PHY 101 ASU
When an air molecule is hit by an approaching, faster moving molecule, does the approaching molecule's rebound speed increase or decrease? How about when it hits a receding molecule?
Increase, decrease
What happens when Dr. Hewitt heats up the strip?
It bends.
Why does the pupil of the eye appear black?
Light that enters the pupil bounces several times before exiting, and is absorbed on each bounce, so little light exits.
Which generally expands more for an equal increase in temperature: solids or liquids?
Liquids generally expand more than solids.
Temperature
Measure of the average translational kinetic energy per molecule of a substance; measured in degrees Celsius or Fahrenheit or in kelvins.
How would decreasing the percentage of greenhouse gases affect the amount of infrared radiation reflected by the atmosphere back toward Earth?
The amount of infrared radiation would decrease.
How would increasing the density of air in the atmosphere affect the amount of infrared radiation returned by the atmosphere back toward Earth?
The amount of infrared radiation would increase.
What would be the consequence of completely eliminating the greenhouse effect?
The average temperature on the surface of Earth would drop to -18°C.
When air rapidly expands, its temperature normally _________.
decreases
What type of gas absorbs and re-emits infrared radiation?
greenhouse gases
Newton's law of cooling applies to objects that undergo _________.
cooling or warming
calorie
the amount of heat required to change the temperature of 1 gram of water by 1 Celsius degree.
kilocalorie (Calorie)
the heat unit used to label foods, 1,000 calories.
You wish to warm 75 kg of water by 20∘C for your bath. Select the correct equation that shows that this is equivalent to about 6300 kJ . (1500kcal)/(4.19kcal/kJ)=6285kJ (1500kcal)(4190kJ/kcal)=6285kJ (1500kcal)/(4190kcal/kJ)=6285kJ (1500kcal)(4.19kJ/kcal)=6285kJ
(1500kcal)(4.19kJ/kcal)=6285kJ
Select the correct equation that shows that 4000cal=16760J, the same quantity of thermal energy in different units. (4000cal)(4190J/cal)=16760J (4000cal)/(4190cal/J)=16760J (4000cal)(4.19J/cal)=16760J (4000cal)/(4.19cal/J)=16760J
(4000cal)(4.19J/cal)=16760J
Radiation
(a) Energy transmitted by electromagnetic waves (b) The particles given off by radioactive atoms such as uranium. Do not confuse radiation with radioactivity
Conduction
(a) in heat, the energy transfer from particle to particle within certain materials, or from one material to another when the two are in direct contact. (b) In electricity, the flow of electric charge through a conductor
Which one of these represents the largest amount of energy? 10 calories 100 calories 1 Calorie 5 joules
1 Calorie
Solar constant
1400J/m^2 received from the Sun each second at the top of Earth's atmosphere; expressed in terms of power, 1.4 kW/m^2
Which one of these cites the lowest temperature? 270 K 273K 32°F 0° C
270 K
At what temperature do the combined effects of contraction and expansion produce the smallest volume for water? 0°C -4°C 4°C -100°C
4°C
What was the precise temperature at the bottom of Lake Michigan, where the water is deep and the winters long, on New Year's Eve in 1901?
4°C because the temperature at the bottom of any body of water containing any 4°C water has a bottom temperature of 4°C, for the same reason that rocks are at the bottom. Rocks are more dense than water, and 4°C water is more dense than water at any other temperature. So both rocks and 4°C water sink to the bottom. Water is also a poor heat conductor, so if the body of water is deep and in a region of long winters and short summers, the water at the bottom likely remains a constant 4°C year-round
At a certain location, the solar power per unit area reaching Earth's surface is 200 W/m2, averaged over a 24-hour day. If the average power requirement in your home is 2.4 kW and you can convert solar power to electric power with 11 % efficiency, how large a collector area will you need to meet all your household energy requirements from solar energy?
A = 110 m^2 2.4kW=2400W 2400W/(200W)(.11)=109.09 2 Significant Figures = 110m^2
Distinguish between a calorie and a Calorie.
