49 Outcome 4: Thermodynamics
Outcome/Competency: You will be able to describe and predict the outcomes of systems involving heat and temperature
Rationale:
Why is it important for you to learn this skill?
Heat and temperature are related, but not the same thing. Understanding the difference, and how heat travels, will explain certain phenomenon you encounter on your job. For example, why do powerlines shrink when it is cold out? How do you need to take this contraction into account when calculating the tension of the line? Understanding heat and temperature will make these calculations possible.
Objectives:
To be competent in this area, the individual must be able to:
- Explain and apply the principals and laws of thermodynamics
Learning Goals
- Understand and apply the first and second law of thermodynamics.
- Identify, describe, and solve problems related to heat, temperature, and their relationship to each other.
- Explain thermal expansion using concepts of matter and heat, and calculate the thermal expansion of solids
Introduction:
This section will cover how to apply the first and second law of thermodynamics, review the relationship between heat and temperature, and explore thermal expansion through heat. This chapter will include class discussions, articles to be reviewed, review questions, and a cumulative test.
Topic 1: The Laws of Thermodynamics
One fundamental rule of the universe is that energy is conserved. This means it is neither created nor destroyed, but it can certainly be converted from one form to another. Remember, some forms of energy are heat, mechanical, chemical, and electrical.
1.1 The First Law of Thermodynamics
The first law of thermodynamics states that work and heat are convertible. In a closed system (no interaction from outside the system) the amount of work equals the amount of heat. Stated another way, the amount of work and the amount of heat represent the total energy in the system.
Key Takeaways
A gas in a closed container is heated, causing the lid of the container to rise. The gas performs 3 J (joules) of work to raise the lid, such that is has a final total energy of 15 J. How much heat energy was added to the system?
Solution
In this example, it is important to remember that heating a gas causes it to expand. Think of a sealed coffee mug. When you put hot coffee in it, and seal it off, the small amount of air at the top of the mug is heated. When you open the mug, it creates a “popping” sound as the pressure is released.
The question is asking how much heat energy was added to the system. We need to solve the heat equation for ∆U by subtracting W from both sides.
Substitute numbers. The work, W , is 3 J. The total energy, Q, is 15 J.
12 Joules of heat was added to the system.
The Second Law of Thermodynamics
Heat energy always moves from a hot area to a cold area. Consider an ice cube in a hot cup of coffee. You may think that the cool ice cube causes the cup of coffee to cool off; that the coolness of th