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

2 nd 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 T H : high temperature reservoir schematic representation of a heat engine T C : 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 = QHQH W Work done by the engine is equal to the net heat. W = Q net = Q H − Q C Maximum efficiency: e = THTH T H – T C

Thermal Power Plants A thermal power plant uses a heat engine to generate electricity from heat obtained from natural resources. efficiency: T H = 600 °C T C = 100 °C e = 57%

2 nd Law of Thermodynamics An engine that does nothing but convert heat to useful work cannot be constructed.

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, Q H, equals the energy put into the engine from both work, W, and heat, Q C. Q H = W + Q C

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.

2 nd Law of Thermodynamics Equivalency of first two statements of the 2 nd law: QCQC 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 > Q H − Q C e ≥ 100%

2 nd Law of Thermodynamics Every isolated system becomes more disordered with time. ORDER VS. DISORDER

2 nd Law of Thermodynamics ORDER VS. DISORDER ICE LIQUID WATER

2 nd 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)

2 nd 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.