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physics
thermodynamics
Conceptual Physics 11th edition Paul G. Hewitt - Solutions
Why can't you establish whether you are running a high temperature by touching your own forehead?
After you measure the dimensions of a plot of land with a steel tape on a hot day, you return and remeasure the same plot on a cold day. On which day do you determine the larger area for the land?
Suppose that water is used in a thermometer instead of mercury. If the temperature is at 4°C and then changes, why can't the thermometer indicate whether the temperature is rising or falling?
A piece of solid iron sinks in a container of molten iron. A piece of solid aluminum sinks in a container of molten aluminum. Why does a piece of solid water (ice) not sink in a container of "molten" (liquid) water? Explain, using molecular terms.
How would the shape of the 0°C-18°C curve in Figure 15.20 differ if density rather than volume were plotted against temperature? Make a rough sketch.
How does the combined volume of the billions and billions of hexagonal open spaces in the structures of ice crystals in a piece of ice compare with the portion of ice that floats above the waterline?
Why is it important to protect water pipes in the winter so that they don't freeze?
If water had a lower specific heat capacity, would ponds be more likely to freeze or less likely to freeze?
If cooling occurred at the bottom of a pond instead of at the surface, would the pond freeze from the bottom up? Explain.
Which has the greater amount of internal energy-an iceberg or a cup of hot coffee? Defend your answer.
What would be the final temperature of a mixture of 50 g of 20°C water and 50 g of 40°C water?
Suppose that a brass rod 1.0 m long expands 0.5 cm when its temperature is increased a certain amount. By how much will a brass rod 100 m long expand with the same change of temperature?
Consider a 40,000-km steel pipe-that forms a ring to fit snugly all around the circumference of the Earth. Suppose people along its length breathe on it so as to raise its temperature 1°C. The pipe gets longer. It also is no longer snug. How high does it stand above ground level? (To simplify,
Many tongues have been injured by licking a piece of metal on a very cold day. Why would no harm result if a clean piece of wood were licked on the same day?
Visit a snow-covered cemetery and note that the snow does not slope upward against the gravestones. Instead, it forms depressions, as shown. Can you think of a reason for this?
Wood conducts heat very poorly-it has a very low conductivity. Does wood still have a low conductivity if it is hot? Could you quickly and safely grasp the wooden handle of a pan from a hot oven with your bare hand? Although the pan handle is hot, is much heat conducted from it to your hand if
Does wood have a low conductivity if it is very hot-that is, in the stage of smoldering, red-hot coals? Could you safely walk across a bed of red-hot wooden coals with bare feet? Although the coals are hot, does much heat conduct from them to your feet if you step quickly? Could you do the same on
Why would you not expect all the molecules of air in your room to have the same average speed?
In a mixture of hydrogen and oxygen gases at the same temperature, which molecules move faster? Why?
One container is filled with argon gas and the other with krypton gas. If both gases have the same temperature, in which container are the atoms moving faster? Why?
Which atoms, on average, move slower in a mixture, U-238 or U-235? How would this affect diffusion through a porous membrane of otherwise identical gases made from these isotopes?
Consider two equal-size rooms connected by an open door. One room is maintained at a higher temperature than the other one. Which room contains more air molecules?
In a still room, smoke from a candle will sometimes rise only so far, not reaching the ceiling. Explain why.
Why does helium, released into the atmosphere, eventually disappear into space?
Ice cubes float in a glass of iced tea. Why would cooling be less if the cubes were instead on the bottom of the drink?
What does the high specific heat of water have to do with convection currents in the air at the seashore?
If we warm a volume of air, it expands. Does it then follow that if we expand a volume of air, it warms? Explain.
Ceiling fans can make you feel cooler in a warm room. Do ceiling fans reduce room temperature?
Some ceiling fans are reversible so that they drive air down or pull it up. In which direction should the fan drive the air during winter? In which direction for summer?
If 70°F air feels warm and comfortable to us, why does 70°F water feel cool when we swim in it?
A number of bodies at different temperatures placed in a closed room share radiant energy and ultimately reach a common temperature. Would this thermal equilibrium be possible if good absorbers were poor emitters and poor absorbers were good emitters? Explain.
The heat of volcanoes and natural hot springs comes from trace amounts of radioactive minerals in common rock in Earth's interior. Why isn't the same kind of rock at Earth's surface warm to the touch?
Even though metal is a good conductor, frost can be seen on parked cars in the early morning even when the air temperature is above freezing. Provide an explanation.
When there is morning frost on the grass in an open park, why is frost unlikely to be found on the ground directly beneath park benches?
At what common temperature will a block of wood and a block of metal both feel neither hot nor cold to the touch?