A Calorie is 1000 calories.
If a fast marble hits a random scatter of slow marbles, does the fast marble usually speed up or slow down? Of the initially fast-moving marble and the initially slow ones, which lose(s) kinetic energy and which gain(s) kinetic energy? How do these questions relate to the direction of heat flow?
A fast-moving marble slows when it hits slower-moving marbles. It transfers some of its kinetic energy to the slower ones. Likewise with the flow of heat. Molecules with more kinetic energy that are in contact with molecules that have less kinetic energy transfer some of their excess energy to the less energetic ones. The direction of energy transfer is from hot to cold. For both the marbles and the molecules, however, the total energy before and after contact is the same.
Does a substance that heats up quickly have a high or a low specific heat capacity?
A low specific heat capacity
Convection
A means of heat transfer by movement of the heated substance itself, such as by currents in a fluid
In desert regions that are hot in the daytime and cold at nighttime, the walls of houses are often made of mud. Why is it important that the mud walls be thick?
A wall of appropriate thickness keeps the house warm at night by slowing the flow of heat from inside to outside, and it keeps the house cool in the daytime by slowing the flow of heat from outside to inside. Such a wall has "thermal inertia."
In what form does radiant energy travel?
As electromagnetic waves
From greatest to least, rank the wires in how much they'll sag on a hot summer day. A: Copper, α=17×10−6/∘C B: Aluminum, α=24×10−6/∘C C: Steel, α=11×10−6/∘C
B>A>C
Which will normally warm faster: a black pot of cold water or a silvered pot of cold water? Which will cool faster?
Black, black
Why does the strip behave as it does when Dr. Hewitt heats it up?
Both metals expand unequally
When the temperature of ice-cold water is increased slightly, does it undergo a net expansion or a net contraction?
Contraction
Three blocks of metal at the same temperature are placed on a hot stove. Their specific heat capacities are listed below. Rank them from greatest to least in how quickly each warms up. Steel, 450 J/(kg⋅∘C) Aluminum, 910 J/(kg⋅∘C) Copper, 390 J/(kg⋅∘C)
Copper > Steel > Aluminum
Global warming
Earth's climate has gotten too warm too fast
Solar power
Energy per unit time derived from the Sun
In winter, why does the road surface on a bridge tend to be more icy than the road surfaces at either end of the bridge?
Energy radiated by roads on land is partly replenished by heat conducted from the warmer ground below the pavement. But there's no thermal contact between the road surfaces of bridges and the ground, so they receive very little, if any, replenishing energy conducted from the ground. This is why road surfaces on bridges get colder than roads on land, which increases the chance of ice formation on bridges. Understanding heat transfer can make you a safer driver!
Do any of the following not give off radiant energy? (a) The Sun, (b) lava from a volcano, (c) red-hot coals, (d) this text that you're reading
Everything that has a temperature about absolute zero emits electromagnetic radiation
True or false? Temperature is a measure of the total kinetic energy in a substance.
False. Temperature is a measure of the average (not total!) translational kinetic energy of molecules in a substance. For example, there is twice as much total molecular kinetic energy in 2 L of boiling water as in 1 L—but the temperatures of the two volumes of water are the same because the average translational kinetic energy per molecule is the same in each.
