chapter 20 second law of thermodynamicspeople.virginia.edu/~ben/hue_physics_152/ben_lect_19.pdf ·...

Copyright © 2009 Pearson Education, Inc.

Chapter 20 Second Law of

Thermodynamics


It is easy to produce thermal energy using work, but how does one produce work using thermal energy?

This is a heat engine; mechanical energy can be obtained from thermal energy only when heat can flow from a higher temperature to a lower temperature.

20-2 Heat Engines


We will discuss only engines that run in a repeating cycle; the change in internal energy over a cycle is zero, as the system returns to its initial state.

The high-temperature reservoir transfers an amount of heat QH to the engine, where part of it is transformed into work W and the rest, QL, is exhausted to the lower temperature reservoir. Note that all three of these quantities are positive.

20-2 Heat Engines


A steam engine is one type of heat engine. 20-2 Heat Engines


The internal combustion engine is a type of heat engine as well.

20-2 Heat Engines


Why does a heat engine need a temperature difference?

Otherwise the work done on the system in one part of the cycle would be equal to the work done by the system in another part, and the net work would be zero.

20-2 Heat Engines


The efficiency of the heat engine is the ratio of the work done to the heat input:

Using conservation of energy to eliminate W, we find:

20-2 Heat Engines


20-2 Heat Engines

Example 20-1: Car efficiency.

An automobile engine has an efficiency of 20% and produces an average of 23,000 J of mechanical work per second during operation.

(a)  How much heat input is required, and

(b) How much heat is discharged as waste heat from this engine, per second?


20-2 Heat Engines

No heat engine can have an efficiency of 100%. This is another way of writing the second law of thermodynamics:

No device is possible whose sole effect is to transform a given amount of heat completely into work.


The Carnot engine was created to examine the efficiency of a heat engine. It is idealized, as it has no friction. Each leg of its cycle is reversible.

The Carnot cycle consists of:

•  Isothermal expansion

•  Adiabatic expansion

•  Isothermal compression

•  Adiabatic compression

20-3 Reversible and Irreversible Processes; the Carnot Engine


From this we see that 100% efficiency can be achieved only if the cold reservoir is at absolute zero, which is impossible.

Real engines have some frictional losses; the best achieve 60–80% of the Carnot value of efficiency.

For an ideal reversible engine, the efficiency can be written in terms of the temperature:




Example 20-2: A phony claim?

An engine manufacturer makes the following claims: An engine’s heat input per second is 9.0 kJ at 435 K. The heat output per second is 4.0 kJ at 285 K. Do you believe these claims?



Automobiles run on the Otto cycle, shown here, which is two adiabatic paths alternating with two constant-volume paths. The gas enters the engine at point a and is ignited at point b. Curve cd is the power stroke, and da is the exhaust plus intake..



Example 20-3: The Otto cycle.

For an ideal gas as working substance, the efficiency of an Otto cycle engine is

e = 1 – (Va/Vb)1-γ

where γ is the ratio of specific heats (γ = CP/CV) and Va/Vb is the compression ratio.

Calculate the efficiency for a compression ratio Va/Vb = 8.0 assuming a diatomic gas like O2 and N2.


These appliances are essentially heat engines operating in reverse.

By doing work, heat is extracted from the cold reservoir and exhausted to the hot reservoir.

20-4 Refrigerators, Air Conditioners, and Heat Pumps



This figure shows more details of a typical refrigerator.


Refrigerator performance is measured by the coefficient of performance (COP):


Substituting:

For an ideal refrigerator,



Example 20-4: Making ice.

A freezer has a COP of 3.8 and uses 200 W of power. How long would it take this otherwise empty freezer to freeze an ice-cube tray that contains 600 g of water at 0°C?


A heat pump can heat a house in the winter:




Example 20-5: Heat pump.

A heat pump has a coefficient of performance of 3.0 and is rated to do work at 1500 W.

(a) How much heat can it add to a room per second?

(b) If the heat pump were turned around to act as an air conditioner in the summer, what would you expect its coefficient of performance to be, assuming all else stays the same?


