Thermal Physics Topic 3.1 Thermal Concepts. Temperature – Macroscopic At a macroscopic level,...
If you can't read please download the document
Thermal Physics Topic 3.1 Thermal Concepts. Temperature – Macroscopic At a macroscopic level, temperature is the degree of hotness or coldness of a body
Temperature Macroscopic At a macroscopic level, temperature is
the degree of hotness or coldness of a body as measured by a
thermometerAt a macroscopic level, temperature is the degree of
hotness or coldness of a body as measured by a thermometer
Temperature is a property that determines the direction of thermal
energy transfer (heat) between two bodies in contactTemperature is
a property that determines the direction of thermal energy transfer
(heat) between two bodies in contact Temperature is measured in
degrees Celsius ( o C) or Kelvin (K)Temperature is measured in
degrees Celsius ( o C) or Kelvin (K) Where Temp in K = Temp in o C
+ 273 Temp in K is known as the absolute temperature
Slide 3
For example, a massive glacier will have more total thermal
energy than a small hot nail (simply because it has more
molecules); however, its temperature is lower because it has less
average thermal energy. Therefore, energy will be transferred from
the nail to the glacier... DIFFERENCE BETWEEN HEAT AND THERMAL
ENERGY Thermal energy is a term often confused with that of heat.
Heat will not flow between two bodies of the same temperature
Simply put, heat is the flow of thermal energy. Thermal energy is
the total internal energy of the system Thermal energy is the total
internal energy of the system. This has to do with the kinetic and
potential energies of the molecules, i.e. how fast the molecules
are vibrating and their chemical bonds. Heat goes from objects with
high temperature to low temperature, not high thermal energy to low
thermal energy. Heat transfer = energy transfer
Slide 4
Thermal Equilibrium When 2 bodies are placed in contact heat
will flow from the body at higher temp to the body with lower
tempWhen 2 bodies are placed in contact heat will flow from the
body at higher temp to the body with lower temp Until the two
objects reach the same temperatureUntil the two objects reach the
same temperature They will then be in Thermal EquilibriumThey will
then be in Thermal Equilibrium This is how a thermometer worksThis
is how a thermometer works Hot black coffeeCold milk Light brown
coffee
Slide 5
Thermometers A temperature scale is constructed by taking two
fixed, reproducible temperaturesA temperature scale is constructed
by taking two fixed, reproducible temperatures The upper fixed
point is the boiling point of pure water at atmospheric pressureThe
upper fixed point is the boiling point of pure water at atmospheric
pressure The lower fixed point is the melting point of pure ice at
atmospheric pressureThe lower fixed point is the melting point of
pure ice at atmospheric pressure These were then given the values
of 100 o C and 0 o C respectively, and the scale between them was
divided by 100 to give individual degreesThese were then given the
values of 100 o C and 0 o C respectively, and the scale between
them was divided by 100 to give individual degrees
Slide 6
Temperature - Microscopic At a microscopic level, temperature
is regarded as a measure of the average kinetic energy per molecule
(associated with its movement average kinetic energy per molecule
(associated with its movement in the substance) in the substance)
Internal Energy Potential energy is associated with intermolecular
forces. (arises from the bonds between molecules) Kinetic energy
includes both translational and rotational motion. Is the total
potential and kinetic energy of the molecules in a substance. Heat
Heat Heat represents energy transfer due to a temperature
differenceHeat represents energy transfer due to a temperature
difference Occurs from higher to lower temperature regionsOccurs
from higher to lower temperature regions
Slide 7
Methods of Heat Transfer Heat can be transferred from one body
to another byHeat can be transferred from one body to another by
Conduction ( Conduction ( Heat transfer through material)
Convection ( Convection (Heat transfer by movement of hot material)
Radiation ( Radiation (Heat transfer by light)
Slide 8
Conduction of heat Conduction in solidsConduction in solids
Heat energy causes atoms to vibrate, a vibrating atom passes this
vibration to the next Conduction in metalConduction in metal Heat
energy causes electrons to gain energy, electrons travel through
metal (conduction) and carry heat energy with them Metals are good
conductors of both heat and electricity The atoms at the bottom are
at a higher temperature and will oscillating more strongly than
those at the top.
Slide 9
Convection of heat Hot air rises (and takes its heat with it!)
Radiators Hot air rises (and takes its heat with it!) Cumulus
clouds
Slide 10
Alternating Land and Sea Breezes Convection of heat Hurricanes
Plate tectonics
Slide 11
Radiation of heat involves the generation and absorption of
photons. Unlike conduction or convection, however, radiation
requires no intervening medium to transport the heat. All objects
radiate energy continuously in the form of electromagnetic waves
The hotter an object the more power it radiate sand the shorter the
wavelength of the peak emission wavelength
Slide 12
Are heat emitter also good absorbers? Black and dull on the
surfaceBlack and dull on the surface Best emitter/absorber Charcoal
Blackbody radiators perfect absorber & emitter White and
polished/shinyWhite and polished/shiny Good Reflectors Stay cool in
the summer
Slide 13
The Thermos Bottle Discuss the operation of a thermos making
reference to methods of heat exchange.