Select the correct equations that show how the food value in calories per gram is 5.7 kcal/g (or 5.7 Cal/g ). Food value per gram=(Food value)/mpeanut=(3.42Cal)/(0.6g)=5.7Cal/g Food value per gram=(Food value)(40)/mwater=(3.42Cal)(40)/(57g)=5.7Cal/g Food value per gram=(Food value)(0.40)/mpeanut=(3420calCal)(0.40)/(0.6g)=5.7Cal/g Food value per gram=(Food value)/mwater=(3420Cal)/(57g)=5.7Cal/g
Food value per gram=(Food value)/mpeanut=(3.42Cal)/(0.6g)=5.7Cal/g
Will Maynez burns a 0.6-g peanut beneath 57 g of water, which increases in temperature from 22∘C to 46 ∘C. (The specific heat capacity of water is 1.0 cal/(g⋅∘C).) Assuming that 40% of the heat released by the burning peanut makes its way to the water (40% efficiency), select the correct equations that show that the peanut's food value is 3420 calories (equivalently, 3.42 Calories). Food value=(cmwaterΔT)/0.40=(1.0cal/(g⋅∘C))(57g)(46∘C−22∘C)/0.40=3420cal Food value=(cmpeanutΔT)(0.40)=(1.0cal/(g⋅∘C))(0.6g)(46∘C)(0.40)=3420cal Food value=(cmpeanutΔT)/40=(1.0cal/(g⋅∘C))(0.6g)(46∘C−22∘C)/40=3420cal Food value=(cmwaterΔT)(40)=(1.0cal/(g⋅∘C))(57g)(46∘C)(40)=3420cal
Food value=(cmwaterΔT)/0.40=(1.0cal/(g⋅∘C))(57g)(46∘C−22∘C)/0.40=3420cal
What is the role of "loose" electrons in heat conductors?
Free electrons can move through a material carrying heat, jostling atoms and other electrons.
How does glass act like a one-way valve for a conventional greenhouse? Does the atmosphere similarly act as a one-way valve?
Glass transmits solar radiation into the greenhouse and blocks infrared radiation from leaving the greenhouse. The same thing happens in the atmosphere.
Why do substances expand when their temperature is increased?
Higher-temperature substances have greater molecular motion.
You can hold your fingers beside the candle flame without harm, but not above the flame. Why?
Hot air travels upward by air convection. Since air is a poor conductor, very little heat travels sideways to your fingers.
Think about the correlation between a growing human population and climate change. Drag the terms on the left to the appropriate blanks on the right to complete the sentences.
Humans consume energy and more humans consume more energy. Energy is mostly obtained by burning fuels. It leads to production of more greenhouse gases that results in climate change.
If a good absorber of radiant energy were a poor emitter, how would its temperature compare with the temperature of its surroundings?
If a good absorber were not also a good emitter, there would be a net absorption of radiant energy and the temperature of the absorber would remain higher than the temperature of the surroundings. Things around us approach a common temperature only because good absorbers are, by their very nature, also good emitters.
In what way did the Industrial Era contribute to climate change? Discuss better uses of fossil fuels than creating heat and smoke. Match the words in the left column to the appropriate blanks in the sentences on the right. Make certain each sentence is complete before submitting your answer.
In the Industrial Era the large-scale burning of fossil fuels caused the increase of amount of greenhouse gases in the atmosphere that resulted in climate change. Apart from burning fossil fuels to produce energy, many modern materials are made of fossil fuels.
Which of the following would best describe the conductive properties of wood?
It is a good insulator.
Which is likely to be colder: a night when you can see the stars or a night when you cannot?
It is colder on the starry night, when Earth's surface radiates directly to frigid deep space. On a cloudy night, net radiation is less because the clouds radiate energy back to Earth's surface.
What determines whether an object is a net absorber or a net emitter of radiant energy at a given time?
It is determined by the temperature of the object relative to its surroundings. Hotter objects are net emitters.
What happens to the conductive properties of wood when it gets very hot?
It will continue to remain a good insulator.
Suppose you apply a flame to 1 L of water for a certain time and its temperature rises by 2°C. If you apply the same flame for the same time to 2 L of water, by how much will its temperature rise?
Its temperature will rise by only 1°C because there are twice as many molecules in 2 L of water, and each molecule receives only half as much energy on average.
If you touch the metal sides in an oven with your bare hand, you're in trouble. But hold your hand briefly in the oven air and you're okay. What does this tell you about the relative conductivities of metal and air?
Metal is a good conductor of heat, whereas air is a terrible conductor of heat.
In a glass of water at room temperature, do all the molecules have the same speed?
No
Since a hot cup of tea loses heat more rapidly than a lukewarm cup of tea, is it correct to say that a hot cup of tea will cool to room temperature before a lukewarm cup of tea will?
