Thermodynamics Added to Chapter 4

Thermodynamics Thermal energy, heat, and work are related, and the study of the relationship among them is thermodynamics Heat and Work increase Thermal Energy Ex. Heat hands by fire (radiation) Rub hands together & by fire = greater thermal energy System: anything you can imagine a boundary around

Heat & Work Increase Thermal Energy of A System The heat of a system is the amount of thermal energy added to the system that crosses the boundary. The work done on a system is the work done by something outside the system’s boundary

First Law of Thermodynamics According to the first law of thermodynamics, the increase in thermal energy of a system equals the work done on the system plus the heat to the system The temperature of a system can be increased by adding heat to the system, doing work on the system, or both. The increase in energy of a system equals the energy (total) added to the system.

Isolated & Non-isolated Systems A system is non-isolated if there are thermal energy transfers across the boundary or if work is done across the boundary. Then energy is added to or taken away from the system. If there are no thermal energy transfers across the boundary and there is no work done across the boundary, then the system is an isolated system

Isolated & Non-isolated Systems The thermal energy of an isolated system does not change. Because energy cannot be created or destroyed, the total energy stays constant in an isolated system.

Second Law of Thermodynamics the second law of thermodynamics is that it is impossible for thermal energy to be transferred from a cool object to a warmer object unless work is done. The transfer of thermal energy spontaneously from a cool object to a warm object never happens.

Converting Mechanical Energy in Thermal Energy If you give a book sitting on a table a push, the book will slide and come to a stop. Friction between the book and the table converted the book’s mechanical energy to thermal energy. As a result, the book and the table became slightly warmer.

Converting Mechanical to Thermal Energy ???? Is it possible to do the reverse, and convert thermal energy completely into mechanical energy? The second law of thermodynamics makes it impossible to build a device that converts thermal energy completely into mechanical energy.

Converting Thermal Energy to Mechanical Energy A device that converts thermal energy into mechanical energy is a heat engine. A car’s engine converts the chemical energy in gasoline into thermal energy. The engine then transforms some of the thermal energy into mechanical energy by rotating the car’s wheels.

However, only about 25 percent of the thermal energy obtained by burning the gasoline is converted into mechanical energy, and the rest is transferred to the engine’s surroundings. NOT VERY EFFICIENT!