Slide 14
The colour of heat Peak wavelength of light emitted depends on
temperature Spectrum includes all wavelength longer than the peak
but not many above 20C - peak in infrared (need thermal imaging
camera to see body heat) 800C - peak in red (electric coil, fire
glows reds) 3000 - peak in blue (but includes green and red light
hence appears white) 2.7K peak in micro-wave (background emission
in the universe left over from the Big Bang)
Slide 15
Topic 3.2 Thermal Properties of Matter Thermal Physics
Slide 16
Heat Capacity/Thermal Capacity, C A body with a high heat
capacity will take in thermal energy at a slower rate than a
substance with a low heat capacity because it needs more time to
absorb a greater quantity of thermal energyA body with a high heat
capacity will take in thermal energy at a slower rate than a
substance with a low heat capacity because it needs more time to
absorb a greater quantity of thermal energy They also cool more
slowly because they give out thermal energy at a slower rateThey
also cool more slowly because they give out thermal energy at a
slower rate
Slide 17
Specific Heat Capacity, c
Slide 18
Unit masses of different substances containUnit masses of
different substances contain different numbers of molecules of
different types of different masses If the same amount of internal
energy is added to each unit massIf the same amount of internal
energy is added to each unit mass it is distributed amongst the
molecules The average energy change of each molecule will be
different for each substanceThe average energy change of each
molecule will be different for each substance Therefore the
temperature changes will be differentTherefore the temperature
changes will be different So the specific heat capacities will be
differentSo the specific heat capacities will be different
Slide 19
Heat Capacity Whereas specific heat capacity is per kg of a
material, heat capacity (or thermal capacity) is the energy
required to raise a certain objects temperature by one Kelvin,
irrespective of its mass. C = mc m = mass (kg) C = heat capacity
(JK -1 ) c = specific heat capacity(Jkg -1 K -1 ) T = Change in
temp (K) Q = Amount of heat energy (J) Amount of energy needed to
raise temperature of an object by T K is Q = CT Amount of energy
needed to raise temperature of 1 kg of a substance by T K is Q =
cmT Heat capacity = mass x specific heat capacity The same amount
is released if the temperature decreases
Slide 20
1st law of thermodynamics: heat and work are both forms of
energy heat and work are both forms of energy Sir James Joule
Stirring water made it warm Change in temperature proportional to
work done Showing equivalence of heat and mechanical energy Also
that electrical current flow through a resistor causes heating
Zeroth law of thermodynamics If two thermodynamic systems are each
in thermal equilibrium with a third, then they are in thermal
equilibrium with each other. First law of thermodynamics Energy can
neither be created nor destroyed. It can only change forms. In any
process, the total energy of the universe remains the same. For a
thermodynamic cycle the net heat supplied to the system equals the
net work done by the system. Applying all these to problems:
Slide 21
1. A 2.00-kg metal object requires 5.02 x 10 3 J of heat to
raise its temperature from 20.0 C to 40.0 C. What is the specific
heat capacity of the metal? 126 J/(kg C) 2. A 0.20-kg lead shot is
heated to 90.0 C and dropped into an ideal calorimeter containing
0.50 kg of water initially at 20.0 C. What is the final equilibrium
temperature of the lead shot? The specific heat capacity of lead is
128 J/(kg C); and the specific heat of water is 4186 J/(kg C). 20.8
C PROBLEMS: 3. Given that the specific heat capacity of water is 11
times that of copper, calculate the mass of copper at a temperature
of 100 C required to raise the temperature of 200 g of water from
20.0 C to 24.0 C, assuming no energy is lost to the
surroundings.
Slide 22
4. Three litres of water at 100 C are added to 15 litres of
water at 40 C. Calculate the temperature of the mixture. Take the
mass of 1 litre of water to be 1 kg and the specific heat capacity
of water to be 4.2 10 3 J kg -1 K -1 5. A heater of 800W is use to
heat a 600 g cast iron cooker plate. How long will it take to raise
the temperature of the plate by 200 o C? Specific heat capacity of
iron = 500 J/(kg K) 75 s = 1 minute 15 s Pt = m c T or V I t = m c
T
Slide 23
6. A hole is drilled in an 800g iron block and an electric
heater is placed inside. The heater provides thermal energy at a
constant rate of 600 W. a) Assuming no thermal energy is lost to
the surrounding environment, calculate how long it will take the
iron block to increase its temperature by 15 0 C. b) The
temperature of the iron block is recorded as it varies with time
and is shown at right. Comment on reasons for the shape of the
graph. begins at room temp increases linearly as Q = cmT as gets
hotter, more energy lost to environment levels out when heat gained
by heater = heat lost to room 9.0 s c) Calculate the initial rate
of increase in temperature. 1.7 0 C/s
Slide 24
Example of finding c of a liquid using electrical current Using
a calorimeter of known heat capacityUsing a calorimeter of known
heat capacity ThermometerCalorimeter Heating coil LiquidInsulation
Stirrer To voltmeter and ammeter Electrical energy = V I
tElectrical energy = V I t Energy gained by liquid = m l c l T
lEnergy gained by liquid = m l c l T l Energy gained by calorimeter
= m c c c T cEnergy gained by calorimeter = m c c c T c Using
conservation of energyUsing conservation of energy Electrical
energy input =Electrical energy input = thermal energy gained by
liquid thermal energy gained by liquid + thermal energy gained by
calorimeter + thermal energy gained by calorimeter V I t = m l c l
T l + m c c c T cV I t = m l c l T l + m c c c T c The only unknown
