Transcript Slide 1

CLB 10102
PHYSICS
CHAPTER 9
Heat Energy
CLB 10102 Physics
CHAPTER 9 Heat Energy
9.1 Heat Transfer
• Heat energy is energy contained in the vibrational energy of
atoms and molecules in a substance that can be transferred by:
 Conduction
 Convection
 Radiation
• When heat energy is transferred to or from an object, the
temperature of the object increases or decreases relative to the
amount of heat added or removed.
• The specific heat of a substance is the amount of heat energy
required to raise the temperature of one gram of the substance
by 1oC (or 1 K).
CLB 10102 Physics
CHAPTER 9 Heat Energy
• To find the total amount of heat energy released or absorbed
by a substance, the equation is as follows:
Heat energy = Mass x Specific Heat x Temperature
Q = mCθ
CLB 10102 Physics
CHAPTER 9 Heat Energy
Conduction
• usual method of heat transfer in solid.
• is the heat transfer from a warmer part of a substance to a
cooler part as a result of molecular collisions, which cause
the slower-moving molecules to move faster.
• when one end of a metal rod is heated, the molecules in
that end move faster than before.
• these molecules collide with other molecules and cause
them to move faster also.
Heat flow
H = Heat conduction
H = kA∆T
L
k = thermal conductivity
(J/s.m.oC)
L = Length
CLB 10102 Physics
CHAPTER 9 Heat Energy
• Energy conducted in a particular time:
Q = Ht
Good Heat Conductor
Poor Heat Conductor
Copper
Aluminum
Steel
Silicon
Zink
Asbestos
Glass
Wood
Air
Polyurethane foam
CLB 10102 Physics
CHAPTER 9 Heat Energy
Convection
• is the heat transfer by the movement of warm molecules
from one region of a gas or a liquid to another.
• convection current are caused by the expansion of liquids
or gases as they are heated or cooled.
• the expansion makes hot gas or liquid less dense than the
surrounding fluid and forced upward by the heavier fluid,
which then flows in to replace it.
• Example:
 the wind carries along with it.
 the water coolant in an engine carries hot water from
the engine block to the radiator by a convection
process.
CLB 10102 Physics
CHAPTER 9 Heat Energy
Radiation
• is heat transfer through energy being transmitted in the
form of rays, waves or particles.
• Radiant heat is similar to light – passes through air, glass
and the vacuum of space.
• e.g.: when we put our hand several inches from a hot iron,
the heat we feel is transferred through radiation.
• Dark objects absorb more radiant heat than light objects.
CLB 10102 Physics
CHAPTER 9 Heat Energy
9.2 The Measurement of Temperature
Temperature and molecular motion:
• The temperature of 0.0 K is known as the absolute zero,
the lowest temperature which can be obtained.
• In order to understand the nature of heat and temperature,
it is necessary to appreciate the fact that matter consists of
moving particles (atoms or molecules) which can interact
more or less strongly with one another.
• The motion of the particles is increased by raising the
temperature.
• Conversely, the motion of the particles is reduced by
lowering the temperature, until, at the absolute zero (0 K),
the motion of the particles ceases altogether.
CLB 10102 Physics
CHAPTER 9 Heat Energy
• Because the particles are in motion, they will have kinetic
energy.
• The particles will not all have the same energy, and the
energy of the 3 particles is constantly changing as they
undergo changes in speed.
• Thus, for a given sample of matter, we can only talk
about the average kinetic energy of the particles.
• Temperature is a measure of the average kinetic energy of
the particles.
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CHAPTER 9 Heat Energy
• We can look at three different phases of matter:
 Solid
 Liquid
 Gas
• In a solid, a metal for example, the particles are atoms,
arranged in an orderly array.
• The atoms are relatively close to one another, and the
motion of each atom is restricted by its interaction with
other atoms.
CLB 10102 Physics
CHAPTER 9 Heat Energy
• In a liquid, the atoms or molecules, are further apart than in a
solid, and are not arranged in any special order.
• There is less interaction between the molecules, and they are
free to move in any direction, but as interactions between
the molecules are still present, most molecules are confined
to the volume occupied by the liquid sample.
• In a gas, the atoms or molecules are further apart and have
little interaction with one another.
