Chapter 24Thermodynamics

Absolute Zero

Absolute zero is a theoretical temperature at which atoms that make up matter have zero kinetic energy. They are at rest.

Watch the NOVA program “Absolute Zero” on Youtube.

Keywords “Absolute Zero”

First Law of Thermodynamics

• Heat that is added to a system can be converted to equal amounts of some other form of energy.

• Another way to state the first law of thermodynamics is:

Energy cannot be created or destroyed but can be transformed from one form to another in a thermal system

• The first law of thermodynamics is simply the law of conservation of energy applied to thermal systems

Adiabatic Processes

The process of compression or expansion of a gas so that no heat enters or leaves a system is said to be adiabatic.

change in air temperature ~ pressure change

As air pressure decreases with altitude, so does the temperature. Air temperature decreases by about 5 °F for every 1000 feet of elevation.

Since cold air holds less water vapor than warm air, the relative humidity gradually increases until it is saturated:

100% relative humidity

The temperature at which the air is saturated is called the dew point.

change in air temperature ~ pressure changechange in air temperature ~ pressure change

Adiabatic cooling of the air that results in the formation of clouds often occurs around mountains (orographic lifting).

Adiabatic cooling of the air results in a “Rain Shadow” effect:

Where are the driest places on earth?

• Atacama Desert:

Second Law of Thermodynamics

• Heat flows from HOT to COLD

Efficiency of a Heat Engine

• Carnot Efficiency: The efficiency of a heat engine depends on the temperature difference between the hot reservoir and the cold sink.

Ideal Efficiency = Thot – Tcold

Thot

Entropy

• Natural systems tend to proceed toward a state of greater disorder

• Entropy - is the measure of the amount of disorder in a system.

• Whenever a physical system is allowed to distribute its energy freely, it always does so in a manner such that entropy increases while the available energy of the system for doing work decreases.

Summary:

First Law of Thermodynamics:

Energy cannot be created or destroyed, but can be transformed to other forms within a thermal system

Second Law of Thermodynamics

Entropy: Natural systems tend to proceed to a state of greater disorder