is the specific heat capacity of the liquidThe only unknown is the
specific heat capacity of the liquid
Slide 25
Example of finding c of a solid using electrical current Using
a specially prepared block of the materialUsing a specially
prepared block of the material The block is cylindrical and has 2
holes drilled in itThe block is cylindrical and has 2 holes drilled
in it one for the thermometer and one for the heater Heater hole in
the centre, so the heat spreads evenly through the block
Thermometer hole, way between the heater and the outside of the
block, so that it gets the averge temperature of the block V I t =
m s c s T sV I t = m s c s T s The only unknown is the specific
heat capacity of the solidThe only unknown is the specific heat
capacity of the solid
Slide 26
Method of mixtures Sometimes called the method of
mixturesSometimes called the method of mixtures In the case of
solid, a known mass of solid is heated to a known temperature
(usually by immersing in boiling water for a period of time)In the
case of solid, a known mass of solid is heated to a known
temperature (usually by immersing in boiling water for a period of
time) Then it is transferred to a known mass of liquid in a
calorimeter of known massThen it is transferred to a known mass of
liquid in a calorimeter of known mass The change in temperature is
recorded and from this the specific heat capacity of the solid can
be foundThe change in temperature is recorded and from this the
specific heat capacity of the solid can be found Energy lost by
block = Energy gained by liquid and calorimeterEnergy lost by block
= Energy gained by liquid and calorimeter m b c b T b = m w c w T w
+ m c c c T cm b c b T b = m w c w T w + m c c c T c the SHC of
water and the calorimeter are needed
Slide 27
Phases (States) of Matter Matter is defined as anything that
has mass and occupies spaceMatter is defined as anything that has
mass and occupies space There are 4 states of matterThere are 4
states of matter Solids, Liquids, Gases and PlasmasSolids, Liquids,
Gases and Plasmas Most of the matter on the Earth in the form of
the first 3Most of the matter on the Earth in the form of the first
3 Most of the matter in the Universe is in the plasma stateMost of
the matter in the Universe is in the plasma state Macroscopic
properties Macroscopic properties are all the observable behaviours
of that material such as shape, volume, compressibilityMacroscopic
properties are all the observable behaviours of that material such
as shape, volume, compressibility The many macroscopic or physical
properties of a substance can provide evidence for the nature of
that substanceThe many macroscopic or physical properties of a
substance can provide evidence for the nature of that
substance
Fluids LiquidsLiquids GasesGases are both fluidsare both fluids
Because they FLOWBecause they FLOW
Slide 30
Arrangement of Particles SolidsSolids Closely packed Closely
packed Strongly bonded to neighbours Strongly bonded to neighbours
held rigidly in a fixed position held rigidly in a fixed position
the force of attraction between particles gives them PE the force
of attraction between particles gives them PE LiquidsLiquids Still
closely packed Still closely packed Bonding is still quite strong
Bonding is still quite strong Not held rigidly in a fixed position
and bonds can break and reform Not held rigidly in a fixed position
and bonds can break and reform PE of the particles is higher than a
solid because the distance PE of the particles is higher than a
solid because the distance between the particles is higher between
the particles is higher GasesGases Widely spaced Widely spaced Only
interact significantly on closest approach or collision Only
interact significantly on closest approach or collision Have a much
higher PE than liquids because the particles are furthest apart
Have a much higher PE than liquids because the particles are
furthest apart
Slide 31
Changes of State A substance can undergo changes of state or
phase changes at different temperaturesA substance can undergo
changes of state or phase changes at different temperatures Pure
substances have definite melting and boiling points which are
characteristic of the substancePure substances have definite
melting and boiling points which are characteristic of the
substance When the solid is heated the particles of the solid
vibrate at an increasing rate as the temperature is increased The
vibrational KE of the particles increases At the melting point a
temperature is reached at which the particles vibrate with
sufficient thermal energy to break from their fixed positions and
begin to slip over each other As the solid continues to melt more
and more particles gain sufficient energy to overcome the forces
between the particles and over time all the solid particles are
changed to a liquid The PE of the system increases as the particles
move apart (actually it absolutely decreases towards zero) As the
heating continues the temperature of the liquid rises due to an
increase in the vibrational, rotational and translational energy of
the particles At the boiling point a temperature is reached at
which the particles gain sufficient energy to overcome the
inter-particle forces and escape into the gaseous state. PE
increases. Continued heating at the boiling point provides the
energy for all the particles to change
Slide 32
Heating Curve Solid Liquid Gas Solid - liquid phase change
Liquid - gas phase change Temp / o C Time /min
Slide 33
Changes of State GASSOLIDLIQUIDFreezing/solidification
vaporisation condensation Melting/fusion sublimation Thermal energy
given out Thermal energy added Deposition/
Deposition/desublimation
Slide 34
Mom melted chocolate and poured it into molds. She then put it
in the fridge to cool. She left it overnight, so it stayed in the
fridge even after the chocolate became hard. Which graph best shows
the temperature of the chocolate while in the fridge?
