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Chapter 22

Heat Engines, Entropy, and the Second Law of

Thermodynamics (cont.)

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Outline

Carnot theorem and maximum efficiency (22.3)

Entropy (22.6)

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Carnot theorem and maximum efficiency

In 1824, Sadi Carnot: Under what conditions will a heat engine have maximum efficiency?

Carnot’s Theorem: If an engine operating between two constant-

temperature reservoirs is to have maximum efficiency, it must be an engine in which all processes are reversible. In addition, all reversible engines operating between the same two temperatures, Tc and Th have the same efficiency.

No real engine can ever be perfectly reversible. The concept of a reversible engine is a useful idealization.

Maximum efficiency of a heat engine: emax = 1 – Tc/Th (Temperatures must be in Kelvin)

Sadi Carnot

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Examples 22.3 The steam engine A steam engine has a boiler that operates

at 500 K. The energy from the burning fuel changes water to steam, and this steam then drives a piston. The cold reservoir’s temperature is that of the outside air, approximately 300 K. (A) What is the maximum thermal efficiency

of this steam engine? (B) Determine the maximum work that the

engine can perform in each cycle if it absorbs 200 J of energy from the hot reservoir during each cycle.

Answer: 40% and 80 J.

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Entropy (conceptual discussion) Entropy: a fundamental quantity that is

related to the amount of disorder in a system.

Entropy in the universe: The total entropy of the universe increases

whenever an irreversible process occurs. The total entropy of the universe is unchanged whenever a reversible process occurs.

Since all real processes are irreversible, the total entropy of the universe continually increases.

A “directionality” in nature.

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Order, disorder, and entropy Entropy can be thought of as a measure of the

amount of disorder in the universe. As the entropy of a system increases, its

disorder increases as well; that is, an increase in entropy is the same as a decrease in order.

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Homework

Ch. 22, P. 700, Problems: #9, 10, 12.

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Chapter 23

Electric Fields

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Outline

Properties of electric charges (23.1)

Charging objects by induction (23.2)

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Properties of electric charges

There are two kinds of electric charges in nature, “positive” and “negative”.

Example: Electrons possess “-” charge and protons possess “+” charge.

Charges of the same sign repel one another and charges with opposite signs attract one another.

Total charge in an isolated system is always conserved.

Charge is quantized: q = Ne N: some integer; e: a fundamental amount

of charge Charge of an electron = -e; charge of a

proton: + e.

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Electrical conductors, insulators and semiconductors Electrical conductors: materials in which some

of the electrons are free electrons that are not bound to atoms and can move relatively freely through the material.

Copper, aluminum, silver. Electrical insulators: materials in which all

electrons are bound to atoms and cannot move freely through the material.

Glass, rubber, wood. Semiconductors: between insulators and

conductors. Silicon, germanium

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Charging objects by induction

Charging an object by induction requires no contact with the object inducing the charge. In contrast to charging an object by

rubbing.

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Charging a conductor by induction

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Electrical polarization