Book cover for Thermodynamics: An Engineering Approach

Thermodynamics: An Engineering Approach

Yunus A. Cengel, Michael A. Boles

ISBN #9781259822674

9th Edition

2,694 Questions

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Homework Questions

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Summary
Learning Objectives
Key Concepts
Example Problems
Explanations
Common Mistakes

Summary

This chapter introduces the foundational concepts of thermodynamics, emphasizing its applications in both natural and engineered processes. It covers the critical role of proper unit treatment, dimensional homogeneity, and conversion between SI and English units. The chapter also reviews various temperature scales, including the ITS-90, and demonstrates practical measurement techniques using devices like manometers and barometers. Additionally, the systematic approach to problem solvingā€”from defining the problem to verifying resultsā€”is highlighted, preparing students for real-world engineering analysis.

Learning Objectives

1

Understand the application areas of thermodynamics in natural processes and engineered systems.

2

Explain the importance of proper use of SI and English units, including dimensional homogeneity.

3

Analyze measurement devices such as manometers and barometers for pressure and temperature assessments.

4

Interpret various temperature scales including the ITS-90 and perform unit conversions.

5

Apply systematic problem-solving steps in thermodynamic analyses from problem statement through verification.

Key Concepts

CONCEPT

DEFINITION

Thermodynamics

The branch of physics that deals with energy forms, energy conversion processes, and the relationship between work and heat.

Dimensional Homogeneity

A principle stating that all additive terms in a physical equation must have the same dimensions, ensuring consistency in units.

SI Units

The International System of Units which provides a standard set of measurements (such as meters, kilograms, seconds) for scientific calculations.

English Units

A system of measurement originally used in the United States, including units like feet, pounds, and Fahrenheit.

Temperature Scales

Various scales for measuring temperature (e.g., Celsius, Kelvin, Fahrenheit), each with its own reference points.

ITS-90

The International Temperature Scale of 1990, a standardized calibration scheme for temperature measurement.

Manometer

A device used to measure pressure differences in fluids, often relying on the balance of liquid column heights.

Barometer

An instrument that measures atmospheric pressure, important in both weather forecasting and engineering applications.

Unity Conversion Ratios

Standard conversion factors that relate different units while ensuring consistency in mathematical operations.

Continuum

The concept that physical properties or variables change continuously without abrupt jumps, applicable in material and fluid analysis.

State Postulate

A fundamental principle stating that the state of a simple compressible system is defined by a limited number of independent properties.

Engineering Software Packages

Simulation and calculation tools that aid engineers in solving complex thermodynamic problems.

Equation Solvers

Software or algorithms used to solve mathematical equations, facilitating thermodynamic analysis.

Significant Digits

The digits in a number that contribute to its precision, important in ensuring accuracy in thermodynamic calculations.

Example Problems

Example 1

Why does a bicyclist pick up speed on a downhill road even when he is not pedaling? Does this violate the conservation of energy principle?

Example 2

One of the most amusing things a person can experience is when a car in neutral appears to go uphill when its brakes are released. Can this really happen or is it an optical illusion? How can you verify if a road is pitched uphill or downhill?

Example 3

An office worker claims that a cup of cold coffee on his table warmed up to $80^{\circ} \mathrm{C}$ by picking up energy from the surrounding air, which is at $25^{\circ} \mathrm{C}$. Is there any truth to his claim? Does this process violate any thermodynamic laws?

Example 4

What is the difference between the classical and the statistical approaches to thermodynamics?

Example 5

Explain why the light-year has the dimension of length.
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Step-by-Step Explanations

QUESTION

How do you verify that a thermodynamic equation is dimensionally homogeneous?

STEP-BY-STEP ANSWER:

Step 1: Identify each term in the equation and write down its units using SI or consistent units.
Step 2: Break down composite units into their base dimensions (e.g., mass, length, time).
Step 3: Compare the dimensions on both sides of the equation to ensure they match.
Step 4: Confirm that any conversion factors used do not violate the dimensional consistency.
Final Answer: The equation is dimensionally homogeneous if all terms match in their base dimensions.

Dimensional Homogeneity

QUESTION

Describe the process of using a manometer to measure pressure differences in a fluid system.

STEP-BY-STEP ANSWER:

Step 1: Set up the manometer so that both ends are connected to the points where pressure is to be measured.
Step 2: Observe the difference in the height of the fluid columns in the manometer.
Step 3: Use the known density of the fluid and gravitational acceleration to calculate the pressure difference using the formula Ī”P = ĻgĪ”h.
Step 4: Convert the measured pressure into the desired unit if necessary.
Final Answer: Manometers provide a direct measurement of pressure differences by relating the fluid column height difference to pressure through Ī”P = ĻgĪ”h.

Pressure Measurement Devices

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Common Mistakes

  • Confusing or mixing SI and English units without proper conversion.
  • Neglecting the requirement for dimensional homogeneity when formulating or verifying equations.
  • Misinterpreting temperature scales and failing to apply the correct conversion factors.
  • Assuming that measurement devices such as manometers and barometers work identically in all conditions.
  • Overlooking assumptions and approximations during step-by-step problem solving, leading to inaccurate computations.