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Temperature & Heat
Physical Science Chapter 5 Cont2
What are we going to study?
Temperature
Heat
Specific Heat and Latent Heat
Heat Transfer
Phases of Matter
The Kinetic Theory of Gases
Thermodynamics
Chapter 5: Temperature
“Hot” & “Cold” are relative terms.
Temperature depends on the kinetic (motion) energy of the molecules of a substance.
Temperature is a measure of the average kinetic energy of the molecules of a substance.
Section 5.1
Thermometer
Thermometer - an instrument that utilizes the physical properties of materials for the purpose of accurately determining temperature
Section 5.1
Thermometer
Thermal expansion is the physical property most commonly used to measure temperature. – Expansion/contraction of metal
– Expansion/contraction of mercury or alcohol
Section 5.1
Bimetallic Strip and Thermal Expansion
Brass expands more than iron.
The degree of deflection is proportional to the temperature.
A/C thermostat and dial-type thermometers are based on bimetal coils.
Section 5.1
Liquid-in-glass Thermometer
Thermometers are calibrated to two reference points (ice point & steam point.) – Ice point – the temperature of a mixture of
pure ice and water at normal atmospheric pressure
– Steam point – the temperature at which pure water boils at normal atmospheric pressure
Usually contains either mercury or red (colored) alcohol
Section 5.1
Temperature Scales Celsius, Kelvin, Fahrenheit
Section 5.1
Temperature Scales Celsius, Kelvin, Fahrenheit
Scale Absolute
Zero
Ice
Point
Steam
Point
Diff.
(Boil –
Ice)
Fahrenheit - 460o 32o 212o 180
Celsius -273o 0o 100o 100
Kelvin 0o 273o 373o 100
Section 5.1
Converting Temperatures is Easy!
TK = TC + 273 (Celsius to Kelvin)
TC = TK – 273 (Kelvin to Celsius)
TF = 1.8TC + 32 (Celsius to Fahrenheit)
Section 5.1
TF – 32
1.8 • TC = (Fahrenheit to Celsius)
Converting a Temperature Example # 1
Convert the Celsius temperature of -30oC into Fahrenheit.
EQUATION: TF = 1.8TC + 32
Section 5.1
Thermal Energy and Heat Temperature – a measure of the AVERAGE kinetic
energy of the individual particles of a substance.
Temperature – Perception of how cold or hot something
is.
Thermal energy – TOTAL energy of all of the particles
Heat
Kinetic and Potential energy both exist at the molecular level. – Kinetic – motion of molecules
– Potential – bonds that result in the molecules oscillating back and forth
Heat is energy that is transferred from one object to another as a result of a temperature difference.
Heat – thermal energy moving from a warmer object to a cooler object, trying to reach thermodynamic equilibrium.
Section 5.2
Heat Unit SI - Calorie Since heat is energy, it has a unit of joules. (J)
A more common unit to measure heat is the calorie.
Calorie - the amount of heat necessary to raise one gram of pure water by one Celsius degree at normal atmospheric pressure
1 cal = 4.186 J (or about 4.2 J)
Kilocalorie – heat necessary to raise 1kg water by 1oC
1 food Calorie = 1000 calories (1 kcal)
1 food Calorie = 4186 J (or about 4.2 kJ)
Section 5.2
Heat Unit British - Btu
British thermal unit (Btu) – the amount of heat to raise one pound of water 1oF
1 Btu = 1055 J = 0.25 kcal = 0.00029kWh
A/C units are generally rated in the
number of Btu’s removed per hour.
Heating units are generally rated in the number of Btu’s supplied per hour.
Section 5.2
1. Energy in transit because of a temperature difference
2. The same as temperature
3. The internal energy of a substance
4. Unrelated to energy
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Conceptual Question 1: Which of the following phrases best describes heat?
1. Energy is transferred from the cold object to the hot object
2. Energy is transferred from the hot object to the cold object
3. The temperature is transferred from cold object to hot object
4. The temperature is transferred from hot object to cold object
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Conceptual Question 2: How does the energy is transferred?
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Example 2
How much energy have you gained after eating a peanut that contains 10 Calories?
