ANSWER KEY
Overview of Heat and Thermodynamics Unit – Part 2 (CP)
Review Sheet – Mrs. Travers (Introductory Physics, PS II)
Text Reference: Chapter 7.2 – 7.4, Temperature, Energy and Matter; CPO Physics A First
Course
In this unit, we focused on temperature, thermal energy, heat, thermal expansion,
calorimetry, specific heat, phase changes, First and Second Law of Thermodynamics.
Terms to Know from Text and Classroom Lecture and Activities:
Temperature
Kelvin Scale
First Law of Thermodynamics
Fahrenheit scale
Absolute Zero
Second Law of Thermodynamics
Celsius Scale
Heat of Fusion
Intermolecular Forces (IMFs)
Heat Transfer
Heat of Vaporization
Thermal conductivity
Conduction
Boiling
Evaporation
Radiation
Melting
Condensation
Convection
Freezing
Performance Based Standards (what you should be able to do at the end of this unit):
1. I can convert between Celsius, Fahrenheit and Kelvin
2. I understand how energy is transferred through radiation, convection and
conduction and I can provide and recognize examples of each.
3. I understand that energy is absorbed when a substance undergoes the following
phase changes: solid to liquid or liquid to gas
4. I understand that energy is released when a substance undergoes the following
phase changes: gas to liquid or liquid to solid
5. I can draw the heating curve for a substance and identify areas where each
phase change occurs and where the substance is increasing in temperature.
6. I understand what heat of fusion is the energy required to melt one gram of a
substance and the heat of vaporization is the energy required to boil one gram
of a substance.
7. I understand the changes in potential and kinetic energy that substances
undergo as they absorb or release thermal energy during phase changes and
temperature changes.
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8. I can explain how evaporation cools its surroundings and condensation warms its
surroundings.
Data/Equations You’ll be Given on this Test:
The specific heat of water is 1 cal/g°C or 4.184 J/g°C.
The heat of fusion of water is 80. cal/g. The heat of vaporization of water is 540. cal/g.
Q = mcΔT
°F = (9 x °C) +32
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°C = 5 (°F - 32)
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Test will consist of multiple choice questions, temperature conversions, knowledge of common
temperatures in Fahrenheit, Celsius and Kelvin scales, identifying parts of a heating curve,
calculation of energy absorbed or released during temperature increases or phase changes,
and short essay.
Conceptual Review Questions
Temperature Conversions
1.
Fill in the table of common temperatures in each of the three scales we studied:
Fahrenheit
Kelvin
Boiling Point of Water
212 ˚F
Celsius
Freezing Point of Water
32 ˚F
0 ˚C
273 K
Normal Body Temperature
98.6 ˚F
37 ˚C
Approximate room
temperature
68 ˚F
20 ˚C
2.
100˚C
373 K
Convert 173 degrees Celsius to Fahrenheit.
343 ˚F
`
(9/5 x 173) +32 Be Careful of Order of Operations! ADD 32 at END
3. Convert 95 degrees Fahrenheit to Celsius.
35 ˚C
5/9*(95-32)
Be Careful of Order of Operations! Subtract 32 FIRST
4. What is the temperature of absolute zero in Celsius? -273 ˚C
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Phase Change Questions
5. Use the attached graphic and draw the heating curve for water. Identify the following
on the graphic:
SEE PHASE CHANGE KEY – separate file
-Solid
-freezing
-boiling point of water
-liquid
-2 places where PE of
-freezing point of
-gas
molecules is increasing
water.
-melting
-boiling
-condensing
-3 places where KE of
molecules is increasing
6. What is evaporation and why is it a cooling process? What cools during evaporation?
Evaporation is a change from liquid to gas that takes place at the surface of a liquid.
The remaining liquid cools as a result since molecules with greater KE leave the liquid to
go into the gas phase. Evaporation increases with temperature.
7. Why is perspiration a cooling process?
Perspiration is a cooling process because molecules absorb energy from you gaining a
higher kinetic energy (which contribute to a higher ave. KE, or temperature), leave the
surface of your skin, leaving molecules behind that have a lower ave. KE).
8. Why is a steam burn more damaging than a burn from boiling water at the same
temperature?
Steam has more energy than water at the same temperature. Steam gives up considerably more
energy as it condenses when coming into contact with your skin.