Slide 35
Latent Heat The thermal energy absorbed or released in phase
change is calledThe thermal energy absorbed or released in phase
change is called Latent Heat because it does not produce a change
in temperature Latent Heat because it does not produce a change in
temperature There is no temperature change during a phase change,
thus there is no change in the kinetic energy of the particles in
the material. The energy released/absorbed comes from the potential
energy stored in the bonds between the particles. Thermal energy
absorbed by a body results in decrease of PE of the particles as
they move closer togetherThermal energy absorbed by a body results
in decrease of PE of the particles as they move closer together
Thermal energy released by a body, increases PE of the particles as
they move further apart move further apartThermal energy released
by a body, increases PE of the particles as they move further apart
move further apart
Slide 36
L = Q at const. P unit: J L = Q at const. P unit: J Specific
latent heat Specific latent heat is the thermal energy required to
change the phase of 1 kg of a substance (at constant pressure).
Latent heat Latent heat is the thermal energy absorbed or released
by a body during a change of phase (at constant pressure). Types of
Latent Heat FusionFusion VaporisationVaporisation
SublimationSublimation The latent heat of fusion of a substance is
less than the latent heat of vaporisation or the latent heat of
sublimationThe latent heat of fusion of a substance is less than
the latent heat of vaporisation or the latent heat of
sublimation
Slide 37
Methods of finding Latent Heat Using similar methods as for
specific heat capacityUsing similar methods as for specific heat
capacity The latent heat of fusion of ice can be found by adding
ice to water in a calorimeterThe latent heat of fusion of ice can
be found by adding ice to water in a calorimeter Block of ice
ThermometerCalorimeterWater Block of ice Insulation The change in
temperature is recorded and from this the latent heat of fusion of
the ice can be foundThe change in temperature is recorded and from
this the latent heat of fusion of the ice can be found Energy
gained by block melting = Energy lost by liquid and
calorimeterEnergy gained by block melting = Energy lost by liquid
and calorimeter m b L b = m w c w T w + m c c c T cm b L b = m w c
w T w + m c c c T c the SHC of water and the calorimeter are
neededthe SHC of water and the calorimeter are needed
Slide 38
Latent Heat of Vaporisation The initial mass of the liquid is
recordedThe initial mass of the liquid is recorded The change in
temperature is recorded for heating the liquid to boilingThe change
in temperature is recorded for heating the liquid to boiling The
liquid is kept boilingThe liquid is kept boiling The new mass is
recordedThe new mass is recorded Energy supplied by heater = energy
to raise temperature of liquidEnergy supplied by heater = energy to
raise temperature of liquid + energy use to vaporise some of the
liquid + energy use to vaporise some of the liquid (The calorimeter
also needs to be taken in to account.(The calorimeter also needs to
be taken in to account. V I t = m l c l T l + m e L e + m c c c T
cV I t = m l c l T l + m e L e + m c c c T c
Slide 39
Scattered thoughts Under extreme conditions of heat and
exercise, an individual may sweat more than a liter of liquid per
hour. The interior of roasted meat can never reach temperatures
higher than the boiling point of water until all the water is
cooked out of it, at which point it would resemble shoe leather.
The outside is quickly dried out, however, and can reach the
temperature of the surrounding cooking medium. Cocoa butter is
unique among the fats in that it is very regular in composition;
whereas most other fats are actually mixtures. This gives it a very
definite point; unlike butter, which softens gradually. As it melts
in your mouth, it absorbs latent heat. This makes chocolate bars
taste "cool". Cocoa butter is remarkably uniform in composition and
structure: only three fatty acids in the majority of its
triglycerides, with the same one occupying the middle position.
Pure cocoa butter is quite brittle up to about 34 (93 ), at which
point it melts quite quickly.
Slide 40
Changes in size: An increase in temperature generally causes
bodies to expand, while a reduction in temperature causes bodies to
contract. Engineers must take expansion into account when they are
designing various structures (bridges,). When concrete roads are
laid down, gaps (normally filled with tar) are left between
sections in order to allow for expansion. Why do solids expand with
increasing temperature? The atoms or molecules in a solid vibrate
at all temperatures above the absolute zero. As the temperature
increases, the vibrations increase in amplitude, and this pushes
the atoms further apart. This occurs in all three dimensions.
Slide 41
Why do liquids expand with increasing temperature? As the
temperature increases, the kinetic energy of the molecules of the
liquid increases. The movement of the molecules gradually overcomes
forces of attraction between the molecules, with the result that
they have greater freedom to move, over greater volumes. Thus the
liquid expands. Water behaves in an anomalous manner. It contracts
as the temperature increases from 0 C to 4 C, reaching its highest
density at that temperature. This has enormous significance for
aquatic life in regions with severe winters. The abnormal behaviour
of water: At 4 C, water has its highest density. It actually
contracts as the temperature rises from 0 C to 4C. This means that
in regions with severe winters, lakes cool at the surface, and when
the surface water reaches 4 C, that water, being more dense, sinks.
