THERMAL ENERGY

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Chapter 5
TEMPERATURE AND HEAT Dr. Babar Ali
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CHAPTER OUTLINE
Temperature Heat Kinetic Theory of Matter Specific Heat Calculating Heat Phase Changes Latent Heat Heat Transfer Thermal Expansion Thermal Properties of Water
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TEMPERATURE

• Temperature is a measure of how hot or cold a
  substance is.
• Thermometer is an instrument used for measuring
  temperature, and is based on thermometric
  properties of matter.
• Three scales are used for measuring temperature.

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TEMPERATURE SCALES
Fahrenheit
32 - 212
Celsius
0 - 100
Kelvin
273 - 373
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Example 1

• The melting point of silver is 961ºC. What is
  this temperature in Kelvin?

TK TC 273
TK 961 273
1234 K
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Example 2

• Pure iron melts at 1800 K. What is this
  temperature in Celsius?

TC TK - 273
TC 1800 - 273
1527 ?C
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Example 3

• On a winter day, the temperature is 5 ºF. What
  is this temperature on the Celsius scale?

TC (TF 40) 1.8- 40
See also Eq. 5.2 (a, b)
TC (5 40) 1.8- 40
-15
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KINETIC THEORY OF MATTER

• All matter is composed of tiny particles
  (molecules) that are in constant motion.

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KINETIC THEORY OF MATTER

• Solid particles are closely packed with little
  motion. Solids have definite volume and definite
  shape.
• Liquid particles are loosely packed and have some
  motion. Liquids have definite volume, but
  indefinite shape.
• Gas particles are far apart from one another and
  have great motion. Gases have indefinite shape
  and volume.

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KINETIC THEORYOF MATTER

• Increase in T increases the motion of molecules
  in any state.
• For example, evaporation of liquids results from
  loss of fast moving molecules _at_ surface.

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TEMPERATURE HEAT

• Heat is the Ethermal that is transferred from one
  body to another because of T difference between
  the bodies.
• Heat flow occurs from high T to low T.
• T (temperature) is the average Ekinetic of
  molecules in a substance.
• See, Kinetic Molecular Theory - Heat

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CONCEPTCHECK 1

• Consider a cup of hot coffee and a frozen lake.

Which has higher T?
Which has more heat?
Hot coffee has a higher temperature than a frozen
lake
Frozen lake has more heat because it contains
many more particles
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CONCEPTCHECK 2

• On a cold day when someone leaves a door open, it
  is often said that the person is letting in the
  cold air. Is this correct?

No, heat travels from higher to lower T. ? heat
(H) is going out rather than cold in!
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MEASURING HEAT

• Heat (H) is a physical quantity that can be
  measured. The science of heat measurement is
  known as calorimetry.
• Quantity of H is ? to
• 1) the amount of substance
• 2) its T, and
• 3) the nature of the substance.
• The SI unit of H is joules (J). Another
  convenient unit for H is calorie.

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Concept Check

• It takes 5 as much heat to boil 5 kg of water
  than 1 kg.

Mass and heat are directly proportional
Which container requires more heat to reach the
same temperature?
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Concept Check

• Although the same amount of heat is added to both
  containers, the container with less water gets
  hotter.

Mass and temp. are indirectly proportional
Using the same amount of H, which container
reaches a higher T?
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SPECIFIC HEAT

• Different materials absorb or lose H differently.
• i.e., the filling of a hot apple pie may be too
  hot to eat, whereas the crust is not.

• The Specific heat of a substance is the amount of
  H required to ?T of 1 gram of that substance by 1
  ?C.
• ? Units are J/g?C (or cal/g?C)

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CALCULATINGHEAT

• Heat changes are measured using a calorimeter.
• Conservation of E the amount of heat lost by one
  body is gained by another.

