Heat Engine: Converts Heat into Mechanical Energy.

Temperature: Measurement of Internal Energy...how fast molecules are moving or vibrating in gas, liquids & solids. Commonly called Kinetic Theory.

Measure Temperature in degrees Celsius.

0^oC ==> ICE

100^oC ==> Boiling Water

23^oC ==> Room Temperature (70^oF)

Kelvin Scale based on Absolute 0.

Absolute 0 ==> Temperature where all molecular motion stops.

Lecture Demonstration on Temperature Scale: Extrapolating Absolute Zero

Absolute Temperature K, Kelvin Scale

0K = -273^oC

0K = -460^oF

273K = 0^oC

Temperature in Kelvin = Temperature in degrees Celsius + 273

2nd Law - Heat flows from Hot to Cold. Convert some of this heat flow into Mechanical Energy.

The term heat engine is a generic description of any device that exploits a temperature difference to perform work. When two systems at different temperatures are placed in contact with one another, heat energy transfer will take place between them. If left alone these systems would eventually reach thermal equilibrium with one another. A heat engine converts some of the heat energy that is transferred into useful work.

So the first thing we need for a heat engine to work is two systems, or reservoirs, at different temperatures. For example, the hot gas resulting from combustion of the gasoline vapor/air mixture fed into your automobile's engine has a higher temperature than the water in the cooling system. The hot gas can be called the heat source and the water in the cooling system the heat sink. The maximum amount of work that can be performed by the heat engine is the difference between heat out of the source and heat flowing into the heat sink. Diagrammatically we can represent a heat engine as follows:

Heat Engine Diagram:

A really Cool Looking Heat Engine Schematic

The function of a heat engine is analogous to the conversion of the gravitational potential energy of water stored behind a hydroelectric dam into electrical energy. In order to "generate" hydroelectricity we need an upper reservoir (behind the dam) and a lower reservoir, which may simply be a stream or river. There must be a place at lower elevation (a "heat sink") where the water can go in order to extract work (in the form of electrical energy) from the upper reservoir. Just as one can't imagine generating electricity within the reservoir of water behind a dam, work can not be extracted from a heat engine without a second reservoir at a lower temperature. This principle is summarized in the second law of thermodynamics, which states:

It is impossible for a machine (heat engine) to take heat energy from a reservoir at a certain temperature, to produce work, and to exhaust heat to a reservoir at the same temperature.

Example of Heat Engines:

In Class Demo of Thermoelectric Engines, Converts a temperature difference into a voltage which turns a motor. Temp Difference: Hot Water and Ice Water...Motor turns. Can we get work out of ice water? It is at 0^oC.

Remember 0^oC = 273K. There is still energy there. Use a second container with dry ice and alcohol = -80^oC = 193K. Heat will flow from the ice water to the dry ice solution and the propeller will turn.

You can extract work as long as there is a temperature difference between two thermal reservoirs.

Recall The Falling Mass Generator

If Q is the heat transfer, W is the work, and T is the temperature we can calculate the efficiency of a heat engine.

Maximum Efficiency: Carnot Cycle

Entropy: Measurement of Disorder: Natural progression of the universe. It takes work to decrease entropy. Example, cleaning your room or desk takes work. Its natural tendency is to go to disorder.

Heat flowing from a hot reservoir to a cold reservoir is entropy increasing. Ordered system: Fast and slow molecules are separated. Heat flow from Hot to Cold. The fast moving molecules are interacting with the slow. The ordered separation is decreasing and disorder is increasing.

A small discussion on reversible and irreversible process. Demo: Reversible and Irreversible Processes

Carnot Cycle: Reversible Process; Heat Engine with Maximum Efficiency

Maximum Efficiency = (1 - T_C/T_H)*100

100% efficiency is not possible because T_C cannot be 0K (3rd Law).

Generation of Electricity from a Heat Engine: A typical Fossil Fuel Power Plant.

Study Aid. These lecture have stated the Second Law of Thermodynamics a number of different ways. Write a brief definition of the 2nd Law as you now understand it.