No! Although the rate of cooling is greater for the hotter cup, it has farther to cool to reach thermal equilibrium. The extra time is equal to the time it takes to cool to the initial temperature of the lukewarm cup of tea. Cooling rate and cooling time are not the same thing.
Since all objects emit energy to their surroundings, why don't the temperatures of all objects continuously decrease?
Objects that are good emitters are equally good absorbers, so they absorb radiation as well as emit it.
Why a 1-ft^2 hole in a ceiling that admits sunlight when the Sun is overhead is like having a 100-W bulb in the room? Assume the atmosphere reduces solar energy at Earth's surface as 1.0 kW/m2. One square meter is a bit more than 10 square feet, thus PSun=(1000W/m2)⋅(1ft2)⋅((1m2)/(10ft2))=100W, as with 100-W bulb. One square meter is a bit more than 100 square feet, thus PSun=(1000W/m2)⋅(1ft2)⋅((1ft2)/(100m2))=100W, as with 100-W bulb. One square meter is a bit more than 100 square feet, thus PSun=(1000W/m2)⋅(1ft2)⋅((1m2)/(100ft2))=100W, as with 100-W bulb. One square meter is a bit more than 10 square feet, thus PSun=(1000W/m2)⋅(1ft2)⋅((1ft2)/(10m2))=100W, as with 100-W bulb.
One square meter is a bit more than 10 square feet, thus PSun=(1000W/m2)⋅(1ft2)⋅((1m2)/(10ft2))=100W, as with 100-W bulb.
Calculate the quantity of heat absorbed by 80 g of water that warms from 30∘C to 90∘C.
Q =4800cal Q=(90-30)*(80)=4800
You wish to warm 75 kg of water by 20∘C for your bath. Select the correct equations that show that the amount of heat needed is 1500 kcal ( 1500 Cal). Q=cΔT/m=(1kcal/(kg⋅∘C))(20∘C)/(75kg)=1500kcal Q=cm/ΔT=(1kcal/(kg⋅∘C))(75kg)/(20∘C)=1500kcal Q=cΔT=(1kcal/(kg⋅∘C))(20∘C)=1500kcal Q=cmΔT=(1kcal/(kg⋅∘C))(75kg)(20∘C)=1500kcal
Q=cmΔT=(1kcal/(kg⋅∘C))(75kg)(20∘C)=1500kcal
The specific heat capacity of steel is 450 J/kg⋅∘C. Select the correct equations that show that the amount of heat needed to raise the temperature of a 22-kg piece of steel from 0 ∘C to 100 ∘C is 990,000 J. Q=cmΔT=(450J/kg⋅∘C)(22kg)(100∘C−0∘C)=990,000J Q=cΔT/m=(450J/kg⋅∘C)(100∘C−0∘C)/(22kg)=990,000J Q=cm/ΔT=(450J/kg⋅∘C)(22kg)/(100∘C−0∘C)=990,000J Q=mΔT=(22kg)(100∘C−0∘C)=990,000J
Q=cmΔT=(450J/kg⋅∘C)(22kg)(100∘C−0∘C)=990,000J
Select the correct equations that show that 20950 joules are required to raise the temperature of the mass 0.50 kg of water from 22∘C to 32∘C. For the specific heat capacity c, use 4190 J/kg⋅∘C. Q=cΔT=(4190J/kg⋅∘C)(32∘C−22∘C)=20950J Q=cmΔT=4190J/kg⋅∘C(0.50kg)(32∘C)=20950J Q=cmΔt=(4190J/kg⋅∘C)(0.50kg)(32∘C−22∘C)=20950J Q=cΔTm=(4190J/kg⋅∘C)(32∘C−22∘C)0.50kg=20950J
Q=cmΔt=(4190J/kg⋅∘C)(0.50kg)(32∘C−22∘C)=20950J
Terrestrial radiation
Radiant energy emitted from Earth
Relatively speaking, do high-frequency waves have long wavelengths or short wavelengths? Name an electromagnetic wave with higher frequency than blue light.
