AP Physics 1- “All About Energy” (Power, Thermal Physics, & Heat Transfer) Review Power Concepts (Integrated Through the Unit): Students will be able to relate the concepts of power and energy via time. Students will be able to analyze systems/machines (simple machines or energy production) via work, energy, and power concepts. (Some from Unit 4) 1. A 100 Watt incandescent light bulb is on for 8 hours. a) How much energy is “used” during this time? b) What is the main energy transfer of the light bulb? c) Using energy ideas, why can fluorescent & LED bulbs produce the same light intensity use so much less power? d) If 400 Joules of useful light energy is used during this time, what is the efficiency of the light bulb? 2. A 10 kg bucket is lifted up 10 meters by a pulley & motor system in a time of 8 seconds. a) What is the minimum average power of the motor? b) Why is “minimum” used in the question above? c) If this is a 0.5 horsepower motor, find the efficiency. d) List 2 ways a simple machine like a pulley can help a HUMAN be more efficient. e) If a person could push this bucket up a 20 degree slope (no friction) to a height of 10 meters find the work done on the bucket by the person AND by gravity. f) What was the advantage of doing this? g) What did the human have to give “more of” to make this work happen? h) An 1kW wind turbine spins for 6.5 hours. The electricity that reaches your home is 3 kWhr. Find the efficiency of the system. 3. A car (m=2000kg) slows down from 66 mph to a stop in a time of 2 seconds. a) Find the work done on the car by the brakes. b) How much power do the brakes have in Watts and h.p.? c) If the four disc brakes are made of 10 kg of iron (Heat capacity is 1/10 th that of water), and all of the KE from part a) went into heating them up, what would be the temperature change in Celsius? (Use the “mc” rapper equation.) Heat & Temperature Concepts: Students will be able to distinguish between the concepts of heat & temperature. Students will be able to relate mechanical energies (GPE, EPE, KE) to changes in energies on the atomic level. (Kinetic Molecular Theory-KMT Ch.13) 1. A person sweats (sweat is just water) 600 grams of water during intense exercise. How much heat is removed from the body if 500 grams of this sweat evaporates off the body by changing phase from a liquid to a solid? 2. Calculate the amount of energy that needs to be removed to freeze 1000 grams of 20 degree Celsius water to ice at -10 degrees Celsius. (3 steps!)
Transcript
AP Physics 1- “All About Energy” (Power, Thermal Physics, & Heat Transfer) Review
Power Concepts (Integrated Through the Unit):
Students will be able to relate the concepts of power and energy via time. Students will be able to analyze systems/machines (simple machines or energy production)
via work, energy, and power concepts. (Some from Unit 4) 1. A 100 Watt incandescent light bulb is on for 8 hours. a) How much energy is “used” during this time? b) What is the main energy transfer of the light bulb? c) Using energy ideas, why can fluorescent & LED bulbs produce the same light intensity use so much less power?
d) If 400 Joules of useful light energy is used during this time, what is the efficiency of the light bulb?
2. A 10 kg bucket is lifted up 10 meters by a pulley & motor system in a time of 8 seconds. a) What is the minimum average power of the motor? b) Why is “minimum” used in the question above? c) If this is a 0.5 horsepower motor, find the efficiency. d) List 2 ways a simple machine like a pulley can help a HUMAN be more efficient.
e) If a person could push this bucket up a 20 degree slope (no friction) to a height of 10 meters find the work done on the bucket by the person AND by gravity. f) What was the advantage of doing this? g) What did the human have to give “more of” to make this work happen? h) An 1kW wind turbine spins for 6.5 hours. The electricity that reaches your home is 3 kWhr. Find the efficiency of the system.
3. A car (m=2000kg) slows down from 66 mph to a stop in a time of 2 seconds. a) Find the work done on the car by the brakes. b) How much power do the brakes have in Watts and h.p.? c) If the four disc brakes are made of 10 kg of iron (Heat capacity is 1/10th that of water),
and all of the KE from part a) went into heating them up, what would be the temperature change in Celsius? (Use the “mc” rapper equation.)
