Chapter 16. The second law of thermodynamics

Goals

• Study inherently one way processes and entropy

• Heat engines: Otto-, Diesel, and Carnot engine

• The second law of thermodynamics

heat engines- cycle process

H

c

H

c

H Q

Q

C

Q

Q

We 11

Thermodynamical efficiency

The ottocycle = gasoline engine

3 types of heat engines:1.Gasoline or otto engine e = 56%2.Diesel engine e= 68%3.Carnot engine most efficient heat engine e= 90% for large temp difference

e=0.56 r=8 ottoe=0.70 r=15-20 Diesel

For Diesel there is no fuel in the compression stroke and r can be large

Preigintion cannot occur since only hot air is present before the fuel is injected immediately before the power stroke

Refrigerator Air conditioner

Refrigerator= heat engine in reverse

Second law of thermodynamics

Avoid all irreversible processesBottom line: every process of the cycle must be isothermal or adiabatic

Carnot Engine

The thermal efficiency formula is valid for which of the following heat engines? (There may be more than one

correct choice)

A.Carnot engineB.Otto engineC.Diesel engineD.Any other type of heat engine.

HQWe /

For an Otto engine with a compression ratio of 7.50, you have your choice of using an Ideal monatomic or ideal diatomic gas. Which one would give greater efficiency?

In one cycle a freezer uses 785 J of electrical energy in order to remove 1750 J of heat From its freezer compartment at 10 F. a) what is the coefficient of performanceOf this freezer? b) how much heat does it expel into the room during this cycle?

A refrigerator has a coefficient of performance of 2.10. Each cycle, it absorbs3.4 E4 J of heat from the cold reservoir. A) how much mechanical energy is required Each cycle to operate the refrigerator? B) During each cycle, how much heat is Discarded to the high-temp reservoir?

An insulated box has a barrier that confines a gas to only one side of the box. The barrier springs a leak, allowing the gas to flow and occupy both sides of the box. Which statement best

describes the entropy of this system?

A.The entropy is greater in the first state, with all the gas on one side of the box.B.The entropy is greater in the second state, with the gas on both sides of the box.C.The entropy is the same in both states, since no heat was added to the gas and its temperature did not change.

If you mix cold milk with hot coffee in an insulated Styrofoam cup, which of the following things happen? (There may be

more than one correct answer)

A.The entropy of the milk increases.B.The entropy of the coffee decreases by the same amount that the entropy of the milk increased.C.The net entropy of the coffee-milk mixture does not change, because no heat was added to the system.D.The entropy of the coffee-milk mixture increases.