Energy in Thermal Processes

1
Energy in Thermal Processes
2

• Heat and internal energy
• Internal energy U is the energy associated with
  the microscopic components of a system- the atoms
  and molecules of the system. The internal energy
  includes kinea and potential energy associated
  with the random translational., rotational, and
  vibration motion of the particles that make up
  the system, and any potential energy bonding the
  particles together.

3

• Heat is the transfer of energy between a system
  and its environment due to a temperature
  difference between them.
• Calorie- the energy necessary to raise the
  temperature of 1g of water from 14.5 to 15.5oC
• 1cal 4.186J (mechanical equivalent of heat)

4

• If a quantity of energy Q is transferred to a
  substance of mass m, changing its temperature by
  ?T Tf-Ti, the specific heat c of the substance
  in defined by
• cQ/(m ?T )
• SI unit Joule per kilogram degree Celsius (J/kg
  oC)
• Q mc ?T
• - when temperature increase, Q and ?T are
  positive, energy flowing into system
• - when temperature decrees, Q and ?T are
  negative, energy flows out of the system

5

• Calorimeters- vessel that is assumed to be a good
  isolator, so that energy doesnt leave the system
• Calorimetry -the analysis using calorimeters
• Qcold -Qhot
• The energy needed to change the phase of a given
  pure substance is
• Q m L
• L-latent heat of the substance, depends on the
  nature of the phase change as well as on the
  substance

6

• Latent heat of fusion Lf- when a phase change
  occurs during melting or freezing
• Latent heat of vaporization Lv when a phase
  change occurs during boiling and condensing (see
  table 11.2/360)
• Energy Transfer
• Thermal conduction (or conduction)-the energy
  transfer process associated with a temperature
  difference
• The temperature difference drives the flow of
  energy, from the region with higher temperature
  to a region with lower temperature

7

• The rate of energy transfer
• P Q/ ?t A ?T/ ?x
• ?T/ ?x (Tf-Ti )/L
• P k A (Tf-Ti )/L
• k- termal conductivity (see table 11.3/367)
• Convection-the transfer of energy by the movement
  of a substance
• When the movement results from differences in
  density (air around the fire) is natural
  convection

8

• When the substance is forced to move by a fan or
  a pump, as in some hat air and hot water heating
  systems is forced convection
• Radiation- (no conduction no convection) all
  objects radiate energy continuously in the form
  of electromagnetic waves due to thermal
  vibrations of their molecules (ex thermometer in
  the doctors office)

9

• Stefans law the rate at which an objet radiates
  energy is proportional to the fourth power of its
  absolute temperature
• P sA eT4
• P power in watts (or J/s) radiate by an object
• s - Stefan-Boltzman constant
• s 5.6696x10-8W/m2K4
• A- surface area T- temperature
• e- emissivity of the object (constantbetween 0
  and 1)

10

• An ideal absorber -is an object that absorbs all
  the light radiation incident on it, including
  infrared and ultraviolet light (black body)
• A black body have emissivity e1
• e0 absorbs none of the energy incident on it ,
  reflecting it all, is an ideal reflector
• The amount of energy radiated by an object can be
  measure via thermography