According to the first law of Thermodynamics [Mechanical engineering]

# According to the first law of Thermodynamics

1st Law of Thermodynamics

stLaw of Thermodynamics

The st Law of Thermodyamics simply states that energy can be neither created nor destroyed (conservation of energy). Thus power generation processes and energy sources actually involve conversion of energy from one form to another, rather than creation of energy from nothing. For example:

terms of their energy conversion processes.

 Automobile Engine Chemical ® Kinetic Heater/Furnace Chemical ® Heat Hydroelectric Gravitational ® Electrical Solar Optical ® Electrical Nuclear Nuclear ® Heat, Kinetic, Optical Battery Chemical ® Electrical Food Chemical ® Heat, Kinetic Photosynthesis Optical ® Chemical

As you can see conversion between chemical energy and other forms of energy are extremely important, whether you are veterinarian or a mechanical engineer. That is what we will focus on for the remainder of this chapter.

System and Surroundings

The 1st Law of Thermodynamics tells us that energy is neither created nor destroyed, thus the energy of the universe is a constant. However, energy can certainly be transferred from one part of the universe to another. To work out thermodynamic problems we will need to isolate a certain portion of the universe (the system) from the remainder of the universe (the surroundings).

For example consider the pendulum example given in the last section. In real life there is friction and the pendulum will gradually slow down until it comes to rest. We can define the pendulum as the system and everything else as the surroundings. Due to friction there is a small but steady transfer of heat energy from the system (pendulum) to the surroundings (the air and the bearing upon which the pendulum swings). Due to the 1st law of thermodynamics the energy of the system must decrease to compensate for the energy lost as heat until the pendulum comes to rest. [Remember though the total energy of the universe remains constant as required by the 1st Law.]

When it comes time to work homework, quiz and exam problems (not to mention to design a power plant or computer chip) the 1st Law of Thermodynamics will be much more useful if we can express it as an equation.

DE = q + w (1st Law of Thermodynamics)

• DE ® The change internal energy of the system,
• q ® The heat transferred into/out of the system,
• w ® The work done by/on the system.

This reformulation of the 1st Law tells us that once we define a system (remember we can define the system in any way that is convenient) the energy of the system will remain constant unless there heat added or taken away from the system, or some work takes place.

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