Available Energy

First Law of Thermodynamics

The increase in internal energy of a system is equal to the heat added plus the work done on the system.

Apparently, the availability of energy depends on the form that it takes.

Heat Engines

We can extract some of an object’s internal energy under certain circumstances.

All heat engines involve the flow of energy from a hotter region to a cooler one.

A heat engine is a device placed in the path of this flow to extract mechanical energy.

Ideal Heat Engine

What kind of engine would get the maximum amount of work from a given amount of heat?

Carnot’s Answer: even under ideal conditions, the heat engines must exhaust some heat!!!!

Second Law of Thermodynamics

It is impossible to build a heat engine to perform mechanical work that does not exhaust heat to the surroundings.

Perpetual Motion Machines

Real Engines

In the case of heat engines, the efficiency of an engine is equal to the ration of the work produced divided by the heat extracted from the hot region.

Q

W

Theoretical Efficiency

h

c

T

T1

Refrigerators

Refrigerators move heat in direction opposite to its natural flow.

Second Law of Thermodynamics

It is impossible to build a refrigerator that can transfer heat from a lower-temperature region to a higher-temperature region without using mechanical work.

Order and Disorder

Understanding the microscopic form of the second law of thermodynamics depends on realizing that the order of real systems is constantly changing.

Entropy

Entropy is a measure of a system’s organization.

Entropy Form of the Second Law of Thermodynamics

The entropy of isolated system tends to increase.

1. It is possible to float heat engines on the ocean and extract some of the internal energy of the water by extending a tube well beneath the ocean surface. Why is it necessary for the heat engine to have this tube in order to satisfy the second law of thermodynamics?

2. Heat engine A operates between 20C and 300C, whereas heat engine B operates between 80C and 300C. Which engine has the greater theoretical efficiency? Explain.

1. You are building a heat engine in which the temperature difference between the hot and cold regions is 100K. Will it be more efficient to have your cold region as cold as possible or as hot as possible? Explain.

2. Would it be possible to keep a room cool by leaving the refrigerator door open? Why or why not?

3. An air-conditioner mechanic is testing a unit by running it on the workbench in an isolated room. What happens to the temperature of the room?

1. Describe the system in which entropy is decreasing. Is this system isolated from its surroundings?

2. When water freezes to ice, does the order of the water molecules increase or decrease? What does this imply about the change in entropy in the rest of the universe?

3. A cold piece of metal is dropped into an insulated container of hot water. After the system has reached an equilibrium temperature, has the entropy of the universe increase or decrease?