ANSWER KEY
Overview of Heat and Thermodynamics Unit – Part 2 (H)
Review – 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:
Heat Transfer
Heat of Fusion
Thermal conductivity
Heat of Vaporization
Radiation
Boiling
Convection
Melting
Conduction
Freezing
Evaporation
First Law of Thermodynamics
Condensation
Second Law of Thermodynamics
Performance Based Standards (what you should be able to do at the end of this unit):
1.
I understand how energy is transferred through radiation, convection and conduction
and I can provide and recognize examples of each.
2. I understand that energy is absorbed when a substance undergoes the following phase
changes: solid to liquid or liquid to gas
3. I understand that energy is released when a substance undergoes the following phase
changes: gas to liquid or liquid to solid
4. 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.
5. I understand what heat of fusion and heat of vaporization are and know how to use
these values to calculate the energy required to melt or boil a substance (or the energy
released to solidify or liquefy a substance).
6. 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.
7. I can explain how evaporation cools its surroundings and condensation warms its
surroundings.
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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
Test will consist of multiple choice questions, identifying parts of a heating curve, calculation
of energy absorbed or released during temperature increases or phase changes, and short
essay.
Conceptual Review Questions
Phase Change Questions
1.
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.
2. 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).
3. 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.
4. 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
intermolecular forces between them are able to cause cohesion.
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5. 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.
6. 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.
7. 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 overcome the intermolecular forces between them.
80 calories of energy is required to melt 1 gram of ice (the heat of fusion of water).
8. Calculate the amount of energy released when 20 grams of steam at 100 °C becomes a liquid
at 80 °C.
Ans: -11,200 cal
Heat Transfer
Define heat.
Heat is the transfer of thermal energy from a higher temperature substance to a lower
temperature substance
In what direction does heat flow?
Always from the higher temperature substance to the lower temperature substance (2 nd Law of
Thermodynamics)
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.
Radiation
True or False?
1.
Radiation can travel through a vacuum T
2.
Radiation requires particles to travel F
3.
Radiation travels at the speed of light T
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4. Four containers were filled with warm water. Which container would have the warmest water
after ten minutes?
The __shiny metal__ container would be the warmest after ten minutes because its shiny surface
reflects heat radiation back into the container so less is lost (THINK THERMOS). The _dull black_
container would be the coolest because it is the best at emitting heat radiation.
5. Four containers were placed equidistant from a heater. Which container would have the
warmest water after ten minutes?
The dull black__ container would be the warmest after ten minutes because its surface absorbs
heat radiation_ the best. The shiny metal container would be the coolest because it is the poorest
at absorbing heat radiation.
6. 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.
7. 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
9.
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.
10. 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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11. 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
12. 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.
13. 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. Gasses 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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