EA Notes (Scen 101), Tillery
Chapter 4
Heat and Temperature
Introduction
• Heat is an important part of our study of energy.
• Using and controlling it has built our civilization.
• All energy forms can be converted into heat.
• Before studying the concept of HEAT, you must understand the
basic structure of matter and its underlying theory.
The Kinetic Molecular Theory
(Model)
• The name of this basic theory of matter.
• The word "KINETIC" means the molecules are ALWAYS moving
EVEN IN SOLID MATERIALS.
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Atoms
• The basic building blocks of matter.
• About 100 different CHEMICAL types called ELEMENTS.
( See Periodic Table, Just before back cover of Text Book)
• Tiny, indivisible particles.
• Atoms in matter are separated by very large spaces.
Molecules
• One or more different atoms combined in specific ratios.
• He,
• O2 ,
• H 2O .
• Molecule is the smallest entity that retains the PHYSICAL and CHEMICAL
properties of a substance.
• Molecules are starting point for describing structure of matter.
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Molecules Interact
• Cohesion: the attractive force between LIKE molecules.
• Solids and liquids have stronger cohesion than gases.
• Adhesion: the attractive force between UNLIKE molecules.
• Glues and liquids that wet solids have strong adhesion.
Phases of Matter
• There are 4 phases that matter can be in:
• Solid:
• Keeps its own volume and shape.
• Molecules are very cohesive.
• Fig.4.3,p.93: is a MODEL to help describe what we can't see.
The Equations describing Balls connected by Springs give correct results.
• All except helium are solid at lowest possible temperature.
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• Liquid:
• Keeps its volume, but takes shape of bottom of container.
• Molecules SLIGHTLY less cohesive.
• As temperature is increased most solids become liquids.
• Gas:
• Expands in volume and fills entire container.
• Molecules not cohesive.
• As temperature is increased all materials become gases.
• Fluid: (NOT one of the 4 phases).
• A name that includes both liquids and gases,
both of which flow freely through holes.
• Plasma:
• At VERY HIGH temperatures, molecules break apart into freely moving charged
atoms.
• The STARS are mostly plasma. (99% of the universe.)
• We won't study plasmas any further in this course.
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Molecules Move
• The 3 kinds of motion a molecule can have are:
• Vibration:
• Atoms alternate closer together and further apart.
• Occurs in ALL Phases. The ONLY motion possible in solids.
• Rotation:
• Atoms keep their spacing and rotate around common center.
• Occurs in LIQUIDS and GASSES (but not in SOLIDS).
• Translation:
• Atoms keep their spacing and move together in the same direction.
• Occurs in LIQUIDS (short distance) and GASSES (larger distance).
• Each molecule has its own KE within a wide range of values.
• An average KE for all the molecules can be calculated (by spring model).
• Experiments show average KE increases linearly with temperature.
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Temperature
• Temperature Scales were originally defined with "available" standards.
Today there's a scientific standard alongside the earlier ones:
• The
∴
AVERAGE
Internal KE of the molecules of the object.
Temp. does NOT change with mass (number of molecules).
Thermometers
• Devices in which some physical property changes with temperature.
• Most use length or volume, which increases with temperature.
• liquid in glass,
• bimetallic strip.
( Demo on Board. )
( Demo on Board. )
• Electronic properties are now becoming more important.
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Thermometer Scales
• We'll study 3 temperature scales.
(First two on this slide.)
• Fahrenheit scale:
• Originally defined with 0° as the freezing point of brine and 100° as the human
body temperature. Not easily reproducible.
• Celsius scale:
• Defined with 0°C and 100°C standard points at the freezing and boiling
temperatures of pure water (at standard atmospheric air pressure of 760 Torr).
Easy to purify water.
• Fahrenheit scale (again):
• Then redefined with water standard temperatures at 32°F & 212°F.
(This choice
provides a fractional multiplier, 5/9 or 9/5, to convert between scales.)