Heat & Temperature Concepts: Students will be able to distinguish between the concepts of heat & temperature. Students will be able to relate mechanical energies (GPE, EPE, KE) to changes in energies
on the atomic level. (Kinetic Molecular Theory-KMT Ch.13)
1. A person sweats (sweat is just water) 600 grams of water during intense exercise. How
much heat is removed from the body if 500 grams of this sweat evaporates off the body
by changing phase from a liquid to a solid?
2. Calculate the amount of energy that needs to be removed to freeze 1000 grams of 20
degree Celsius water to ice at -10 degrees Celsius. (3 steps!)
3. If it takes energy to melt ice, but it doesn’t increase the temperature, where is the
energy going on the atomic level?
4. Why is boiling a cooling process? (Remember the paper cup never got hot enough to
start on fire!?) What is happen to the Internal energy of the water when some of it
vaporizes?
5. How does a refrigerator work? (Bike pump and Spray Can! Go Aussies!) This helps
understand the ideas of Latent Heat and PV=nRT=NkT)
6. How does t he drinking bird demo work? (Super Smart bonus here. . . Extra Credit
Question on the test for sure,. . . also prepare a Haiku about KMT.)
7. As Mr. Mac writes this test, it is 12 Celsius degrees. Convert this to: a) Fahrenheit b) Kelvin c) The Fahrenheit and Celsius scales are based upon what physical properties of water/Hg? d) What is the kinetic energy of a Helium atom at this temperature? e) What is the “rms speed” of an average Helium atom (.004 kg per mole)? f) If the rms speed of Nitrogen is much less (7x more kg per mole) Why is there very little
Helium still left in our atmosphere but a lot of nitrogen and other “heavy” molecules? g) A balloon is filled with 0.02 moles of pure Helium. What is the TOTAL kinetic energy of
the gas? (remember, 3/2kT is for only 1 MOLECULE. Know Avogadro’s Number!)
8. A student adds 450 Joules of Heat to a 50 gram mass of an unknown substance. The
substance experiences a temperature change of 10oC. Find the specific heat capacity, cp.
(5)
9. A student decides he wants to add a chunk of steel (cp= 0.4 J/goC) that has been in
boiling water (95oC) in the classroom to a perfectly insulated cup full of 100 grams of water
at room temperature. The mixture comes to an equilibrium temperature of 37oC.
a) Find the mass of the steel. (10) b) Suggest 3 reasons (using all 3 heat Transfer mechanisms), that the experiment would have systematic errors.
10. To convert 100 grams of -5oC ice to water at 50oC, find
a) The least amount of Joules of heat needed to make this series of phase changes. b) Sketch a Temperature vs. time graph for this series of phase changes, labeling the 3
steps in the process. c) Explain, using each of the 3 heat transfer mechanisms, why the energy required to do
this series of phase changes/heating steps would be MUCH bigger in real life. d) Why is the Latent Heat of Fusion (Solid to Liquid or vice versa) so much LESS energetic
than the Latent Heat of Vaporization (Liquid to Solid or vice versa)? Use KMT. Remember water is very POLAR molecularly (Lv=7xLf)
Power Production & Global Energy Sources & Greenhouse Effect
-Outline the energy transfers/Sankey Diagram for electricity generation
-What is the efficiency of various turbine based systems
-What is a renewable and non-renewable energy source? Fossil fuel?
-Why is too much CO2 in the atmosphere leading to an increase in global temperatures?
-What are the more common forms of energy production (renewable and non)
-What are disadvantages and advantages of all major types of energy production?
9. For a 1 m2 size window that is made out of 1 cm thick glass (k=0.84) find the heat transfer rate P in Watts (Sometimes called the “heat current or heat transfer rate”) :
a) When it conducts because of a 30oC temperature difference across it.
b) When it radiates as a perfect “blackbody” (e=1) from a 30oC surface temperature to 0oC surroundings.
c) List 2 things you could do to decrease the rate of radiation through the window. d) Microscopically, what is going on between molecules during conduction in a solid?
