Document Sample

```					Thermal Physics

Chapter 11
States of Matter
 The state of matter of an object depends on
   Molecular separation
   Large in gases
   Close to the same for liquids and solids
   Intermolecular forces
   Increase from gas to liquid to solid
   Speed of Molecules
   Generally decrease from gas to liquid to solid
   Exceptions can occur if pressure changes
Temperature (Pass 1)
 General use definition:
   Temperature is a number assigned to an
object to indicate its degree of warmth
   more useful than comparative warmth
   Measured with thermometer (more later)
 Initial definition:
   Objects are in thermal equilibrium if they are
at the same temperature
Thermometers
 Thermometer: an physical system whose
properties change predictably with
temperature and is calibrated and marked off
to assign numbers with those changes
   Changing properties can include:
   Volume of gas or liquid
   Length of a solid
   Electrical resistance of a conductor
   Light transmission of a crystal
Common Temperature Scales
 Fahrenheit:
   BP H2O = 212°F
   FP H2O = 32°F
 Celsius:
   BP H2O = 100°C
   FP H2O = 0°C
 Kelvin:
   BP H2O = 373.15 K
   FP H2O = 273.15 K
Temperature Scale Conversions
 Fahrenheit to Celsius:

TC  9 TF  32 
5

 Celsius to Kelvin:

TK  TC  273 .15
Thermal Expansion of Solids
 Most solids expand when heated and contract when
cooled
   Predictable in linear, area, and volume
 Mathematical analysis:
 Linear expansion

L  L0T
where α is the linear thermal expansion coefficient
 determined for each material experimentally

 units of °C
-1
Thermal Expansion of Solids (cont.)
   Volumetric expansion
V  V0 T
where β is volume thermal expansion coefficient and
  3
   same unit as α
   note equations have same form
Mechanical Equivalent of Heat
 Heat: energy transfer when a temp.
difference exists between two objects
   Heat is measured in metric by the calorie
   Amount of heat required to raise the temp. of 1 g
of water by 1°C.
   We’ll want to use SI joule to be consistant
   1 calorie = 4.187 joules
 Since heat is a form of energy, we can
convert between heat and the other forms of
energy we’ve used
Calorimetry and Change of Phase
 Heat capacity: amount of heat needed to change
an object’s temp. by 1°C
 Heat required to change an object’s temp. by ΔT°C
Q  mcT
where
   Q is heat
   c is specific heat
Calorimetry and Change of Phase
(cont.)
 Heat of transformation: heat required or released
as an object transitions between one phase and
another
   Heat of fusion: heat for solid-liquid phase change
   Heat of vaporization: heat for liquid-gas phase
change
 Heat absorbed or released during phase change
Q  mL
where L is heat of transformation
Heat Transfer
 Three methods
 Conduction:
 transfer w/o net motion of the material

 transfer through direct contact

 relatively slow

 Convection:
 transfer through mass motion or flow of fluid

 transfer through direct contact

 more rapid than conduction

 transfer without contact or mass motion

 more rapid than convection
Rate of Heat Flow
 Conduction:
 Heat flows through a uniform material according to
the following:
Q
 KA
T2  T1 
t          L
where ΔQ/Δt is time rate of heat flow, A is the area and L
is the thickness of material, and K is the thermal
conductivity (characteristic of material) in units of
W/m·°C
Rate of Heat Flow (cont.)
   Effectiveness of insulation (resistance to heat flow)
can be expressed as an R value
L
R
K
   Used to rate home insulation, windows, and coats
 Rate of radiation is expressed as power

P  eAT     4
where σ = 5.67 × 10-8 W·m-2·K-4, e is the emissivity
constant (dimensionless, between 0 and 1), and A
is surface area of object

```
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
 views: 37 posted: 4/29/2012 language: English pages: 14