Principles of Technology Waxahachie High School

1. Principles of TechnologyWaxahachie High School Energy in Thermal Systems PIC Chapter 5.4 PT TEKS

2. Energy in Electrical Systems Objectives: Define the internal energy of a system. Describe two ways you can change a system’s internal energy. Explain the first law of thermodynamics. Use the first law to solve problems involving internal energy, heat, and work. Describe the operation of a heat engine and a refrigerator. Explain the second law of thermodynamics. Describe the processes that are prohibited by the second law. Explain the differences between the Celsius and Kelvin temperature scales. Calculate the Carnot efficiency of a heat engine.

3. Energy in Electrical Systems Thermal Energy is the total kinetic energy of the motion of atoms in an object. Molecules in an object are constantly moving in a randommotion.

4. Energy in Electrical Systems All molecules have 3 types of motion: Translational movement – forward or backward movement Rotationalmovement – spinning motion Vibration– small, fast movements back and forth

5. Energy in Electrical Systems Temperature is decided by how much a molecule or atom bounces around a container hitting other molecules and atoms. Temperature of an object is dependent upon translational movement only.

6. Energy in Electrical Systems The total energy of an object is called the internal energy. As atoms collide, the Potential energy and Kinetic energy are constantly being transferred. However, the total energy or the internal energy of the object always stays the same.

7. Energy in Electrical Systems A body’s internal energy depends on: Material Composition Mass Starting Temperature Physical State – Solid, liquid, gas, or plasma

8. Energy in Electrical Systems Internal energy can be transferred from one object to another with a temperature difference or by heat. Another way to transfer energy is by doing work to produce friction. Example: Rubbing hands together.

9. Energy in Electrical Systems Other forms of energy can also be converted into internal energy. Example: Electric stove uses resistance to convert electrical energy to thermal energy

10. Energy in Electrical Systems The science dealing with relationships between internal energy, heat, and work is called thermodynamics.

11. Energy in Electrical Systems 1st Law of Thermodynamics: The law of conservation of energy says that energy cannot be created or destroyed but can change from one form of energy to another form.

12. Energy in Electrical Systems Internal energy is represented by the letter U. Heat is represented by the letter Q. 1st Law of Thermodynamics equation = Internal energy = Heat – Work U = Q - W

13. Energy in Electrical Systems Heat is positive if it enters the system and negative if it leaves the system. Work is positive when the system does work and work is negative when work is done on the system.

14. Energy in Electrical Systems A system that has 50 units of heat entering the system and 40 units of work are done on the system, what is the internal energy? Heat is positive; work is negative U = Q - W U = 50 – (-40) = 50 + 40 U = 90

15. Energy in Electrical Systems A system that has 50 units of heat entering the system and the system does 40 units of work, what is the internal energy? Heat is positive; work is positive U = Q - W U = 50 – (+40) = 50 - 40 U = 10

16. Energy in Electrical Systems A system that has 50 units of heat leaving the system and 40 units of work are done on the system, what is the internal energy? Heat is negative; work is negative U = Q - W U = -50 – (-40) = -50 +40 U = -10

17. Energy in Electrical Systems A system that has 50 units of heat leaving the system and the system does 40 units of work, what is the internal energy? Heat is negative; work is positive U = Q - W U = -50 – (+40) = -50 – 40 U = -90

18. Energy in Electrical Systems A process in which there is no heat transfer to or from the system is called an adiabatic process. Two ways to create an adiabatic process: Isolate the system with insulation Do work quickly so there is no time for heat transfer

19. Energy in Electrical Systems A device that converts thermal energy into mechanical energy is called a heat engine. Example of a heat engine that burns gas is an internal combustion engine.

20. Energy in Electrical Systems Example of a heat engine that uses steam - Train Example of a heat engine that uses chemical reactions - Rocket

21. Energy in Electrical Systems Every heat engine must: Absorb thermal energy from a high-temperature source Convert some of the thermal energy to work Discard the remaining thermal energy into a low-temperature reservoir (Earth, Earth’s Atmosphere, or a body of water)

22. Energy in Electrical Systems 2nd Law of Thermodynamics: Heat will flow from a body at a higher temperature to a body at a lower temperature. Applied to heat engines: When work is done by an engine operating in a cycle, only some of the heat taken from a reservoir can be converted into work. The rest is rejected as heat at a lower temperature.

23. Energy in Electrical Systems A heat engine cannot be 100% efficient in turning heat to work. The maximum efficiency of a heat engine is called the Carnot efficiency. It depends on only the absolute temperatures of the hot and cold reservoirs, TH and TL. Carnot efficiency = 1 - TL / TH

24. Energy in Electrical Systems The absolute temperature scale is also called the Kelvin scale. The conversion between Celsius and Kelvin temperatures is: TKelvin = TCelsius + 273

25. Energy in Electrical Systems High-pressure steam enters a turbine at a temperature of 525ºC. The steam expands in the turbine and pushes on the blades of the turbine shaft, causing the shaft to rotate and do work. The steam exits the turbine at a lower pressure and a temperature of 110ºC. What is the maximum efficiency of the turbine?

26. Energy in Electrical Systems The steam transfers energy to the turbine at a high temperature of TH = 525ºC Convert to Kelvin: TKelvin = TCelsius + 273 TKelvin = 525 + 273 = 798 K

27. Energy in Electrical Systems The steam exits the turbine at a low temperature of TL = 110ºC Convert to Kelvin: TKelvin = TCelsius + 273 TKelvin = 110 + 273 = 383 K

28. Energy in Electrical Systems Carnot efficiency = 1 - TL / TH Carnot efficiency = 1 – 383 K / 798 K Carnot efficiency = 1 – .48 Carnot efficiency = .52 or 52% The maximum efficiency of the turbine is 52%