20-5 Entropy

Definition of the change in entropy S when an amount of heat Q is added:

if the process is reversible and the temperature is constant. Otherwise,


20-5 Entropy

Any reversible cycle can be written as a succession of Carnot cycles; therefore, what is true for a Carnot cycle is true of all reversible cycles.


20-5 Entropy Since for any Carnot cycle QH/TH + QL/TL = 0, if we approximate any reversible cycle as an infinite sum of Carnot cycles, we see that the integral of dQ/T around a closed path is zero. This means that entropy is a state variable—the change in its value depends only on the initial and final states.


20-6 Entropy and the Second Law of Thermodynamics

Example 20-6: Entropy change when mixing water.

A sample of 50.0 kg of water at 20.00°C is mixed with 50.0 kg of water at 24.00°C. Estimate the change in entropy.



The total entropy always increases when heat flows from a warmer object to a colder one in an isolated two-body system. The heat transferred is the same, and the cooler object is at a lower average temperature than the warmer one, so the entropy gained by the cooler one is always more than the entropy lost by the warmer one.



Example 20-7: Entropy changes in a free expansion.

Consider the adiabatic free expansion of n moles of an ideal gas from volume V1 to volume V2, where V2 > V1. Calculate the change in entropy

(a) of the gas and

(b) of the surrounding environment.

(c) Evaluate ΔS for 1.00 mole, with V2 = 2.00 V1.



Example 20-8: Heat transfer.

A red-hot 2.00-kg piece of iron at temperature T1 = 880 K is thrown into a huge lake whose temperature is T2 = 280 K. Assume the lake is so large that its temperature rise is insignificant. Determine the change in entropy

(a) of the iron and

(b) of the surrounding environment (the lake).



The fact that after every interaction the entropy of the system plus the environment increases is another way of putting the second law of thermodynamics:

The entropy of an isolated system never decreases. It either stays constant (reversible processes) or increases (irreversible processes).


Entropy is a measure of the disorder of a system. This gives us yet another statement of the second law:

Natural processes tend to move toward a state of greater disorder.

Example: If you put milk and sugar in your coffee and stir it, you wind up with coffee that is uniformly milky and sweet. No amount of stirring will get the milk and sugar to come back out of solution.

20-7 Order to Disorder


Another example: When a tornado hits a building, there is major damage. You never see a tornado approach a pile of rubble and leave a building behind when it passes.

Thermal equilibrium is a similar process—the uniform final state has more disorder than the separate temperatures in the initial state.

20-7 Order to Disorder


Another consequence of the second law:

In any natural process, some energy becomes unavailable to do useful work.

If we look at the universe as a whole, it seems inevitable that, as more and more energy is converted to unavailable forms, the ability to do work anywhere will gradually vanish. This is called the heat death of the universe.

20-8 Unavailability of Energy; Heat Death


20-11 Thermal Pollution, Global Warming, and Energy Resources

Over 80% of the energy used in the world is generated using heat engines to drive turbines and generators—even nuclear power plants use the energy generated from fission heat water for a steam engine. The thermal output QL of all these heat engines contributes to warming of the atmosphere and water. This is an inevitable consequence of the second law of thermodynamics.


Summary of Chapter 20 •  Heat engine changes heat into useful work

•  Efficiency: work/heat input

•  Maximum efficiency: 1 – TL/TH

•  Refrigerators and air conditioners are heat engines, reversed; COP = heat removed/work

•  Heat pump: COP = heat delivered/work

•  Second law of thermodynamics: Natural processes always tend to increase entropy


Summary of Chapter 20

•  Change in entropy gives direction to “arrow of time”

•  As time goes on, energy becomes degraded.

•  Heat engines cause thermal pollution.

•  Entropy change in reversible process:


Study problems: Chapter 20 – 6, 17, 27, 29, 39

chapter 20 second law of thermodynamicspeople.virginia.edu/~ben/hue_physics_152/ben_lect_19.pdf ·...

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