Short, ultraviolet
Why do fish benefit from water being most dense at 4°C?
Since water is most dense at 4°C, colder water rises and freezes on the surface, which means that fish remain in relative warmth!
What eventually happens to the solar energy that falls on Earth?
Sooner or later, it will be radiated back into space. Energy is always in transit—you can rent it, but you can't own it.
In a 25∘C∘C room, hot coffee in a vacuum flask cools from 73 ∘C∘C to 46 ∘∘ in 8 hours. Find its temperature after another 8 hours.
T =34∘C (73-46)=27 27/2=13.5 46-13.5=32.5 =33 (it gave me credit for 33 degrees, but said the answer is 34 degrees due to rounding and significant figures)
Why is it advisable to allow telephone lines to sag when stringing them between poles in summer?
Telephone lines are longer in summer, when they are warmer, and shorter in winter, when they are cooler. They therefore sag more on hot summer days than in winter. If the telephone lines are not strung with enough sag in summer, they might contract too much and snap during the winter.
An iron thumbtack and a big iron bolt are removed from a hot oven. Both are red-hot and have the same temperature. When dropped into identical containers of water of equal temperature, which one raises the water temperature more?
The big iron bolt has more internal energy to impart to the water and warms it more than the thumbtack. Although both objects have the same initial temperature (the same average kinetic energy per molecule), the more massive bolt has more molecules and therefore more total energy—internal energy. This example underscores the difference between temperature and internal energy.
Why is one able to walk on red-hot coals without getting one's feet burned?
The coals are not good conductors of heat.
Heat
The energy that "flows" from one object to another by virtue of a difference in temperature; measured in calories or joules
How is the energy value of foods determined?
The food is burned and the energy released is measured.
Northeastern Canada and much of Europe receive about the same amount of sunlight per unit area. Why, then, is Europe generally warmer in the winter?
The gulf stream carries warm, high heat capacity water past the west coast of Europe.
Absolute zero
The lowest possible temperature that any substance can have; the temperature at which the atoms of a substance have their minimum kinetic energy. the temperature of absolute zero is -273.15 C, which is -459.7 F, and 0 K
Specific heat capacity
The quantity of heat required to raise the temperature of a unit mass of a substance by 1 degree Celsius (or, equivalently, by 1 kelvin); often simply called specific heat
Newton's law of cooling
The rate of cooling an object - whether by conduction, convection, or radiation - is approximately proportional to the temperature difference between the object and its surroundings
Why does the direction of coastal winds change from day to night?
The specific heat of water is greater than the specific heat of land. Water cools more slowly at night. The temperature difference drives convection and offshore winds.
Internal energy
The total energy stored in the atoms and molecules within a substance. Changes in internal energy are of principal concern in thermodynamics.
A farmer turns on the propane burner in his barn on a cold morning and heats the air to 20°C (68°F). Why does he still feel cold?
The walls of the barn are still cold. The farmer radiates more energy to the walls than the walls radiate back, and he is chilled. (Inside your home or your classroom, you are comfortable only if the walls are warm, not just the air.)
What's inside the open spaces of the water crystals shown in Figures 15.18 and 15.19? Is it air, water vapor, or nothing?
There's nothing at all in the open spaces. It's empty space—a void. If there were air or vapor in the open spaces, the illustration should show molecules there—oxygen and nitrogen for air and H2O for water vapor.
Is there a distinction between thermal energy and internal energy? Which term do physicists prefer?
Thermal energy emphasizes heat flow, whereas internal energy is the grand total of all energies inside a substance. Physicists prefer internal energy.
Which of the following changes would make the water balloon more likely to pop? (Ignore effects of convection within the fluid.) Use a liquid that has a lower heat capacity than water. Use a liquid that has a higher heat capacity than water. Use a thinner balloon. Use a thicker balloon.
Use a liquid that has a lower heat capacity than water. Use a thicker balloon.