• C —> F:
TF = 95 T C + 32° F
• F —> C:
TC =
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5
9
(T F − 32° F )
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• Kelvin scale:
• No known upper limit to temperature, but TWO experiments show there is a
lower limit. ( DRAW CURVES )
• At –273°C :
1. A gas's volume goes LINEARLY to zero;
2. An object's internal KE goes LINEARLY to zero
(molecular motion stops).
• Temperatures lower than this are IMPOSSIBLE!!
• –273°C is made equal to the zero of the SI temperature scale, the Absolute or
Kelvin scale [K] .
• By convention, we do not use the ° with the K scale.
• K and C degrees are the same size, thus:
TK = TC + 273
• C —> K:
• Thus the Water Standard Temperatures are also 273 K and 373 K.
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Heat
• Heat is the energy added to or removed from an object that causes its
temperature to change. (MUCH more accurate definition next slide.)
• There are many ways to add or remove heat from an object.
Here's one:
• If you do work against friction, one surface slides against another.
( The surfaces may be any combination of solids, liquids, or gases. )
• Sliding increases average KE of molecules —> temperature increases.
• For surfaces moving at ordinary speeds, the increase is small.
• For meteorites, the increase is enough to burn them.
• Work against friction always ADDS heat to BOTH surfaces.
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Internal and External Energy of Object
• External: PE+KE of the entire OBJECT'S position and motion.
• Internal: KE+PE Summed up for all of object's MOLECULES.
• Adding Heat increases internal energy,
removing Heat decreases it.
Accurate Definition of Heat as Energy Transfer
• Heat is the Internal energy transferred into or out of an object that causes its
temperature (or "Phase" see slide # 15) to change.
Difference between Temperature & Heat
• Temperature:
• Measures Average Internal KE of the molecules of the object.
• This Average does NOT vary with mass (number of molecules).
• Heat:
• Energy that changes Total Internal KE & PE of object.
• This Energy Total DOES vary with mass (and substance).
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Measures of Heat
• Symbol
Q
Joule [J] (since it is energy)
• First heat theory was that a fluid called calor flowed from hot to cold.
• These older Units are still used: They are Defined using a Water standard.
• 1 calorie [cal] = heat to raise 1 g water 1°C (= 4.184 J)
• 1 kilocalorie [kcal] = heat to raise 1 kg water 1°C (= 4184 J)
{ Dietitian's Calorie = 1 kilocalorie }
• 1 British Thermal Unit [Btu] = 252 cal
Mechanical Equivalent of Heat (1849)
• 1 cal = 4.184 J
(4.184 J cal )
• Rumford's experimental measurement of this value showed heat is a form of
energy that can be converted back and forth from other forms.
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Heat vs. ∆T Formula & Specific Heat
• Find Q needed to change temperature of m kg of Water:
• From the kilocalorie definition:
• Formula for water:
Q [kcal] = m [kg] ∆T [° C] .
{Board Demo}
• For most other materials, less heat is needed.
• Specific Heat of a substance = heat needed to change the temperature of
one unit mass of a substance by 1°C. Specific Table value for each substance.
• Values are in Table.4.2, p.102.
• Symbol: c ( lower case )
• Units:
( for mass in kg ): kcal/kg·°C
( for mass in g ): cal/g·°C
• The numerical value is the same for each set of units.
• Heat vs. m and ∆T:
• Do
Q = m c ∆T
(Use with ALL substances.)
Ex. B-4, p.121 & Ex. B-9, p.121.
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Heat Flow
• Rule:
• IF there is a temperature difference between two points,
• AND there is a way to transfer energy,
• Heat will be transferred from higher to lower temperature.
3 METHODS OF TRANSFER:
• Conduction:
• Uses collisions between molecules (or electron flow in metals)
• Hotter molecules vibrate faster and transfer KE to neighbors.
