Transcript Chapter 21 Temperature, Heat and Expansion

Chapter 21
Temperature, Heat and Expansion
CONCEPTUAL PHYSICS
HEWITT, 1999
BLOOM HIGH SCHOOL
21.1 Temperature
 Temperature- a measure of how hot or cold something is

Demonstrated by the expansion or contraction of a liquid
Red thermometer- colored alcohol
 Silver thermometer- mercury
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 Celsius scale- 100 degrees difference between freezing and
boiling of pure water
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0°C corresponds to the freezing point of pure water
100°C corresponds to the boiling point of pure water
 Fahrenheit scale- 180 degrees difference between freezing
and boiling of pure water
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0°F corresponds to a mixture of ice, water, ammonium chloride and salt
100°F corresponds to human body temperature
Kelvin
 Kelvin- 100 degrees difference between freezing and
boiling of pure water

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273K corresponds to the freezing point of pure water
373K corresponds to the boiling point of pure water
0K (absolute zero) represents zero kinetic energy of a substance
 Fahrenheit is an English unit
 Celsius is not SI, but a derived unit
 Kelvin is SI (metric)
 We don’t use “degrees” with Kelvin
Temperature Conversions
 °F = (1.8 x °C) +32
 °C = (°F-32) x 0.555
 Kelvin = °C + 273.2
Temperature & Kinetic Energy
 Temperature is related to the
motion of molecules
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Proportional to the average kinetic
energy
Not a measure of the total KE of the
molecules
 Example: There is twice as much
energy in 2L of 90°C H2O as 1L of
90°C H2O
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Even though the temperature is the
same, there is twice as much mass
21.2 Heat
 Heat- the energy transfer that takes
place because of the temperature
difference between two objects
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Energy always flows from high to low
Matter does not contain heat
 Thermal energy- energy resulting
from heat flow

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Chapter 8
Internal energy- term used in this
chapter for the same thing
 Thermal contact- when heat flows
due to direct contact
21.3 Thermal Equilibrium
 Thermal equilibrium- when objects in contact reach
the same temperature

No heat flows from one to the other any more
 A thermometer is read at thermal equilibrium
 When the alcohol or mercury stops moving
 The glass of the thermometer is too small to affect the
temperature
21.4 Internal Energy
 Internal energy- grand total of all of the energies in the
substance

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Translational energy (moving in a straight line)
Rotational energy of the molecule
Kinetic energy within the molecule
Potential energy due to forces within molecule
 As a substance absorbs energy, the temperature
increases
 As a substance gives off energy, temperature decreases
and one or more of the energies decrease
21.5 Measurement of Heat
 Temperature- a measure of
heat transfer, not heat content
 Calorie (cal)- amount of heat
required to increase 1g of H2O
by 1°C
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
Metric, derived unit of heat energy
kilocalorie (kcal or Cal)- 1000
calories

Found on food wrappers
 Joule (J)- 4.18J are required to
increase 1g of H2O by 1°C

Standard SI unit of heat energy
Hungry Man Classic Fried
Chicken Dinner
21.6 Specific Heat Capacity
 Specific heat capacity (or just specific heat)- the
quantity of heat required to raise the temperature of
1g by 1°C

Ability to store internal energy
 A measured value for each substance (see table)
Using Specific Heat (chemistry review)
Label
Symbol
Unit
Specific Heat
c
cal/(g°C) or J/(g°C)
Heat
Q
cal or J
Mass
m
g
Change in temp.
DT
°C
q=mcDT
cal=(g)(cal/(g°C))(°C)
J=(g)(J/(g°C))(°C)
21.7 The High Specific Heat Capacity of H2O
 Absorbs more than would be expected
 Slow to gain heat
 Slow to lose heat
 Air near water areas are cooler than far inland because
water resists a change in temperature
“Cooler near the lake”
21.8 Thermal Expansion
 As temperature increases, kinetic energy of the
molecules increase and they “wiggle” further apart
 Gases expand the most when heated
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Liquids expand less than gases
Solids expand less than liquids
Your Thermostat & You
 Bimetallic strip- a strip made of two metals that
expand at different rates
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Brass & iron, typically
One side expands at a different rate than the other
 Thermostat- practical application of bimetallic strip to
turn on and off your furnace/AC

In the diagram, closing the circuit (touching) turns the furnace on
Thermostat Cat
21.9 Expansion of Water
 All liquids expand when
heated
 Ice cold (0°C) H2O does
the opposite!
 As H2O warms from 0°C
to 4°C, it contracts

H2O is more dense at 4°C
than 0°C, therefore sinks to
the bottom of the container
Solid versus Liquid H2O
 In liquid H2O, some of the O’s
are attracted to the H’s

H2O molecules are able to slide past
each other with minimal interaction
 In solid H2O, each O is attracted
to two H’s
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This causes large, fixed gaps, which
decreases density
Ice floats because of this decreased
density
Lakes/ponds freeze from the top
down