The specific heat capacity of steel is 450 J/kg⋅∘C. How does this compare with the heat needed to raise the temperature of the same mass of water through the same temperature difference?
This value is less than the heat needed to raise the temperature of the same mass of water through the same temperature difference.
What does it mean to say that the greenhouse effect is like a one-way valve?
Transparent material—atmosphere for Earth and glass for the greenhouse— allows only incoming short wavelengths and blocks outgoing long wavelengths. As a result, radiant energy is trapped within the "greenhouse."
How would you walk on red-hot coals without getting your feet burned?
Walk fast.
Greenhouse effect
Warming caused by short-wavelength radiant energy from the Sun that easily enters the atmosphere and is absorbed by Earth, but when radiated at longer wavelengths cannot easily escape Earth's atmosphere
Which has a higher specific heat capacity: water or sand?
Water has a higher specific heat capacity. Water has greater thermal inertia and takes a longer time to warm in the hot sunlight and a longer time to cool on a cold night. Sand has a low specific heat capacity, as evidenced by how quickly the surface warms in the morning sunlight and how quickly it cools at night. (Walking or running barefoot across scorching sand in the daytime is a much different experience than walking on cool sand in the evening.)
Which of the following most prevents the escape of terrestrial radiation? Water vapor Carbon monoxide Air Carbon dioxide
Water vapor
Why can you place your hand briefly inside a hot oven without harm, but you are burned if you touch the metal sides of the oven?
When your hand is in the air of the hot oven, you're not harmed mainly because air is a poor conductor—heat doesn't travel well between the hot air and your hand. Touching the hot metal sides of the oven is another story because metal is an excellent conductor and considerable heat flows into your hand.
What kind of strip is Dr. Hewitt holding in his hand?
a strip consisting of brass on one side and steel on the other, welded together
If you are able to grab a hot pan and pull it out by the handle without burning yourself, what kind of handle must the pan have, and why?
a wooden handle, because it is the better insulator
Each of the following items states a temperature, but does not tell you whether the temperature is measured on the Fahrenheit, Celsius, or Kelvin scale. Match the items to the appropriate temperature scale. a) Water freezes into ice at 0 b) Water boils into gas phase at 373.15. c) Liquid water boils at 100 d) Ice cream is stored in freezers at 26. e) The coldest possible temperature is 0 f) A typical room temperature is 24 g) A hot summer day might be 100
a) C b) K c) C d) F e) K f) C g) F
Rank the magnitudes of the following units of thermal energy from greatest to least: a) calorie b) Calorie c) 5 joule
b>a>c
Rank from greatest to least the volumes of water of the same mass at the following temperatures: a) 0-C b) 4-C c)1 10-C
c>a>b
A thermos bottle controls heat transfer by _________.
conduction, convection, and radiation.
Suppose that we replace the aluminum with a mystery metal and repeat the experiment in the video. As in the video, the mass of the metal is the same as that of the water. Room temperature is about 20∘C before the start of the experiment. The water heats up to 40∘C, and the mystery metal heats up to 80∘C. Compared to that of water, the heat capacity of our mystery metal is
one-third as great Given the same input of energy, the temperature of the metal increased three times as much as the temperature of the water. Therefore, the metal has one-third the heat capacity of water. (Recall that the heat Q delivered to a substance can be written Q = mc ΔT , where m is the mass of the substance and c is its heat capacity.)
Climate change
results from global warming
Radioactive decay of granite and other rocks in Earth's interior provides sufficient energy to keep the interior molten, to heat lava, and to provide warmth to natural hot springs. This is due to the average release of about 0.03 J per kilogram each year. Find an increase in temperature for a thermally insulated chunk of granite that takes about 12.9 million years to change temperature. (Assume that the specific heat capacity c of granite is 800 J/kg⋅C∘. Use the equation Q=cmΔT.)
ΔT =484∘C 0.03J/(800J/kg)(1kg)=3.75*10^-5 3.75*10^-5(12.9 million)=483.75