Activity A5-CI due by 4 pm Friday 02/29

Chapter 4 Mallard HW quiz – new due date: end of the day next Monday 03/03

Chapter 4 quiz in class next Tuesday 03/04

Exam 2 postponed to Thursday 03/06

Tuesday, February 26

Spring 2008

Heat and the Second Law of Thermodynamics

Chapter 4

Great Idea:Heat is a form of energy that flows from

warmer to cooler objects

2nd Law of Thermodynamics

Heat will not flow spontaneously from a cold to a hot body.

Like the flow of air from high pressure regions to low pressure regions, heat tends to flow from hot objects

(regions of “higher” energy) to cold objects (regions of “lower” energy).

Heat Engines

An engine converts energy to mechanical work

TH: high temperature reservoir

schematic representation of a heat engine

TC: low temperature reservoir

working body

Heat Engines

The efficiency of an engine is the work done by the engine divided by the energy put into it.

Engine efficiency:

e = QH

W

Work done by the engine is equal to the net heat.

W = Qnet = QH − QC

Maximum efficiency:

e =TH

TH – TC

Thermal Power Plants

A thermal power plant uses a heat engine to generate electricity from heat obtained from natural resources.

efficiency:

TH = 600 °C

TC = 100 °Ce = 57%

2nd Law of Thermodynamics

An engine that does nothing but convert heat to useful work cannot be constructed.

http://auto.howstuffworks.com/engine1.htm

Sample Exercise

A typical nuclear power plant delivers heat from the reactor to the turbines at a temperature of 540 °C.

If the turbines release heat at a temperature of 200 °C, what is the maximum possible efficiency of these turbines?

Sample Exercises

In one cycle, a heat engine does 500 J of work and releases 700 J of heat to a lower-temperature reservoir.

a. How much heat does it take in from the higher-temperature reservoir?

b. What is the efficiency of the engine?

Refrigerators and Heat Pumps

A heat pump, or refrigerator, is a device that moves heat from a cooler reservoir to a warmer reservoir by means of

work supplied from an external source.

The heat released, QH , equals the energy put into the

engine from both work, W , and heat, QC.

QH = W + QC

Refrigerators and Heat Pumps

A refrigerator moves heat from the cooler air from the interior to the warmer room.

A heat pump moves heat from the cooler air outdoors into the warmer

interior of a house or building.

2nd Law of Thermodynamics

Equivalency of first two statements of the 2nd law:

QC

A 100% efficient engine would require the spontaneously flow

of heat from cold to hot.

The spontaneously flow of heat from cold to hot would make the

engine 100% efficient.

Violations of Laws of Thermodynamics

W > QH − QC e ≥ 100%

2nd Law of Thermodynamics

Every isolated system becomes more disordered with time.

ORDER VS. DISORDER

2nd Law of Thermodynamics

ORDER VS. DISORDER

ICE LIQUID WATER

2nd Law of Thermodynamics

Probability

Number of ways to arrange three orange and three green numbered balls:

720 total; 36 ordered (3 orange then 3 green)

2nd Law of Thermodynamics

Entropy is a quantity that describes the extent to which a system loses the ability to do useful

work. It is a measure of the disorder or randomness of a system

The entropy of the universe or of an isolated system can only increase or remain constant.

Its entropy cannot decrease.