A temperature gradient is set up, and when the water freezes, it
does so at the surface. Eventually, the ice, which floats on water,
acts as an insulator, protecting the water below it from further
cooling. This results in lakes freezing from the top down, and not
from the bottom up. This means that fishes can survive below the
ice even if the air temperature is far below 0 C for prolonged
periods. The reason for this anomalous behaviour of water is beyond
the scope of the IB Physics curriculum.
Slide 42
Explain why supply of latent heat causes a change in potential
energy Q. Explain why supply of latent heat causes a change in
potential energy but not kinetic energy. but not kinetic energy. A.
Kelvin temperature is proportional to the KE of the molecules. Thus
if the temperature hasnt increased then the KE has also not
increased. Work has been done against forces, changing the position
of the molecules. Thus the PE of the molecules must change.
EXAMPLE: Let: i. Specific latent heat of vaporisation (L v ) - from
liquid to vapour ii. Specific latent heat of fusion (L f ) - from
solid to liquid Then if0.5kg of ice at 0C is heated i. until it has
all melted to water at 0C ii. until it reaches its boiling point at
100C iii. until it has vaporised to steam at 100C Calculate the
heat supplied at each stage and in total. Q = Amount of heat energy
(J) L = Specific latent heat (Jkg -1 ) m = mass (kg) Q = mL f Q =
mL v
Slide 43
A i. Q= mL f = 0.5 x 334 x 10 3 = 1.67 x 10 5 J ii. Q= mcT =
0.5 x 4200 x 100 = 2.10 x 10 5 J iii.Q= mL v = 0.5 x 2258 x 10 3 =
1.13 x 10 6 J Total = 1.51 x 10 6 J ( = 1.51 MJ)
Slide 44
Example: Which one of the following statements is UNTRUE? 1.
Impurities generally decrease the melting point of substances. 2.
Water can boil at room temperature. 3. Ice does not evaporate. 4.
Steam may attain temperatures above 100 C. 5. The melting point of
ice decreases with increasing pressure. 3. is correct. A great many
solids, including ice, can evaporate without having first to turn
into a liquid. This process is called SUBLIMATION.
Slide 45
Example: An iceberg has a mass of 10 000 tons. How much heat
will be required to melt the iceberg (initially at 0 C) and bring
the resulting water to a to a temperature of 6 C? (Take the
specific heat capacity of water as 4.2 kJ.kg -1.C -1 and the
specific latent heat of fusion of ice as 334 kJ.kg -1.) Heat
required = Heat to melt the iceberg + heat to raise the water
temperature Heat required to melt the iceberg = mass of iceberg x
latent heat of fusion of ice = 10000 (tons) x 1000 (kg.ton -1 ) x
334 (kJ.kg -1 ) = 3.34 x 10 9 kJ. Heat required to raise the water
temperature = mass of water x specific heat capacity x temperature
rise = 1.0 x 10 7 (kg) x 4.2 (kJ.kg -1.C -1 ) x 6 (C) = 2.52 x 10 8
kJ Total heat = 3.34 x 10 9 kJ + 2.52 x 10 8 kJ = 3.6 x 10 9 kJ =
3.6 x 10 6 MJ
Slide 46
Example: An immersion heater supplies heat at a rate of 5 J.s
-1 to an insulated vessel containing a certain liquid. The liquid
was brought to its boiling point and kept boiling for 2 minutes,
during which time the liquid lost 4.0 g. What value for the
specific latent heat of vaporization of the liquid can be
calculated from this experiment? We assume that the heat supplied
by the heater is the heat required to convert 4.0 g of the liquid
at its boiling point to the vapour state. P t= m L 5 (J.s -1 ) x
120 (s) = 4.0 (g) x specific latent heat (J.g -1 ). The specific
latent heat = 5 (J.s -1 ) x 120 (s)/4.0 (g) = 150 J.g -1
Slide 47
Example: A certain substance is heated at a constant rate, and
its temperature measured as a function of time. The graph of the
results obtained is shown in the diagram below. Which one of the
following statements is FALSE? 1. No liquid exists in the region
labelled A 2. Region C involves the heating of a liquid only 3. No
liquid exists in the region labelled B 4. No solid exists in the
region labelled D 5. Region E involves the heating of a gas only 4.
Correct. The statement is FALSE. In the flat region labelled B,
latent heat of fusion is absorbed without a change in temperature.
The substance is melting, thus both solid and liquid are
present
Slide 48
Example: Calculate the amount of heat required to completely
convert 50 g of ice at 0 C to steam at 100 C. The specific heat
capacity of water is 4.18 kJ.kg -1.K -1. The specific latent heat
of fusion of ice is 334 kJ.kg -1, and the specific heat of
vaporization of water is 2260 kJ.kg -1. 5. Answer: Heat is taken up
in three stages: 1. The melting of the ice, 2. the heating of the
water, and 3. the vapourization of the water. The heat taken up in
the complete process is the sum of the heat taken up in each stage.