10. Answer these questions using justifications based on the Kinetic Molecular Theory a) When a pot of water boils on the stove, why doesn’t the temperature increase? b) If there is no such thing as caloric fluid, then what does it really mean when we say “heat flows” from one object to another? (In terms of energy and motion.)
c) Why is the latent heat of fusion so much lower than the latent heat of vaporization for
water? (From a molecular standpoint.)
d) What are the Laws of Thermodynamics from a microscopic energy point of view? If you
can nail this one, you get KMT. Use the following “poker game” analogy by C.P. Snow
Law 0) You have to bet to stay in the game.(Thermal Equilibrium stops internal energy
transfer.)
Law 1) You can never win, you can only break even (in terms of $ total at the table. Heat is
microscopic work! Why?)
Law 2) Statistically you CAN win, if you get just the right cards. (Some molecules in a cup of
hot water ARE speeding up, but most aren’t. The deck is stacked against you. “Entropy
can decrease in a system, as long as external work is being done- ie The poker table is not
a CLOSED system. Refrigerators DECREASE entropy, or disorder. How?)
Law 3) You will never live that long. (Absolute 0K can never be reached. There will always
be motion of molecules. Why? (Think radiation here!)
e) Solar PV Panels are only 15% efficient BEFORE the electricity transfers. In Colorado, on
a perfect day, there is about 300 W/m^2 of Intensity at the surface of the Earth for 8
hours. What area of solar panels would be needed to power a 14 kWhr electrical energy
need on that day?
f) If you could use a solar hot water heater/collector which may be 50% efficient with the
same area as the last problem and same intensity from the Sun, how long would it take to
heat up 180 liters (180kg) of water from 20 deg. C to 40 deg. C?
g) What is the efficiency of a steam powered turbine generator? A Hydro Powered?
h) Why is minimizing the energy transfer steps important?
i) What must happen with magnets and coils of wire to make electricity in general?
j) What % of the world’s energy is fossil fuel power production?
k) What is the most common “renewable” energy source and why is it “sort of” renewable?
l) Why is hydroelectric (reservoirs/dams) and wind turbines a perfect match?
Bonus:
Elephants have a very high volume to surface area ratio. If African elephants have much
larger ears (which radiate a lot of heat) than their Asian counterparts, what does this tell
you about their respective environments?
Explain why a Thermos is vacuum sealed and painted silver on the inside to keep liquids
inside hot as long as possible.
Why does a wooden fencepost feel warmer than a steel one on a cold day?
Why does a dog pant on a hot day but horses don’t? (Hint: One of them actually sweats!)
11. Match the definition with the correct term.
A. kinetic energy H. radiation O. insulation
B. potential energy I. Infrared (I.R.) P. metals
C. specific heat capacity J. Pressure Q. Heat
D. latent heat of fusion K. Pascal R. Temperature
E. latent heat of vaporization L. 3rd S. Thermometer
F. conduction M. 1st T. Joules
G. convection N. 2nd U. molecules
1. The energy needed to melt 1 gram of a solid.
2. The energy needed to raise the temperature of 1 gram of a substance by 1 degree.
3. The force exerted on something per square inch.
4. Temperature measures the ________________ of an average molecule in a gas.
5. The law that says entropy will always increase in a closed system _____
6. The law that says heat is a form of energy.
7. This law of thermodynamics says you cannot reach -273.15 Celsius.
8. The heat transfer process that made the blue cylinder heat up more than the silver
one on the hot plate.__________
9. ________________ helps prevent conduction between things.
10. The heat transfer process that is caused by molecules bumping into each other.
11. ______________ does not flow when two objects are at the same temperature.
12. The specific heat capacity of water is 4.18 _________ per gram degree Celsius.
13. Ocean currents are created by warm water rising and colder water sinking.
________________
14. The unit of pressure which is 1 Newton per meter squared is called _________.
15. Air pressure exerts 14 pounds of force on every square inch of our bodies! This
pressure is due to ______________ bouncing off of us at very high speeds, millions
and billions of times per second.
