Chapters 12 & 13 & 14

Thermodynamics

Change of State

Melting and Freezing

At the melting point the atoms of a crystalline substance are moving so vigorously that any additional energy goes into breaking the intermolecular bonds that hold the solid together. The internal energy of the system increases, although the temperature remains constant throughout the melting process.

The amount of heat that must be added to a kilogram of any solid at its melting temperature in order to liquefy is the latent heat of fusion. (J/kg)

For water the heat of fusion is .

To change the phase of a mass already at its melting point requires the addition of an amount of heat:

(11)

 

Vaporization

Transforming a liquid or solid into a gas.

The amount of heat that must be added to a kilogram of any liquid to evaporate it is the heat of vaporization.

For water the heat of vaporization is .

To change the phase of a mass already at its vaporization point requires the addition of an amount of heat:

(12)

 

Transfer of Thermal Energy

Radiation:

Thermal radiation is characterized by a broad, continuous range of wavelengths, which arises from interactions among the atoms of solids, liquids, and dense gases.

The amount of thermal radiation emitted by a body depends on its surface condition (color, texture, exposed area, etc.) and on its temperature.

The amount of thermal radiation absorbed by a body depends on its surface condition and on the nature of the incident radiant energy (wavelength, intensity, etc.) which, in turn, depends on the temperature of the source.

 

Convection:

When a region of fluid is heated and its density decreases so that it rises. The colder fluid around sinks and a current is set in motion.

 

Conduction:

Heat is only conducted only from high to a low temperature. The measure of the ability of a substance to transmit heat is known as its thermal conductivity (). This constant depends on the atomic structure of the material.

Fourier's Conduction Law

(13)

 

On to the First Law of Thermodynamics