Mon. Apr. 20 – Physics Lecture #39Heat Engines and the Second Law of Thermodynamics

1. Lab 32: It’s Getting Hot In Here – Heat Engine Demos

2. Energy, Work, Heat, and the First Law of Thermodynamics

3. Cyclic Processes, Heat Engines, Heat Pumps, Refridgerators

4. Efficiency, Entropy, and the Second Law of Thermodynamics

Internal Combustion EngineAn internal combustion engine can be modeled using the following four step, cyclic process. Process 1: Fill the piston with an air/gas mixture, and compress it adiabatically to a lower volume.

Process 2: Ignite the air/gas mixture. The temperature rapidly increases without the volume changing very much (due to the rapidity of the process).

Process 3: The high pressure gas pushes the piston back to its starting volume during the adiabatic power stroke.

Process 4: The gas cools down, at constant volume, basically by expelling the hot exhaust and filling the piston with a cool air/gas mixture.

Process 1: adiabatic compression

1

Process 2: T increases, V constant

2

Process 3: adiabatic expansion

3

Process 4: T decreases, V constant

4

V

P

Qh

Qc

Heat Engine

hTHigh temperature reservoir

Engine (cyclic)

Low temperature reservoir cT

cQ

hQW

|Energy In| = |Energy Out|

ch QWQ Efficiency:

h

c

h QQ

QW

1

Heat removed from high temperature reservoir is added to engine.

Heat removed from engine is added to low temperature reservoir.

Engine turns some heat into work; some heat removed from engine.

Heat Engine

hTHigh temperature reservoir

Engine (cyclic)

Low temperature reservoir cT

cQ

hQW

Heat Pump/Refrigerator

hTHigh temperature reservoir

Fridge (cyclic)

Low temperature reservoir cT

cQ

hQW

A power plant produces 100 MJ of electricity. The plant runs from a nuclear fission reactor, which boils water and feeds 400 MJ of energy into a turbine generator. How much heat is dumped into the local river during this process?

1. 0 MJ2. 100 MJ3. 300 MJ4. 400 MJ5. 500 MJ6. Not enough info

What is the efficiency of this engine?

Let’s say we tried to make an engine that would take 300 J of energy from a 600 K hot reservoir and turn it all into work. What is the total entropy change of the universe (the heat reservoir + the engine)?

What is the maximum amount of the 300 J that could be turned into work?

300 J

|W|

600 K

c) If this cycle were used as the engine part of a heat engine, determine the efficiency of the engine.

d) Verify the two entries provided in the table.