CLB 10102 Physics
CHAPTER 9 Heat Energy
Temperature conversions
To convert from Celsius to Fahrenheit:
To convert from Fahrenheit to Celsius:
Example:
a) A patient with heat stroke has a temperature of 106oF.
What does this read on a Celsius thermometer?
5 (106 – 32) = 41.1oC
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CLB 10102 Physics
CHAPTER 9 Heat Energy
Example:
b) Because high fevers can cause convulsions in children, the
doctor wants to be called if the child’s temperature goes
over 40oC. Should the doctor be called if a child has a
temperature of 103oF.?
5 (103 – 32) = 39.4oC
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No because the temperature is lower than 40oC.
CLB 10102 Physics
CHAPTER 9 Heat Energy
9.3 Change of State
• The physical state of materials can be brought about by
increasing temperature, from solid to liquid and from
liquid to gas or the other way round.
• When a substance passes from the solid phase to the liquid
phase, it is said to melt or fuse.
• The temperature at which this occurs is called the melting
point of that substance.
• The reverse process, i.e. the passing of a substance from a
liquid to a solid, is called freezing or solidification.
• For most solids, pressure increases the melting point of
pure substances.
CLB 10102 Physics
CHAPTER 9 Heat Energy
• Some substances, under certain conditions of temperature
and pressure, pass directly from the solid to the vapour
phase.
• This is known as sublimation.
• Solid carbon dioxide ("dry ice") and iodine crystals are
example of substances which sublimate at normal
atmospheric pressure.
Fusion
Vaporization
Solidification
Condensation
Ice (Solid phase)
Water (Liquid phase)
Boiling water
(Vapor/Gas phase)
CLB 10102 Physics
CHAPTER 9 Heat Energy
Heating Curve
A= ice warming
B= ice melting
C= water heating
D= water boiling
E= vapor heating
CLB 10102 Physics
CHAPTER 9 Heat Energy
Effects of Pressure and Impurities on Change of
Phase:
• Impurities in water tend to lower the freezing point.
• An increase in the pressure on a liquid raises the boiling
point.
• A decrease in pressure on a liquid lowers the boiling
point.
CLB 10102 Physics
CHAPTER 9 Heat Energy
9.4 Latent Heat of Fusion and Vaporization
Latent heat of fusion (Lp)
• The specific latent heat of fusion of a substance is the
amount of heat required to convert unit mass of the solid
into the liquid without a change in temperature.
CLB 10102 Physics
CHAPTER 9 Heat Energy
• When a solid substance changes from the solid phase to the
liquid phase, energy must be supplied in order to overcome
the molecular attractions between the constituent particles of
the solid.
• This energy must be supplied externally, normally as heat,
and does not bring about a change in temperature.
• We call this energy latent heat (the word "latent" means
"invisible").
• The latent heat is the energy released or absorbed during a
change of state.
CLB 10102 Physics
CHAPTER 9 Heat Energy
• Example:
 the specific latent heat of fusion of ice at 0 ºC is 334
kJ.kg-1. This means that to convert 1 kg of ice at 0 ºC to 1
kg of water at 0 ºC, 334 kJ of heat must be absorbed by
the ice. Conversely, when 1 kg of water at 0 ºC freezes to
give 1 kg of ice at 0 ºC, 334 kJ of heat will be released to
the surroundings.
• Water has one of the largest specific latent heats of fusion of
all substances.
• Latent heats of fusion vary widely, and values should always
be accompanied by the temperatures at which they were
measured (these are not necessarily the normal melting
points).
CLB 10102 Physics
CHAPTER 9 Heat Energy
Latent heat of vaporization (Lw)
• Latent heat of vaporization is a change of state from liquid
to vapour at constant temperature also requires the input of
energy.
• This implies that while a liquid undergoes a change to the
vapour state at the normal boiling point, the temperature of
the liquid will not rise beyond the temperature of the boiling
point.
CLB 10102 Physics
CHAPTER 9 Heat Energy
• The latent heat of evaporation is the energy required to
overcome the molecular forces of attraction between the
particles of a liquid, and bring them to the vapour state, where
such attractions are minimal.