Is it important to convert temperatures to the Kelvin scale when we use Newton's law of cooling? Why or why not?
Why is the insulation in an attic commonly thicker than the insulation in the walls of a house?
Suppose that, at a restaurant, you are served coffee before you are ready to drink it. In order that it be hottest when you are ready for it, should you add cream to the coffee right away or wait until you are ready to drink it?
If you wish to save fuel and you're going to leave your warm house for a half hour or so on a very cold day, should you turn your thermostat down a few degrees, turn it off altogether, or let it remain at the room temperature you desire?
If you wish to save fuel and you're going to leave your cool house for a half hour or so on a very hot day, should you turn your air-conditioning thermostat up a bit, turn it off altogether, or let it remain at the room temperature you desire?
As more energy from fossil fuels and other fuels is released on Earth, the overall temperature of Earth tends to rise. How does temperature equilibrium explain why Earth's temperature cannot rise indefinitely?
If you hold one end of a piece of metal against a piece of ice, the end in your hand soon becomes cold. Does cold flow from the ice to your hand? Explain.
In terms of physics, why do restaurants serve baked potatoes wrapped in aluminum foil?
In lab, Will burns a 0.6-g peanut beneath 50 g of water, which increases in temperature from 22°C to 50°C. The amount of heat absorbed by the water can be found with the equation Q = cmΔT, where Q is the amount of heat, c the specific heat of water, m the mass of water, and ΔT the change
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. Show that a 500°C increase in temperature
In a 25°C room, hot coffee in a vacuum flask cools from 75°C to 50°C in 8 hours. Explain why you predict that its temperature after another 8 hours will be 37.5°C.
At a certain location, the solar power per unit area reach ins; Earth's surface is 200 W/m2, averaged over a 24-hour day. If the average power requirement in your home is 3 kW and you can convert solar power to electric power with 10% efficiency, how large a collector area will you need to meet all
In lab you submerge 100 g of 40°C iron nails in 100 g of 20°C water (the specific heat of iron is 0.12 cal/g°C.) (a) Equate the heat gained by the water to the heat lost by the nails and show that the final temperature of the water becomes 22.1°C. (b) Your lab partner is surprised by the result
Pretend that all the molecules in a liquid have the same speed, not random speeds. Would evaporation of this liquid cause the remaining liquid to be cooled? Explain.
Porous canvas bags filled with water are used by travelers in hot weather. When the bags are slung on the outside of a fast-moving car, the water inside is cooled considerably. Explain.
The human body can maintain its customary temperature of 37°C on a day when the temperature is above 40°C. How is this done?
How does Figure 17.7 help explain the moisture that forms on the inside of car windows when you're parking with your date on a cool night?
You know that the windows in your warm house get wet on a cold day. But can moisture form on the windows if the interior of your house is cold on a hot day? How is this different?
Why do clouds often form above mountain peaks?
Why will clouds tend to form above either a flat or a mountainous island in the middle of the ocean?
Water will boil spontaneously in a vacuum-on the surface of the Moon, for example. Could you cook an egg in this boiling water? Explain.
Our inventor friend proposes a design for cookware that will allow boiling to occur at a temperature less than 100°C so that food can be cooked with less energy consumption. Comment on this idea.
Your instructor hands you a closed flask partly filled with room-temperature water. When you hold it, the heat transfer between your bare hands and the flask causes the water to boil. Quite impressive! How is this accomplished?
When you boil potatoes, will your cooking time be reduced with vigorously boiling water instead of gently boiling water? (Directions for cooking spaghetti call for vigorously boiling water-not to lessen cooking time but to prevent something else. If you don't know what it is, ask a cook.)
Why does placing a lid over a pot of water on a stove shorten the time for the water to come to a boil, whereas, after the water is boiling, the use of a lid only slightly shortens the cooking time?
Explain why the eruptions of many geysers repeat with notable regularity.
Why does the water in a car radiator sometimes boil explosively when the radiator cap is removed?
Can ice be colder than 0°C? What is the temperature of an ice-water mixture?
Why is very cold ice "sticky"?
People who live where snowfall is common will tell you that air temperatures are higher when it's snowing than when it's clear. Some misinterpret this by stating that snowfall can't occur on very cold days. Explain this misinterpretation.
Would regelation occur if ice crystals did not have an open structure? Explain.
Why does blowing over hot soup cool the soup?
Some old-timers found that when they wrapped newspaper around the ice in their iceboxes, melting was inhibited. Discuss the advisability of this practice.
Why is it that a tub of water placed in a farmer's canning cellar helps prevent canned food from freezing in cold winters?
Why will spraying fruit trees with water before a frost help to protect the fruit from freezing?
Why does a hot dog pant?