Heat taken up for converting ice at C to water at C mass of water x
latent heat of fusion = 0.050 (kg) x 334 (kJ.kg -1 ) = 16.7 kJ Heat
taken up heating the water from 0 C to the boiling point, 100 C
mass of water x specific heat capacity x temperature change = 0.05
(kg) x 4.18 (kJ.kg -1. K -1 )x 100 ( K) = 20.9 kJ Heat taken up
vapourizing the water mass of water x latent heat of vaporization =
0.05 (kg) x 2260 kJ.kg -1 = 113 kJ The sum of these is 16.7 + 20.9
+ 113 = 150.6 kJ (151 kJ)
Slide 49
Slide 50
Slide 51
Evaporation The process of evaporation is a change from the
liquid state to the gaseous state which occurs at a temperature
below the boiling pointThe process of evaporation is a change from
the liquid state to the gaseous state which occurs at a temperature
below the boiling point Explanation A substance at a particular
temperature has a range of particle energiesA substance at a
particular temperature has a range of particle energies So in a
liquid at any instant, a small fraction of the particles will have
KE considerably greater than the average valueSo in a liquid at any
instant, a small fraction of the particles will have KE
considerably greater than the average value If these particles are
near the surface of the liquid, they will have enough KE to
overcome the attractive forces of the neighbouring particles and
escape from the liquid as a gasIf these particles are near the
surface of the liquid, they will have enough KE to overcome the
attractive forces of the neighbouring particles and escape from the
liquid as a gas This energy is needed as gases have more PE than
liquids.This energy is needed as gases have more PE than
liquids.
Slide 52
Cooling Now that the more energetic particles have escapedNow
that the more energetic particles have escaped The average KE of
the remaining particles in the liquid will be loweredThe average KE
of the remaining particles in the liquid will be lowered Since
temperature is related to the average KE of the particlesSince
temperature is related to the average KE of the particles A lower
KE infers a lower temperatureA lower KE infers a lower temperature
Cool This is why the temperature of the liquid falls as an
evaporative cooling takes placeThis is why the temperature of the
liquid falls as an evaporative cooling takes place A substance that
cools rapidly is said to be a volatile liquidA substance that cools
rapidly is said to be a volatile liquid When overheating occurs in
a human on hot days, the body starts to perspireWhen overheating
occurs in a human on hot days, the body starts to perspire
Evaporation of the perspiration cools the bodyEvaporation of the
perspiration cools the body
Slide 53
Factors Affecting The Rate Evaporation can be increased
byEvaporation can be increased by Increasing temperature (more
particles have a higher KE) Increasing surface area (more particles
closer to the surface) Increasing air flow above the surface (gives
the particles somewhere to go to)
Slide 54
Topic 3.3 Ideal Gases
Slide 55
The Mole The mole is the amount of substance which contains the
same number of elementary entities as there are in 12 grams of
carbon-12The mole is the amount of substance which contains the
same number of elementary entities as there are in 12 grams of
carbon-12 Experiments show that this is 6.02 x 10 23
particlesExperiments show that this is 6.02 x 10 23 particles A
value denoted by N A and called the Avogadro Constant (units: mol
-1 )A value denoted by N A and called the Avogadro Constant (units:
mol -1 ) Molar Mass Molar mass is the mass of one mole of the
substanceMolar mass is the mass of one mole of the substance SI
units are kg mol -1SI units are kg mol -1 Example Molar mass of
Oxygen gas is 32x10 -3 kg mol -1Molar mass of Oxygen gas is 32x10
-3 kg mol -1 How many moles and how many molecules is 20g of
Oxygen?How many moles and how many molecules is 20g of Oxygen? 20 x
10 -3 kg / 32 x10 -3 kg mol -120 x 10 -3 kg / 32 x10 -3 kg mol -1
0.625 mol 0.625 mol 0.625 mol x 6.02 x 10 23 molecules 0.625 mol x
6.02 x 10 23 molecules 3.7625 x 10 23 molecules 3.7625 x 10 23
molecules
Slide 56
Thermal Properties of Gases An ideal gas can be characterized
by three state variablesAn ideal gas can be characterized by three
state variables Pressure 1 Pa (pascal) = 1N/1m 2 Volume m 3
Temperature K Experiments use these macroscopic properties of a gas
to formulate a number of gas laws. That is historical
approach.Experiments use these macroscopic properties of a gas to
formulate a number of gas laws. That is historical approach. There
is another way:There is another way: The relationship between them
may be deduced from kinetic theory and is called the ideal gas
law:The relationship between them may be deduced from kinetic
theory and is called the ideal gas law:
Slide 57
The Ideal Gas Equation PV = nRT = NkTPV = nRT = NkT Where n is
the number of molesWhere n is the number of moles R is the
universal gas constant = 8.31 J mol -1 K -1R is the universal gas
constant = 8.31 J mol -1 K -1 N is number of moleculesN is number
of molecules k is Boltzman constant = 1.38066 x 10 -23 J/Kk is
Boltzman constant = 1.38066 x 10 -23 J/K An Ideal Gas Is a
theoretical gas that obeys the gas lawsIs a theoretical gas that
obeys the gas laws And thus fit the ideal gas equation exactlyAnd
thus fit the ideal gas equation exactly Real Gases Real gases
conform to the gas laws under certain limited conditionsReal gases
conform to the gas laws under certain limited conditions But they
condense to liquids and then solidify if the temperature is
loweredBut they condense to liquids and then solidify if the
temperature is lowered Furthermore, there are relatively small
forces of attraction between particles of a real gasFurthermore,
there are relatively small forces of attraction between particles
of a real gas This is not the case for an ideal gasThis is not the
case for an ideal gas
Slide 58
The Kinetic Theory of Gases "the theory of moving molecules";
Rudolf Clausius, 1857 The ideal gas equation is the result of
experimental observations about the behavior of gases. It describes