16. The ____________ of water is about 7 times bigger than the energy needed to melt
one gram of Ice.
17. Heat flows from a high _____________ to a lower one.
18. When you feel the “heat” from a fire without touching it, what type of light is your
body absorbing?
19. What type of elements conduct heat very easily?
20. Fahrenheit was a glass blower and made the first accurate ____________.
21. Latent heat is related to the _________________ of molecules.
Multiple Choice: Select the BEST answer.
1. Two different objects are in thermal contact with one another. The objects are at different
temperatures. The temperatures of the two objects determine
A. the process by which thermal energy is transferred.
B. the heat capacity of each object.
C. the direction of transfer of thermal energy between the objects.
D. the amount of internal energy in each object. (1)
2. The specific latent heat of vaporization of a substance is the quantity of energy required to
A. raise the temperature of a unit mass of a substance by one degree Celsius.
B. convert a unit mass of liquid to vapour at constant temperature and pressure.
C. convert a unit mass of solid to vapour at constant temperature and pressure.
D. convert a unit mass of liquid to vapour at a temperature of 100 °C and a pressure of
one atmosphere. (1)
3. A substance changes from solid to liquid at its normal melting temperature. What change, if any,
occurs in the average kinetic energy and the average potential energy of its molecules?
Average kinetic energy Average potential energy
A. constant constant
B. increases constant
C. increases decreases
D. constant increases
(1)
4. Which two values of temperature are equivalent to the nearest degree when measured on the
Kelvin and on the Celsius scales of temperature?
Kelvin scale Celsius scale
A. 40 313
B. 273 100
C. 313 40
D. 373 0
(1)
5. The specific latent heat of vaporization of a substance is greater than its specific latent heat
of fusion because
A. boiling takes place at a higher temperature than melting.
B. thermal energy is required to raise the temperature from the melting point to the
boiling point.
C. the volume of the substance decreases on freezing but increases when boiling.
D. the increase in potential energy of the molecules is greater on boiling than on melting. (1)
6. Thermal energy may be transferred
I. in a fluid as a result of density changes of the fluid.
II. in a non-metallic substance as a result of lattice vibrations.
Which of the following correctly identifies each of these energy transfers?
Transfer I Transfer II
A. Convection Convection
B. Evaporation Convection
C. Convection Conduction
D. Evaporation Conduction
(1)
7. During an experiment, a solid is heated from 285 K to 298 K.
Which one of the following gives the rise in temperature, in deg C, and the final temperature,
in °C, of the solid?
Rise in temperature in deg C Final temperature in °C
A. 13 571
B. 13 25
C. 286 571
D. 286 25
(1)
8. A liquid is evaporating, causing the liquid to cool.
The temperature of the liquid decreases because
A. the number of liquid molecules is decreasing.
B. the mean kinetic energy of the liquid molecules is decreasing.
C. the pressure above the liquid surface is increasing.
D. the rate of evaporation is increasing.
9. A temperature scale is to be constructed using the property X of a substance. Which of the
following must be a characteristic of the property X?
A. The value of the property must be zero at zero kelvin.
B. The property must increase with increase of temperature.
C. The property must have a different value at each temperature to be measured.
D. The value of the property must vary linearly with kelvin temperature.
10. Which of the following is the internal energy of a system?
A. The total thermal energy gained by the system during melting and boiling.
B. The sum of the potential and the kinetic energies of the particles of the system.
C. The total external work done on the system during melting and boiling.
D. The change in the potential energy of the system that occurs during melting and
boiling.
11. A container holds 20 g of neon (mass number 20) and also 8 g of helium (mass number 4).
What is the ratio ?
A. 0.4
B. 0.5
C. 2.0
D. 2.5
Part 2: Free-Response Ap and IB Style Questions
1. This question is about thermal physics.
(a) Explain why, when a liquid evaporates, the liquid cools unless thermal energy is