• Rate of Conduction: Table 4.3 is on p.104.
• Metals are the best heat conductors,
• other solids & liquids are in the middle,
• gases and foams the poorest conductors. ( Called insulators ).
• Vacuum = 0, ( no molecules ).
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• Convection:
• Carried by Flow of a liquid or gas. (THUS NOT BY A SOLID !!)
• Heat enters the convecting fluid by Conduction. This then starts:
• Natural Convection:
• Warmed fluid expands slightly.
• Lower density gas now rises in surrounding cooler, more dense, fluid.
• Risen gas moves along ceiling and eventually returns to floor.
• Fan forced convection speeds distribution of heat (home heating systems).
• Radiation:
• Transfer by Radiated Waves.
• Every object Radiates energy (as Electromagnetic Waves).
4
2
−8
• Radiated Watts m ≈ 10 TK , so you don't notice the heat until the
temperature gets well above 1000 K, like a light bulb filament or the surface of
the Sun.
•
DEMO: ( Describe Vacuum Thermos bottle. )
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Energy, Heat, and Molecular Theory
Phase Change
• Recall: Heat changes Total Internal KE & PE of object.
• Now we'll explain how phase changes occur:
( Draw Curve )
• MELTING AND BOILING:
• Start with a block of frozen water below 0°C and continually add heat.
• Added Q is now increasing Internal KE (KE is proportional to T).
• When T reaches the melting point, it temporarily stops increasing.
• Added Q is now increasing Internal PE – Separating the molecules by
breaking cohesive bonds one by one, until all ice has melted.
• Latent Heat of Fusion (Lf ): The Q needed to melt unit mass of material {
80 cal/g or kcal/kg for water }
• When all ice has melted to water, added Q again increases the Internal KE;
and T again rises proportional to Q.
Continued on Next Page
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• MELTING AND BOILING: ( Continued )
• When T reaches the boiling point, it temporarily stops increasing.
• Added Q is again increasing Internal PE – and separating the molecules by
breaking cohesive bonds one by one, until all water has boiled.
• Latent Heat of Vaporization (Lv): The Q needed to boil unit mass of
material { 540 cal/g or kcal/kg for water }
• When all the water has boiled into steam, its T starts to rise again.
• FREEZING AND CONDENSING:
• The Phase Change process is completely reversible, with heat being liberated
as we go from steam to water or water to ice. That's why steam is so dangerous.
• Heat vs. Mass & Latent Heat:
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Q = ± m Lf
+ if melting or boiling
Q = ±m Lv
– if freezing or condensing
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Phase Changes
( Continued )
• Using Phase Changes and their Equations:
• The input heat needed to warm, melt, or boil any substance depends on its
individual Specific Heats and Latent Heats as shown in Fig 4.20.
• Forcing a liquid refrigerant to expand in a heat exchanger absorbs heat from its
surroundings, thus cooling them.
• [ Optional: Do Ex. B-13, p.122. ]
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Phase Changes
( Continued )
• SUBLIMATION:
• Direct change from solid to gas. Only a few materials do this.
Evaporation and Condensation
• Evaporation Definition:
• From liquid to vapor at temperatures BELOW boiling point.
• Since Temp. measures the average KE of molecules, some have enough KE to
break their cohesive bond, and leave the liquid state.
• Evap. Rate increases as liquid Temp. increases.
• Condensation Definition:
• Capture by a surface of some nearby vapor molecules.
• Cond. Rate increases as number of vapor molecules increases.
• Saturation Definition:
• When Evaporation and Condensation Rates are in balance.
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Optional Readings
• No quiz or test questions on this material.
• Don't try them if you find the text unclear.
Thermodynamics
• This major branch of Physics is important in the design of
• Internal combustion engines,
• Heat pumps (for home heating),
• Refrigerators and Air conditioners.
• The Second Law of Thermodynamics explains why less than 100% of energy
converted to heat can be recovered as useful work.
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