how gases behave.The ideal gas equation is the result of
experimental observations about the behavior of gases. It describes
how gases behave. A gas expands when heated at constant pressureA
gas expands when heated at constant pressure The pressure increases
when a gas is compressed at constant temperatureThe pressure
increases when a gas is compressed at constant temperature But, why
do gases behave this way?But, why do gases behave this way? What
happens to gas particles when conditions such as pressure and
temperature change?What happens to gas particles when conditions
such as pressure and temperature change? That can be explained with
a simple theoretical model known as the kinetic molecular
theory.That can be explained with a simple theoretical model known
as the kinetic molecular theory. The kinetic theory relates the
macroscopic behaviour of an ideal gas to the microscopic behaviour
of its molecules or atomsThe kinetic theory relates the macroscopic
behaviour of an ideal gas to the microscopic behaviour of its
molecules or atoms This theory is based on the following
postulates, or assumptions.This theory is based on the following
postulates, or assumptions.
Slide 59
Gases consist of tiny particles called atoms or moleculesGases
consist of tiny particles called atoms or molecules The total
number of particles in a sample is very largeThe total number of
particles in a sample is very large The particles are in constant
random motionThe particles are in constant random motion The range
of the intermolecular forces is small compared to the average
separationThe range of the intermolecular forces is small compared
to the average separation The size of the particles is relatively
small compared with the distance between them, so they are treated
as pointsThe size of the particles is relatively small compared
with the distance between them, so they are treated as points
Collisions of a short duration occur between particles and the
walls of the containerCollisions of a short duration occur between
particles and the walls of the container Collisions are perfectly
elasticCollisions are perfectly elastic No forces act between the
particles except when they collideNo forces act between the
particles except when they collide Between collisions the particles
move in straight linesBetween collisions the particles move in
straight lines And obey Newtons Laws of motionAnd obey Newtons Laws
of motion
Slide 60
Gas consists of large numbers of tiny particles called atoms or
Gas consists of large numbers of tiny particles called atoms or
molecules that behave like hard, spherical objects in a state
molecules that behave like hard, spherical objects in a state of
constant, random motion. of constant, random motion. The size of
the particles is relatively small compared with The size of the
particles is relatively small compared with the distance between
them, so they are treated as points the distance between them, so
they are treated as points Collisions of a short duration occur
between particles and Collisions of a short duration occur between
particles and the walls of the container the walls of the container
No Intermolecular forces act between the particles except when they
collide, No Intermolecular forces act between the particles except
when they collide, so between collisions the particles move in
straight lines so between collisions the particles move in straight
lines Collisions are perfectly elastic (none of the energy of a gas
Collisions are perfectly elastic (none of the energy of a gas
particle is lost in collisions) particle is lost in collisions)
Energy can be transferred between molecules during collisions.
Energy can be transferred between molecules during collisions. They
all obey Newtons Laws of motion They all obey Newtons Laws of
motion
Slide 61
Macroscopic Behaviour The large number of particles ensures
that the number of particles moving in all directions is constant
at any time moving in all directions is constant at any time With
these basic assumptions we can relate the pressure of a gas
(macroscopic behaviour) to the behavior of the molecules themselves
(macroscopic behaviour) to the behavior of the molecules themselves
(microscopic behaviour). (microscopic behaviour).
Slide 62
Pressure Pressure is the result of collisions between molecules
Pressure is the result of collisions between molecules and the wall
of the container and the wall of the container Focus on one
molecule moving toward the wall and examine Focus on one molecule
moving toward the wall and examine what happens when on molecule
strikes this wall. what happens when on molecule strikes this wall.
Elastic collision no loss of kinetic energy, so speed remains the
same, only direction changes. If you can imagine 3-D picture you
can see that only the component of the molecules momentum
perpendicular to the wall changes. Change in momentum implies that
there must be a force Change in momentum implies that there must be
a force exerted by the wall on the particle. exerted by the wall on
the particle. That means that there is a force exerted on the wall
by that molecule. That means that there is a force exerted on the
wall by that molecule.
Slide 63
The average pressure on the wall is the average of all The
average pressure on the wall is the average of all microscopic
forces per unit area: microscopic forces per unit area: It can be
shown that the pressure on the wall can be expressed as: It can be
shown that the pressure on the wall can be expressed as: Now,
finally we have the pressure in a gas expressed in Now, finally we
have the pressure in a gas expressed in terms of molecular
properties. terms of molecular properties. This is a surprisingly
simple result! The macroscopic pressure This is a surprisingly
simple result! The macroscopic pressure of a gas relates directly
to the average kinetic energy per molecule. of a gas relates
directly to the average kinetic energy per molecule. We got key
connection between microscopic behaviour We got key connection
between microscopic behaviour and macroscopic observables. and
macroscopic observables.