• Example:
 For water at its normal boiling point of 100 ºC, the latent
specific latent heat of vaporization is 2260 kJ.kg-1. This
means that to convert 1 kg of water at 100 ºC to 1 kg of
steam at 100 ºC, 2260 kJ of heat must be absorbed by the
water. Conversely, when 1 kg of steam at 100 ºC
condenses to give 1 kg of water at 100 ºC, 2260 kJ of heat
will be released to the surroundings.
CLB 10102 Physics
CHAPTER 9 Heat Energy
9.5 Specific Heat Capacity
• The specific heat of a substance is the amount of heat
necessary to change 1 kg of it 1oC.
c= Q
m∆T
• To find the amount of heat added or taken away from
substance to produce a certain temperature change, we use
Q = cm∆T
CLB 10102 Physics
CHAPTER 9 Heat Energy
• When 2 substances at different temperatures are mixed
together, heat flows from the warmer body to the cooler
body until they reach the same temperature.
• Part of the heat lost by the warmer body is transferred to
the cooler body and part is lost to the surrounding objects
or the air.
• We assume here that all the heat lost by the warmer body
equals the heat gained by the cooler body.
Qlost = Qgained
clml(Tl -Tf) = cgmg(Tf -Tg)
CLB 10102 Physics
CHAPTER 9 Heat Energy
Example 1:
The specific heat of water is equal to 4.184 J/(goC) or 1
calorie (cal). How much energy does it take to raise the
temperature of 28 g of water by 11oC ?
Q = cm∆T
= 4.184 J/goC x 28 g x 11oC
= 1288.7 J
CLB 10102 Physics
CHAPTER 9 Heat Energy
Example 2:
Calculate the amount of heat required to completely convert
20 g of ice at 0 ºC to steam at 100 ºC. 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.
1. Heat taken up for
converting ice at ºC to
water at ºC
2. Heat taken up heating
the water from 0 ºC to
the boiling point, 100 ºC
= mass of water x latent heat of fusion
= 0.020 kg x 334 kJ.kg-1
= 6.7 kJ
= mass of water x specific heat
capacity x temperature change
= 0.020 kg x 4.18 kJ.kg-1. K-1 x 100 K
= 8.4 kJ
CLB 10102 Physics
CHAPTER 9 Heat Energy
3. Heat taken up vaporizing = mass of water x latent heat of
the water
vaporization
= 0.020 (g) x 2260 kJ.kg-1
= 45.2 kJ
The sum of these is = (6.7 + 8.4 + 45.2) kJ
= 60.3 kJ
CLB 10102 Physics
CHAPTER 9 Heat Energy
Example 3:
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 × 103 J kg -1 K -1.
Qlost = Qgained
clml(Tl -Tf) = cgmg(Tf -Tg)
Tf = clmlTl + cgmgTg
clml +cgmg
Tf = (4.2 × 103 J kg-1K-1 x 3 kg x 373 K) + (4.2 × 103 J kg-1K-1 x 15 kg x 313 K)
(4.2 ×103 J kg-1K-1 x 3 kg) + (4.2×103 J kg-1K-1 x 15 kg)
Tf = 323 K = 50oC
CLB 10102 Physics
CHAPTER 9 Heat Energy
9.6 A Simple Refrigerator
• The basic idea behind a refrigerator is very simple: it uses
the evaporation of a liquid to absorb heat.
• You probably know that when you put water on your skin
it feels cool.
• As the water evaporates it absorbs heat and creates the
cool feeling.
• Rubbing alcohol feels cooler because it evaporates at a
lower temperature.
• The liquid, or refrigerant, used in a refrigerator evaporates
at an extremely low temperature so it can create freezing
temperatures inside the refrigerator.
CLB 10102 Physics
How it works:
CHAPTER 9 Heat Energy
CLB 10102 Physics
CHAPTER 9 Heat Energy
• The refrigerator works by using latent heat.
• A refrigerant is pumped through a system of closed pipes
through a valve.
• A compressor maintain a higher pressure on one side of the
expansion on valve than on the other.
• The pressure of the refrigerant is reduced when it passes
through the valve, so that it will expand and vaporize,
absorbing the necessary latent heat from inside the fridge
(cold body).
• On the high pressure side of the valve, the vapor is
compressed and condensed to a liquid once again,
releasing its tent heat of vaporization to the surroundings
(warm body).
• The cycle repeats.