Chapter 21 Temperature, Heat and Expansion
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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
Celsius scale- 100 degrees difference between freezing and
boiling of pure water
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
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
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
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
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
Energy always flows from high to low
Matter does not contain heat
Thermal energy- energy resulting
from heat flow
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
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
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
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
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
This causes large, fixed gaps, which
decreases density
Ice floats because of this decreased
density
Lakes/ponds freeze from the top
down