Slide 64
If we compare the ideal-gas equation of state: PV = NkT, with
the If we compare the ideal-gas equation of state: PV = NkT, with
the result from kinetic theory: PV = N m (v 2 ) avg we find result
from kinetic theory: PV = N m (v 2 ) avg we find The average
translational kinetic energy of molecules in a gas The average
translational kinetic energy of molecules in a gas is directly
proportional to the absolute temperature. is directly proportional
to the absolute temperature. The higher the temperature, according
to kinetic theory, The higher the temperature, according to kinetic
theory, the faster the molecules are moving on the average. the
faster the molecules are moving on the average. At absolute zero
they have zero kinetic energy. Can not go lower. At absolute zero
they have zero kinetic energy. Can not go lower. This relation is
one of the triumphs of the kinetic energy theory. This relation is
one of the triumphs of the kinetic energy theory.
Slide 65
The absolute temperature is a measure of the The absolute
temperature is a measure of the average kinetic energy of its
molecules average kinetic energy of its molecules If two different
gases are at the same temperature, their molecules If two different
gases are at the same temperature, their molecules have the same
average kinetic energy, but more massive molecules have the same
average kinetic energy, but more massive molecules will have lower
average speed. will have lower average speed. If the temperature of
a gas is doubled, the average kinetic If the temperature of a gas
is doubled, the average kinetic energy of its molecules is doubled
energy of its molecules is doubled Absolute Temperature Although
the molecules of gas have an average kinetic energy (and therefore
Although the molecules of gas have an average kinetic energy (and
therefore an average speed) the individual molecules move at
various speeds an average speed) the individual molecules move at
various speeds Some are moving fast, others relatively slowly Some
are moving fast, others relatively slowly At higher temperatures at
greater fraction of the molecules At higher temperatures at greater
fraction of the molecules are moving at higher speeds are moving at
higher speeds For O 2 molecules at 300 K, the most probable speed
is 390 m/s. For O 2 molecules at 300 K, the most probable speed is
390 m/s. When temperature increases to 1100 K the most probable
speed increases When temperature increases to 1100 K the most
probable speed increases to roughly 750 m/s. Other speed occur as
well, from speeds near zero to to roughly 750 m/s. Other speed
occur as well, from speeds near zero to those that are very large,
but these have much lower probabilities. those that are very large,
but these have much lower probabilities. Molecular Speed
Slide 66
Application of the "Kinetic Molecular Theory" to the Gas Laws
Microscopic justification of the laws
Slide 67
Pressure Law (Gay-Lussacs Law) Effect of a pressure increase at
a constant volume Macroscopically: at constant volume the pressure
of a gas is proportional to its temperature: PV = NkT P = (const) T
example: a closed jar, or aerosol can, thrown into a fire will
explode due to increase in gas pressure inside.
Slide 68
Microscopically: As T increases, KE of molecules increase As T
increases, KE of molecules increase That implies greater change in
momentum when they hit That implies greater change in momentum when
they hit the wall of the container the wall of the container Thus
microscopic force from each molecule on the wall Thus microscopic
force from each molecule on the wall will be greater will be
greater As the molecules are moving faster on average they As the
molecules are moving faster on average they will hit the wall more
often will hit the wall more often The total force will increase,
therefore the pressure The total force will increase, therefore the
pressure will increase will increase
Slide 69
The Charless law Effect of a volume increase at a constant
pressure Macroscopically: at constant pressure, volume of a gas is
proportional to its temperature: is proportional to its
temperature: PV = NkT V = (const) T PV = NkT V = (const) T
Slide 70
An increase in temperature means an increase in the average
kinetic energy of the gas molecules, thus an increase in speed An
increase in temperature means an increase in the average kinetic
energy of the gas molecules, thus an increase in speed
Microscopically: There will be more collisions per unit time,
furthermore, the momentum of each collision increases (molecules
strike the wall harder) There will be more collisions per unit
time, furthermore, the momentum of each collision increases
(molecules strike the wall harder) Therefore, there would be an
increase in pressure Therefore, there would be an increase in
pressure If we allow the volume to change to maintain If we allow
the volume to change to maintain constant pressure, the volume will
increase with increasing temperature
Slide 71
Boyle - Marriotts Law Effect of a pressure decrease at a
constant temperature Macroscopically: at constant temperature the
pressure of a gas is inversely proportional to its volume: PV = NkT
P = (const)/V
Slide 72
Microscopically: Constant T means that the average KE of the
gas molecules remains constant Constant T means that the average KE
of the gas molecules remains constant This means that the average
speed of the molecules, v, remains unchanged This means that the
average speed of the molecules, v, remains unchanged If the average
speed remains unchanged, but the volume increases, this means that
there will be fewer collisions with the container walls over a
given time If the average speed remains unchanged, but the volume
increases, this means that there will be fewer collisions with the
container walls over a given time Therefore, the pressure will
decrease Therefore, the pressure will decrease