Energy in Thermal Processes
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.
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)
If a quantity of energy Q is transferred to a substance of mass m, changing its temperature by ΔT =Tf-Ti, the specific heat c of the substance in defined by: c=Q/(m ΔT ) SI unit Joule per kilogram degree Celsius (J/kg oC) Q = mc ΔT - when temperature increase, Q and ΔT are positive, energy flowing into system - when temperature decrees, Q and ΔT are negative, energy flows out of the system
Calorimeters- vessel that is assumed to be a good isolator, so that energy doesn’t 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
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
The rate of energy transfer: P =Q/ Δt ~ A ΔT/ Δx ΔT/ Δx= (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
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)
Stefan’s law: the rate at which an objet radiates energy is proportional to the fourth power of its absolute temperature P =σA eT4 P – power in watts (or J/s) radiate by an object σ - Stefan-Boltzman constant σ =5.6696x10-8W/m2K4 A- surface area; T- temperature e- emissivity of the object (constantbetween 0 and 1)
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 e=1 e=0 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