system

4. The First Law of Thermodynamics The First Law of Thermodynamics – The change in internal energy of a closed systemequals the heat added to the system minus the work done by the system. (A statement of the conservation of energy.)

5. Possible Processes • Isothermal process – a process where the temperature does not change. PV=constant. • Isobaric process – a process where the pressure does not change. T/V = constant. • Isochoric process – a process where volume does not change. T/P = constant. This is also called an isometric process. • Adiabatic process – process where no heat is added or removed. Q = 0.

6. Isothermal Process • Temperature cannot change in an isothermal process so internal energy cannot change. Internal energy (U) is proportional to temperature. • Heat added to the system equals the work done by the system.

7. Isobaric Process • Pressure does not change. Therefore the work done by the system results in a change in volume (increase). Work on the system results in a decrease in volume. • Temperature and volume must be adjusted to keep pressure constant.

8. Isochoric Process • The volume does not change so no work is done in an isochoric process.

9. Adiabatic Process • No heat is added to the system. Q = 0. • This occurs where the work done on or by the system happens so quickly that heat does not have time to move.

10. Second Law of Thermodynamics • Heat flows naturally from a hot to a cold object; heat will not flow spontaneously from a cold to a hot object. (Clausius) • The total entropy of any system plus that of its environment increases as a result of a natural process. • Natural processes tend to move toward a state of greater disorder

11. Summary • Zeroth Law-If a System is in equilibrium with two other systems that the two other systems are also in equilibrium. • First Law – Energy is conserved • Second Law – Disorder increases in natural processes.