Section 5.2
Heat vs. Temperature
http://www.youtube.com/watch?v=NVXLDPHGYhg
Expansion/Contraction with D’s in Temperature
In general, most matter, solids, liquids, and gases will expand with an increase in temperature (and contract with a decrease in temperature.)
Water is an exception to this rule – (ice floats!)
Section 5.2
Thermal-Expansion Joints in a Bridge
These joints allow for the contraction and expansion of the steel girders during the winter and summer seasons.
Section 5.2
Behavior of Water Strange!
The volume of a quantity of water decreases with decreasing temperature but only down to 4oC. Below this temperature, the volume increases slightly.
With a minimum volume at 4oC, the density of water is maximum at this temperature and decreases at lower temperatures.
Most dense point
Section 5.2
Behavior of Water: Structure of Ice Solid water takes up more volume
a) An illustration of the open hexagonal (six-sided) molecular structure of ice.
b) This hexagonal pattern is evident in snowflakes.
Section 5.2
(Ebbing, Darrell D., General Chemistry, Sixth Edition. Copyright 1999 by Houghton Mifflin Company)
Behavior of water
Liquid water below 4 C is bloated with ice crystals.
Upon warming, the crystals collapse, resulting in a smaller volume for the liquid water.
Above 4oC liquid water expands as it is heated because of greater molecular motion.
Section 5.3
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Yellowstone Lake - Frozen
Photo Source: Copyright © Bobby H. Bammel. All rights reserved.
Section 5.2
Do you know that fish live below frozen lakes? How?
1. Specific heat capacities
2. Energy at different temperatures
3. Rates of thermal expansion
4. All of these
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Conceptual Question 3: A bimetallic strip used in thermostats relies on the
fact that different metals have different…
Specific Heat (Capacity)
If equal quantities of heat are added to equal masses of two metals (iron and aluminum, for example) – would the temperature of each rise the same number of degrees? -- NO!
Different substances have different properties.
Specific Heat – the amount of heat necessary to raise the temperature of one kilogram of the substance 1oC
Section 5.3
Specific Heat (Capacity)
The greater the specific heat of a substance, the greater is the amount of heat required to raise the temperature of a unit of mass.
Put another way, the greater the specific heat of a substance the greater its capacity to store more heat energy
Water has a very high heat capacity, therefore can store large amounts of heat.
Section 5.3
1. Absorbs great quantity of heat for small increases of temperature
2. Absorbs great quantity of heat for a big increases of temperature
3. Absorbs a small quantity of heat for small increases of temperature
4. Absorbs a small quantity of heat.
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Conceptual Question 4: Water is useful in the cooling systems of automobiles, and other engines because it …
Specific Heats of Some Common Substances
The three phases
of water are
highlighted.
Section 5.3
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Sand (700 J/kg-Co) & Water (4186 J/kg-Co)
Photo Source: Copyright © Bobby H. Bammel. All rights reserved.
Section 5.3
Specific Heat Capacity
Youtube: http://www.youtube.com/watch?v=hyPLusD-tyM
Demonstration:
Specific Heat Depends on Three Factors
The specific heat or the amount of heat necessary to change the temperature of a given substance depends on three factors:
1) The mass (m) of the substance
2) The heat (c) of the substance
3) The amount of temperature change (DT)
Section 5.3
Using Specific Heat
H = mcDT
– H = amount of heat to change temperature
– m = mass
– c = specific heat capacity of the substance
– DT = change in temperature
The equation above applies to a substance that does not undergo a phase change.
Section 5.3
Example: Using Specific Heat
How much heat in kcal does it take to heat 80 kg of bathwater from 12oC to 42oC?
GIVEN: m = 80 kg, c = 1.00 kcal/kg.Co (known value for water)
Section 5.3
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Example: How much it costs to heat the bath
water electrically?
Heat needed = 2.4 x 103 kcal
Convert to kWh
1kcal = 0.00116kWh
At 12 cents per kWh, it will cost ??
Section 5.3
Using Specific Heat Confidence Exercise
How much heat needs to be removed from a liter of water at 20oC so that is will cool to 5oC ?
GIVEN: 1 liter water = 1 kg = m
DT = 15oC; c = 1.00 kcal/kg.Co
Section 5.3