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Heat, Temperature, Heat Transfer, Thermal Expansion

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Title: Heat, Temperature, Heat Transfer, Thermal Expansion


1
Heat, Temperature, Heat Transfer, Thermal
Expansion Thermodynamics
2
Heat vs. Temperature
  • Heat
  • A form of energy
  • Measured in calories or Joules
  • There is no coldness energy
  • Any object with temperature above zero Kelvin has
    heat energy
  • Temperature
  • Avg. Kinetic Energy of the particles
  • Measured in ?C, ?F, K, R
  • hot cold are relative terms
  • Absolute zero is zero Kelvin

3
Heat Transfer (3 methods)
1. Conduction - requires direct contact or
particle to particle transfer of energy
usually occurs in solids 2. Convection - heat
moves in currents only happens in fluid states
of matter 3. Radiation - heat waves travel
through empty space, no matter needed IR
4
Thermal Equilibrium
  • A system is in thermal equilibrium when all of
    its parts are at the same temperature.
  • Heat transfers only from high to low temperatures
    and only until thermal equilibrium is reached.

5
Temperature Scales
  • There are four temperature scales Celsius
    (Centigrade), Kelvin, Fahrenheit, Rankine
  • Celsius, ?C metric temp. scale
  • Kelvin, K metric absolute zero temp. scale
  • Fahrenheit, ?F customary (english) temp. scale
  • Rankine, R english absolute zero temp scale

6
Comparing Temperature Scales
All temperatures listed are for water
  • Celsius - Freezing 0C, Boiling 100C
  • Kelvin - Freezing 273K, Boiling 373K
  • Fahrenheit- Freezing 32F, Boiling 212F

Conversions between Scales
F 1.8 C32 K C 273
7
Change of State
steam
vaporization
Heat of fusion
100
condensation
water
Temp C
melting
Heat of vaporization
0
ice
freezing
-20
Increasing Heat Energy (Joules)
As heat is added to a substance it will either be
absorbed to raise the temperature OR to change
the state of matter. It can NEVER do both at the
same time. Temperature will NOT change during a
phase change!
8
Specific Heat
The amount of heat energy needed to raise the
temperature of 1 gram of substance by
1C. Substances with lower specific heats change
temperature faster.
Symbol c units cal/gC or J/kgC
For water c 1 cal/gC 4.18 J/gC
4180 J/kgC
9
Latent Heat
The amount of heat energy required to change the
state of 1 gram of substance.
Heat of fusion - latent heat for changes between
the solid and liquid phases. Lf 80 cal/g for
water
Heat of vaporization - latent heat for
changesbetween the solid and liquid phases. Lv
540 cal/g for water
10
Heat Calculations
Temperature Change
Phase Change
Q mL
Q mc?T
Q heat absorbed or released m mass of
substance changing phase L latent heat of
substance Lf heat of fusion (liquid
solid) Lv heat of vaporization
(liquid gas)
Q heat absorbed or released m mass of
substance being heated c specific heat of
substance ?T change in temperature
11
Thermodynamics
  • The study of changes in thermal properties of
    matter
  • Follows Law of Conservation of Energy
  • 1st Law the total increase in the thermal
    energy of a system is the sum of the work done on
    it and the heat added to it
  • 2nd Law natural processes tend to increase the
    total entropy (disorder) of the universe.

12
1st Law of Thermodynamics
The total increase in the thermal energy of a
system is the sum of the work done on it and the
heat added to it.
?U W Q
?U change in the thermal energy of the system W
work done on the system (W Fd or W?K) Q
heat added to the system (Q is if absorbed, Q
is if released) All measured in Joules
13
Heat engines
  • Convert thermal energy to mechanical energy
  • Require high temp heat source and low temp heat
    sink. (Takes advantage of heat transfer process)
  • Examples Steam engine, Automobile engine

14
Refrigerators and Heat Pumps
  • It is possible to remove heat from a cold
    environment and deposit it into a warmer
    environment.
  • This requires an outside source of energy.
  • Examples Refrigerators, Air conditioning
    units
  • Heat pumps are refrigeration units that work in
    either direction.

15
2nd Law of Thermodynamics
All natural processes go in a direction that
increases the total entropy of the universe.
Entropy is a measure of the disorder of a
system. If heat is added, entropy is
increased. If heat is removed, entropy is
decreased. Work with no ?T, entropy is unchanged
16
Thermal Expansion
  • Substances expand as they heat and contract as
    they cool.
  • The rate of expansion depends on the substances
    coefficient of expansion (a)
  • The exception to this rule is water. As water is
    cooled from 4C to 0C, it expands which explains
    why ice floats (it is less dense than water.

17
Thermal Expansion (Linear) Calculations
?L
Linear expansion objects expand along linear
dimensions such as length, width, height,
diameter, etc.
?L Loa ?T
?L change in length measurement (same units as
original length) Lo original length (may be
in any units) ?T change in temperature
(C) a linear coefficient of expansion (/ C)
18
Thermal Expansion (Volume) Calculations
Volume expansion since objects expand in all
dimensions, volume also expands.
?V Voß ?T
?V change in volume (same units as original
volume) Vo original volume (may be in any
units) ?T change in temperature (C) ß
linear coefficient of expansion (/ C)
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