• H gained or lost is based on three quantities

Mass of substance
Specific heat of substance
Change in its temperature



Heat
H m c
?T
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Example 1

• How much H must be supplied to 20 g of tin to
  raise its T from 25?C to 100?C? Specific heat
  (c) of tin is 0.055 cal/g ºC.

m 20 g ?T 75 ºC c 0.055 cal/g?C H ???
H m c ?T
H (20 g)(0.055 cal/gºC)(75 ºC)
H 82.5 cal
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Example 2

• Calculate the specific heat of a solid if 1638 J
  of heat raises the temperature of 125 g of the
  solid from 25.0 to 52.6 ?C.

m 125 g ?T 27.6 ºC c ??? H 1638 J
c 0.475 J/g ºC
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PHASE CHANGES

• When matter releases or absorbs E without a
  change in T, phase change occurs

Gas
Condensation
Evaporation
Liquid
Sublimation
Deposition
Freezing
Melting
Solid
Arrows up E in Arrows down E out
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T vs. HFor Water

• When heat is added to ice, it absorbs the H
  without a ?T ? causing a phase change

• Similarly, when heat is added to hot water, a
  phase change occurs without an ? in T

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LATENT HEATOF FUSION

• The quantity of H required to melt 1 g of solid
  is called the latent heat of fusion (Lf).

Lf for ice is 80 cal/g
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LATENT HEATOF VAPORIZATION

• The quantity of H required to evaporate 1 g of
  liquid is called the latent heat of vaporization
  (Lv).

Lv for water is 540 cal/g
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HEAT DURING A PHASE CHANGE

• The amount of H, released or absorbed during
  phase change, depends on the amount of substance
  and its latent heat.

H m Lf
H m Lv
and
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Example 1

• How much heat is required to melt 50 g of ice at
  0?C?

H m Lf
m 50g Lf 80 cal/g H ???
H 50 g 80 cal/g
H 4000 cal
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Example 2

• How much heat is required to vaporize 50 g of
  water at 100?C?

H m Lv
m 50g Lv 540 cal/g H ???
H 50 g 540 cal/g
H 27000 cal
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Example 3

• Calculate the amount of H required to change 20 g
  of ice at 0?C to water at 10?C.

Solid-Liquid
Liquid
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Example 3
m Lf
20 g 80 cal/g
1600 cal
m c ?T
20 g 1.0 cal/gºC 10 ºC
200 cal
Htotal
1600 cal
200 cal
1800 cal
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HEAT TRANSFER

• When there is a T difference between two objects,
  heat flows from warmer ? cooler
• Heat (Energy) Transfer occurs by one of 3
  methods

1. Conduction
2. Convection
3. Radiation
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1. Conduction

• Conduction is transfer of heat by contact between
  two objects through molecular collisions
• Metals are good conductors of heat, while glass
  and wood are poor conductors (insulators).

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2. Convection

• Convection is transfer of heat by actual motion
  of molecules
• Liquids and gases transfer heat mainly by
  convection.

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3. Radiation

• Radiation is transfer of heat without molecules
• Heat from the sun reaches the earth through space
  by radiation.

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THERMAL EXPANSION

• All matter expands when heated, due to ?
  molecular motion which causes them to separate
  from each other.
• the more molecular motion, the more expansion
  (volume)

Gases
gt Liquids
gt Solids
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THERMAL EXPANSIONOF SOLIDS

• Solids expand upon ? in T, based on three factors

1) increase in temperature
2) the original length
3) type of material
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THERMAL EXPANSIONOF SOLIDS

• A practical use of thermal expansion of metals is
  use of bimetallic strip in operation of a
  thermostat.

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THERMAL EXPANSIONOF WATER

• When water at 0?C is heated, it first contracts
  and then expands
• Water has its smallest volume at 4?C.

• ? water has its highest density _at_ 4ºC.

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More on WATER

• Thermal behavior of water is important in
  preserving aquatic life in colder climates.

• As water cools on surface, it sinks due to its
  high density, allowing for water circulation

• _at_ T lt 4?C, it becomes less dense and rises to the
  surface
• Freezing occurs at top, allowing for marine life
  to exist below the surface.

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THERMAL CIRCULATIONS

• The difference in the specific heat of water and
  land cause the land to warm up more quickly
  during the day.

• The less dense warm air rises, and is replaced by
  cool air from the sea, causing a sea breeze.

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THERMAL CIRCULATIONS

• During the night, the land cools faster than the
  sea, reversing the process, and causing a land
  breeze.

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• THE END

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Some Symbols Used

• ? all
• ? there is or there exists
• D Change
• ? infinity
• ? multiplication
• ? proportional to
• ? less than or equal to
• ? greater than or equal to
• ? plus or minus
• ? degrees
• ? therefore
• approximately equal to
• ? decreases
• ? increases

• E Energy
• H Heat
• e- - electron
• p - proton