Physics 15: 5-6 Heat Engines and Work

Homework

Reading Preparation
Key Equations
WebLecture
Study Activity
Chat Preparation Activities
Chapter Quiz
Lab Work

Reading Preparation

Text Reading: Giancoli, Physics - Principles with Applications, Chapter 15: 5-6

Study Points

Section 5: A heat engine is any system that converts heat energy to mechanical energy. The basic application of the second law allows us to determine engine efficiency by comparing the amount of heat transferred into the engine (measured as a temperature change) to the amount of work actually done by the engine: efficiency = T_H - T_L / T_H.Efficiency = 100% (1) only if T_L equals absolute zero — a value impossible to achieve. Sadi Carnot described the behavior of an ideal heat engine in which each step is reversible with no energy loss — but no real engine work at 100% efficiency on each process. Another way to state the second law is that no device can completely transform heat energy to work energy.
Section 6: Refrigerators, air conditioners and heat pumps reverse the operation of a heat engine. Instead of pumping heat into a cold area, they pump heat from a cooler area to a hotter one. This requires the expenditure of energy: there is no theoretical way to have a perfect refrigerator.

Key Equations

Engine efficiency $e = \frac{W}{Q_{H}} = \frac{Q_{H} - Q_{L}}{Q_{H}} = 1 - \frac{Q_{L}}{Q_{H}}$
Carnot Ideal Efficiency $e = \frac{T_{H} - T_{L}}{T_{H}} = 1 - \frac{T_{L}}{T_{H}}$
Coefficient of performance $COP = \frac{Q_{L}}{W} = \frac{Q_{L}}{Q_{H} - Q_{L}} \Rightarrow {COP}_{ideal} = \frac{T_{L}}{T_{H} - T_{L}}$

Web Lecture

Read the following weblecture before chat: The Second Law of Thermodynamics

Study Activity

Use the Heat Engine Simulator to change temperatures between the "source" and "sink". What happens to heat flow and efficiency?

Optional Websites:

Even though a bit dated, Isaac Asimov's article on Thermodynamics from his overview of physical science is still one of the best treatments of the subject.

Chat Preparation Activities

Forum question: The Moodle forum for the session will assign a specific study question for you to prepare for chat. You need to read this question and post your answer before chat starts for this session.
Mastery Exercise: The Moodle Mastery exercise for the chapter will contain sections related to our chat topic. Try to complete these before the chat starts, so that you can ask questions.

Chapter Quiz

The chapter quiz is not yet due.

Lab Work

If you want lab credit for this course, you must complete at least 18 labs; you may complete more if you are preparing for the AP exam.. One or more lab exercises are posted for each chapter as part of the homework assignment. We will be reviewing lab work at regular intervals, so do not get behind!

Lab Instructions: Joule's Experiment

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