Can you give two reasons why pouring a cup of hot coffee into a saucer results in faster cooling?
What is the source of energy that keeps the dunking bird in Figure 17.4 operating?
The quantity of heat Q that changes the temperature ΔT of a mass m of a substance is given by Q = cmΔT, where c is the specific heat capacity of the substance. For example, for H20, c = 1 cal/g°C. And for a change of phase, the quantity of heat Q that changes the phase of a mass m is Q = mL,
The specific heat capacity of ice is about 0.5 cal/g°C. Supposing that it remains at that value all the way to absolute zero, calculate the number of calories it would take to change a 1-g ice cube at absolute zero (-273°C) to 1 g of boiling water. How does this number of calories compare with
Find the mass of 0°C ice that 10 g of 100°C steam will completely melt.
Consider 50 g of hot water at 80°C poured into a cavity in a very large block of ice at 0°C. What will be the final temperature of the water in the cavity? Show that 50 g of ice must melt in order to cool the hot water down to this temperature.
A 50-g chunk of 80°C iron is dropped into a cavity in a very large block of ice at 0°C. Show that 5.5 g of ice will melt. (The specific heat capacity of iron is 0.11 cal/g°C.)
If you drop a piece of ice on a hard surface, the energy of impact will melt some of the ice. The higher it drops, the more ice will melt upon impact. Show that to completely melt a block of ice that falls without air drag, it should ideally be dropped from a height of 34 km.
A 10-kg iron ball is dropped onto a pavement from a height of 100 m. If half of the heat generated goes into warming the ball, find the temperature increase of the ball. (In SI units, the specific heat capacity of iron is 450 J/kg°C.) Why is the answer the same for a ball of any mass?
The heat of vaporization of ethyl alcohol is about 200 cal/g. If 2 kg of this fluid were allowed to vaporize in a refrigerator, show that 5 kg of ice would be formed from 0°C water.
A friend said the temperature inside a particular oven is 500 and the temperature inside a particular star is 50,000. You're unsure whether your friend meant Celsius degrees or kelvins. How much difference does it make in each case?
Is it possible to wholly convert a given amount of mechanical energy into thermal energy? Is it possible to wholly convert a given amount of thermal energy into mechanical energy? Cite examples to illustrate your answers.
Everybody knows that warm air rises. So it might seem that the air temperature should be higher at the tops of mountains than down below. But the opposite is most often the case. Why?
The combined molecular kinetic energies of molecules in a cool lake are greater than the combined molecular kinetic energies of molecules in a cup of hot tea. Pretend you partially immerse the teacup in the lake and that the tea absorbs 10 calories from the water and becomes hotter, while the water
Why is thermal pollution a relative term?
The box "Thermodynamics Dramatized" shows the crushing of an inverted steam-filled can placed in a pan of water. Would the water need to be cold? Would crushing occur if the water were hot but not boiling? Would it crush in boiling water?
Why is it advantageous to use steam that is as hot as possible in a steam-driven turbine?
What happens to the efficiency of a heat engine when the temperature of the reservoir into which thermal energy is transferred is lowered?
To increase the efficiency of a heat engine, would it be preferable to produce the same temperature increment by increasing the temperature of the reservoir while maintaining the temperature of the sink constant, or to decrease the temperature of the sink while maintaining the temperature of the
Could you cool a kitchen by leaving the refrigerator door open and closing the kitchen door and windows? Explain.
Strictly speaking, why will a refrigerator containing a fixed amount of food consume more energy in a warm room than in a cold room?
In buildings that are being electrically heated, is it at all wasteful to turn all the lights on? Is turning all the lights on wasteful if the building is being cooled by air-conditioning?
Defend the statement that 100% of the electrical energy that goes into lighting a lamp is converted to thermal energy. Are the first and the second laws of thermodynamics violated?
Molecules in the combustion chamber of a rocket engine are in a high state of random motion. When the molecules are expelled through a nozzle in a more ordered state, will their temperature be more, less, or the same as their initial temperature in the chamber before being exhausted?
Is the total energy of the universe becoming more unavailable with time? Explain.
Comment on this statement: The second law of thermo-dynamics is one of the most fundamental laws of nature, yet it is not an exact law at all.
Water evaporates from a salt solution and leaves behind salt crystals that have a higher degree of molecular order than the more randomly moving molecules in the saltwater. Has the entropy principle been violated? Why or why not?
Water put into the freezer compartment of your refrigerator goes to a state of less molecular disorder when it freezes. Is this an exception to the entropy principle? Explain.
The United States Patent and Trademark Office rejects claims for perpetual motion machines (in which energy output is as great or greater than energy input) without even investigating them. Why is this?
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