9. Why does decreasing the temperature of a liquid make it freeze?
As the temperature of a substance is decreased, energy is withdrawn from a liquid and
molecular motion diminishes until finally the molecules are moving so slowly that the
attractive forces between them are able to cause cohesion.
10. Does liquid give off or absorb energy when it evaporates? When it solidifies?
When a molecule in the liquid phase evaporates it absorbs energy so that it has enough
energy to break free of the liquid and go into the gas phase. When a molecule in the
liquid solidifies it gives off energy, thereby slowing down, become attracted to other
molecules and solidifying.
11. Does a gas give off or absorb energy when it condenses?
A gas gives off energy when it condenses. The release of energy results in the slowing
down of particles where the molecules then attract each other in the liquid phase.
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12. Why doesn’t the temperature of melting ice rise when heat is applied?
During a phase change, the energy added to the ice does not raise its temperature. It
does not increase the kinetic energy of the molecules. It instead increases the
potential energy of the solid water molecules which allows them to break the forces
between them. 80 calories of energy is required to melt 1 gram of ice (the heat of
fusion of water).
13. Can water exist at 0°C? I thought water froze at 0°C? What is the difference
between water at 0°C and ice at 0°C?
Yes, water can exist at 0°C. The difference between water at 0° C and ice at 0° C is
thermal energy that is stored as potential energy in the molecules of water. A gram of
ice at 0° C has 80 cal/g less energy than a gram of water at 0° C. That extra energy in
the water is used to overcome intermolecular forces in the water molecules so that they
can move more fluidly.
Heat Transfer
14. Define heat:
Heat is the transfer of thermal energy from a higher temperature substance to a lower
temperature substance
15. In what direction does heat flow?
Always from the higher temperature substance to the lower temperature substance (2 nd
Law of Thermodynamics)
16. If you wanted to cool a cake quickly would you leave it at room temperature or put it in
the refrigerator? Why?
In the refrigerator. The greater the temperature difference, the greater the heat
flow and the faster it will cool.
17. What are the three methods of heat transfer?
Radiation, Conduction and Convection
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18.
Give an example of each:
a. Radiation – sun warming your home during the day, radiators in your home
b. Conduction – cooking food on a stove or fire, the metal frame of your chair feeling cold as it
conducts heat away from you.
c. Convection – a sea breaze, density changes in water as it warms on a stove and comes to a boil
Radiation
True or False?
19. Radiation can travel through a vacuum T
20. Radiation requires particles to travel - F
21. Radiation travels at the speed of light - T
22. Why are houses painted white in hot countries?
White surfaces are poor absorbers of energy and therefore are also poor emitters and will not emit
energy into the house.
23. Why are shiny foil blankets wrapped around marathon runners at the end of a race?
Shiny surfaces are poor absorbers of energy and will reflect radiant heat back away from the
runner helping him/her cool down.
Convection
24. Why does hot air rise and cold air sink?
When air gains thermal energy it expands and lowers its density creating a buoyant force on it
which raises it up. When air loses thermal energy to the cooler surroundings it becomes more
dense and sinks.
25. Why are boilers placed beneath hot water tanks in people’s homes?
As the water is warmed it becomes less dense and rises through the tank helping to convect
thermal energy to the rest of the water in the tank.
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26. What does a bird’s feathers and a wool sweater have in common?
Both form a barrier for air to be trapped in where convection currents can set up between the
warm organism (avian or human) and the insulating barrier (feathers or sweater).
Conduction
27. Using what you have learned about conductivity, explain why, on a cold winter day, a metal
park bench feels colder than a wooden park bench, even though they are really the same
temperature.
Although both the metal bench and wooden bench are at the same temperature, metal is a
good thermal conductor and the metal conducts thermal energy away from you faster so you
sense something colder. Your body senses change not temperature.
28. Which of the following is a good thermal conductor? Why?
a.
Wood
b. Air
c.
Metal
d. Styrofoam
Metal is a good thermal conductor due to its atomic structure and the freedom of the
electrons to be shared among the metallic bonds. This allows for better communication
between the atoms and energy received by one atom can be quickly transferred to other
atoms though the medium. Air is a gas. Gases in general are poor thermal conductors as
their atoms are far apart. Wood and Styrofoam have large carbon-based molecules where
electrons do not move as freely.
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