CH. 4 | Heat and Temperature

Thermodynamics

Study of macroscopic processes involving heat, mechanical, and other forms of energy

• All energy forms can be converted to heat
  
  

Applications:

• Systems with energy inputs and outputs, heat engines, heat pumps, refrigerators

• Based upon but not concerned with microscopic details

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Cohesion v. Adhesion

Cohesion

• Attractive forces between like molecules (solids and liquids)

Adhesion

• Attractive forces between unlike molecules

• Water wetting skin

• Glue mechanism: adhesives

Interactions can also be repulsive

• Water beading on wax

Phases of Matter (SOLID)

• Definite shape and volume

• Rigid 3-D structure

• Atoms / Molecules bonded in one place

• Allowed motions restricted to vibration in place only

Phases of Matter (LIQUIDS)

• Definite volume, indefinite shape

• Only weak cohesive bonds between component molecules

• Constituent molecules mostly in contact

• Allowed motions

Vibrations

• Bonds within molecule stretch/bend

Rotations

• Molecule spins around center of mass

Limited translation

• Entire molecule moves towards a specific direction

Phases of Matter (GASES)

• Indefinte volume and shape

• Molecules mostly not in contact

• Allowed motions
  
  → Vibration & Rotation (molecules with more than one atom)
  → Translation on random, mostly free paths

Molecular Motions

Characterized by average kinetic energy in a large sample

Temperature

• Measure of average kinetic energy on the molecules making up a substance

• Proportional to average kinetic energy

Evidence

• Gases diffuse quickly at higher temperatures

• Expansion / contraction with increasing / decreasing temperature

Temperature Conversions

• Fahrenheit to Celsius

• Celsius to Fahrenheit

• Celsius to Kelvin

Temperature

A measure of the internal energy of an object

• how hot or cold something is

Thermometers

• Used to measure temperature

• Rely on thermometrics properties
  → bimetialic strips and thermostats

Temperature (cont.)

Reference points

• Fahrenheit

• Celsius

• Kelvin

Heat

A form of energy between two objects

External energy

• total potential and kinetic energy of an everyday sized object

Internal energy

• total kinetic energy of the molecules in that object
  → temperature , density , heat, etc

External energy can be transferred to internal, resulting in a temperature increase

Temperature v. Heat

Temperature

• A measure of hotness or coldness of an object

• Based on average molecular kinetic energy

Heat

• Based on total internal energy of molecules

• Doublng amount at same temperature doubles heat

2 Heating Methods

1) Temperature difference:

• Energy always moves from higher temperature regions to lower temperature regions

2) Energy-form conversion:

• Transfer of heat by doing work

Specific Heat

The amount of heat needed to increase the temperature of 1g of a substance 1C

3 Variables involved in heating

• Temperature change

• Mass

• Type of material

Different materials require different amounts of heat to produce the same temperature change

• Measure = specific heat

• Summarized in one equation

Heat Flow

3 Mechanisms of heat transfer due to a temperature difference

• Conduction

• Convection

• Radiation

Conduction

The transfer of energy from molecule to molecule

• Happens primarily in solid

• Heat flowing through matter

• increased KE from molecule to molecule

Mechanisms

• Hotter atoms collide with cooler ones, transferring some of their energy

• Direct physical contact required; cannot occur in a vacuum

Sample Conductivities

Convection

Energy transfer through the bulk motion of hot material

• high KE molecules are moved from place to place

Examples

• Space heater

• Gas furnace (forced)

Natural convection mechanism - “hot air rises”

Radiation

Radiant energy

• Energy associated with electromagnetic waves
  → Can operate through a vacuum
  → All objects emit and absorb radiation

Temperature determines

• Emission rate

• Intensity of emitted light

• Type of radiation given off

Temperature determined by balance between rates of emission and absorption

→ global warming

Phase Change w/ Heating

Phase change at constant temperature

• Related to changes in internal potential energy

• Latent heat

Phase Changes Diagram

Evaporation v. Condensation

Individual molecules can change phase at any time

Evaporation

• Energy required to overcome phase cohesion

• Higher-energy molecules near the surface can then escape

Condensation

• Gas molecules near the surface lose kinetic energy to liquid molecules and merge

Thermodynamics

The study of heat and its relationship with mechanical and other forms of energy

1st Law of Thermodynamics

The energy supplied to a thermodynamic system in the form of heat, minus the work done by the system, is equal to the change in internal energy

• Conservation of energy

• Components include, internal energy, heat, and work

• Stated in terms of changes in internal energy

• Application: heat engines

Cont.

2nd Law of Thermodynamics

Heat flows from objects with higher temperature to objects with cooler temperatures

• Must use energy

• Can be seen as entropy

• Entropy = thermodynamic measure of disorder
  clean office → messy office