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 the ice goes into the cup of coffee. In reality, heat is energy and coolness is the absence of energy. It is impossible to transfer the lack of energy. Therefore, the energy in the coffee (in the form of heat) always travels into areas of less energy (cold areas.)
On a molecular level, heat is the movement of molecules. In hot systems, there is a large movement of molecules in a substance. In a cold system, the molecules vibrate only slightly. For heat to be transferred, the rapidly moving “hot” molecules bombard with the slowly moving “cold” molecules causing them to move faster.
The Third Law of Thermodynamics.
The third law of thermodynamics is a statement having to do with the order of a system (also called entropy.) Heat causes a change in the ordering of a system. For example, in cold ice, with the absence of heat, molecules form an ordered lattice. Once heat is added, the molecules move randomly, without any order. The third law of thermodynamics has far reached implications that are out of the scope of our learning in this course.
Topic 2: Heat and Temperature
As we talked about, heat is the energy of movement of molecules in a substance. On a molecular level, though, not all molecules move at the same rate or same direction. Some molecules will be barely moving at all until they are bombarded by a fast moving molecule. A good analogy is hitting a ball on a pool table (Figure 26). Once the pool balls are in motion, there will be some moving slower than others.
Figure 26 Pool balls move at random rates of speed. Open source image obtained from https://c.pxhere.com/photos/c0/b7/longexposure_break_ghosts_snooker_autofocus_om2n-269425.jpg!d
Temperature is an average of all the molecules’ velocity. A high temperature means that, on average, the molecules are moving faster than molecules of a lower temperature. When two substances of different temperature are put in contact with one another, the overall temperature will change to some point halfway between the temperature of the hot substance and the cold substance.
2.1 Units of Temperature
Heat is energy and so it is measured in Joules, like all other forms of energy. Temperature is measured by a thermometer in several different units: Celsius, Fahrenheit, Kelvin, and Rankine. The more heat energy in a system, the larger the temperature.
There are many reference points for the measurement of temperature: the boiling point of water is 100 degrees Celsius; the melting point of water is 0 degrees Celsius. The melting and boiling point of water is also a reference for the Fahrenheit system, but the boiling point was originally chosen as 240 (60 x 4) to make for easy calculations without fractions and later adjusted to be more accurate at 212. Absolute zero (the complete absence of molecular movement) is the reference point for the Kelvin scale. Absolute zero is measured as 0 Kelvin, -460 Fahrenheit, or -273 Celsius. It is the “coldest” any substance can be.
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Celsius Boiling 100° C Freezing 0° C (100 divisions between freezing and boiling) Absolute Zero -273° C (Negative values below freezing) |
Kelvin Boiling 373° K Freezing 273° K (100 divisions between freezing and boiling) Absolute Zero 0° K (No negative values)
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Fahrenheit Boiling 212° F Freezing 32° F (180 divisions between freezing and boiling) Absolute Zero -460° F (Negative values below freezing)
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Rankin Boiling 672° R Freezing 492° R (180 divisions between freezing and boiling) Absolute Zero 0° R (No negative values)
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Converting Temperature
A formula is needed to convert Celsius to Fahrenheit, or Fahrenheit to Celsius.
To convert Fahrenheit to Celsius, or
To convert Celsius to Fahrenheit.
Example 2
Convert 35 degrees Celsius to Fahrenheit.
Solution
Use the equation:
Substitute the value 35 degrees Celsius.
Topic 3: Thermal Expansion
Substances, for the most part, get larger when heated. More specifically, conductors will get longer when heated. This is a factor that affects the sag of a line.
The reason for this expansion is explained on a molecular level. With increased temperature, the average movement of the molecules is larger, and the molecules are further apart. If each molecule is further apart, then the overall length of the material is larger. Expansion can be a change in length, width, height, perimeter, circumference, diameter of radius.
Different materials are more susceptible to expansion. The amount of expansion per unit length of material is characterized by a number: the coefficient of expansion. This number is a known quantity (found by experimentation) for each material and can be looked up in charts.
The change in length of a material is given by this formula:
Calculate the change of length of a length of copper wire 23 meters long when the change of temperature is from 32 degrees Celsius to -32 degrees Celsius. (The coefficient of expansion for copper is 0.0000167 m/m˚C).
Solution
The change in temperature is:
32 degrees Celsius – (-32 degrees Celsius) = 64 degrees Celsius.
Note: the double negative turns into a positive. If you consider a thermometer, you can visualize that from +32 to -32 is 64 degrees, and not 0 degrees.
The change in length is 0.02 m or 20 mm.
Review Questions: Thermodynamics
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