Exercises – Chapter 7
1. Your body is presently converting chemical potential energy from food into thermal energy at a rate of about 100
J/s, or 100 W. If heat were flowing out of you at a rate of about 200 W, what would happen to your body temperature?
E.1
Your body temperature would decrease.
2. You can use a blender to crush ice cubes, but if you leave it churning too long, the ice will melt. What supplies the
energy needed to melt the ice?
E.2
The blender blades do work on the ice and this work becomes thermal energy in the ice.
3. When you knead bread, it becomes warm. From where does this thermal energy come?
E.3
The work you do in pushing the bread in the direction it moves.
4. You throw a ball into a box and close the lid. You hear the ball bouncing around inside as the ball’s energy changes
from gravitational potential energy to kinetic energy, to elastic potential energy, and so on. If you wait a minute or two,
what will have happened to the ball’s energy?
E.4
The ball's energy will eventually become thermal energy.
5. Why do meats and vegetables cook much more quickly when there are metal skewers sticking through them?
E.5
Foods are poor conductors of heat. The metal skewer carries heat better.
6. Why do aluminum pans heat food much more evenly than stainless steel pans when you cook on a stove?
E.6
Aluminum is a better conductor of heat than stainless steel, so heat disperses more quickly in an aluminum
pan and the food cooks more evenly on that pan.
7. Use the concept of convection to explain why firewood burns better when it’s raised above the bottom of a fireplace
on a grate.
E.7
The grate lets convection carry air upward past the wood.
8. Explain how convection contributes to the shape of a candle flame.
E.8
The rising hot air of convection draws the flame upward.
9. When you hold a lighted match so that its tip is lower than its stick, the flame travels quickly up the stick. Why?
E.9
Heat rises with convection and ignites the rest of the stick.
10. It’s often a good idea to wrap food in aluminum foil before baking it near the red-hot heating element of an electric
oven. Why does this wrapped food cook more evenly?
E.10 Covering the food with aluminum foil protects the food from the intense radiative heat transfer that the
red-hot heating element can provide. Instead of burning on the heating element side, the food cooks more
slowly via convection and warms more evenly.
11. Why are black steam radiators better at heating a room than radiators that have been painted white or silver?
E.11 Black radiators are very efficient at transferring heat via radiation. White or silver radiators radiate heat
less well and aren't as effective at heating their surroundings.
12. The space shuttle generates thermal energy during its operation in earth orbit. How is it able to get rid of that
thermal energy as heat in an airless environment?
E.12 The space shuttle radiates its excess heat away into cold, dark space.
13. Some air fresheners are solid materials that have strong odors. If you leave these air fresheners out, they slowly
disappear. What happens to the solid material?
E.13 The solid material sublimes and its molecules become gaseous.
14. Frozen vegetables will “freeze-dry” if they’re left in cold, dry air. How can water molecules leave the frozen
vegetables?
E.14 The water molecules sublime from the frozen vegetables, going directly from the solid phase to the
gaseous phase.
15. If you remove ice cubes from the freezer with wet hands, the cubes often freeze to your fingers. How can the ice
freeze the water on your hands? Shouldn’t they melt instead?
E.15 The ice is colder than its melting temperature and must warm up before it starts melting.
16. On a bitter cold day, the snow is light and powdery. This snow doesn’t begin melting immediately when you bring
it into a warm room. Why?
E.16 The cold snow is well below its melting temperature and it must absorb a moderate amount of heat in order
to warms up to that melting temperature.
17. When you put a loaf of bread in a plastic bag, there is still air around the bread. Why doesn’t the bread dry out as
quickly in the bag as it does when there’s no bag around it?
E.17 The bag traps steam so that the humidity soon rises to 100% so that net evaporation ceases.
18. Even on a very humid day the hot air from your blow dryer can extract moisture from your hair. Why is heated air
able to dry your hair when the air around you can’t?
E.18 Heating the air lowers its relative humidity. Increasing the temperature stabilizes the gaseous phase of
water relative to the liquid phase.
19. If you add a little hot tea to ice water at 0 °C, the mixture will end up at 0 °C so long as some ice remains. Where
does the tea’s extra thermal energy go?
E.19 It goes into the ice’s latent heat of melting.
20. If you put a warm bottle of wine in a container of ice water, the wine will cool but the ice water won’t become
warmer. Where is the wine’s thermal energy going?
E.20 The wine’s thermal energy is providing the latent heat of melting necessary to transform ice into water.
21. Why is it that you can put a metal pot filled with water on a red-hot electric stove burner without fear of damaging
the pot?
E.21 The boiling water will remove heat quickly enough to protect the pot.
22. Why does a pot of water heat up and begin boiling more quickly if you cover it?
E.22 Trapping the moisture allows the relative humidity to reach 100% so that net evaporation ceases.
23. Putting a clear plastic sheet over a swimming pool helps keep the water warm during dry weather. Explain.
E.23 It prevents evaporation from cooling the water.
24. If you try to cook vegetables in 100 °C air, it takes a long time. But if you cook those same vegetables in 100 °C
steam, they cook quickly. Why does the steam transfer so much more heat to the vegetables?
E.24 The condensing steam releases its latent heat of evaporation and that enormous heat cooks the vegetables
quickly.
25. Why does it take longer to cook pasta properly in boiling water in Denver (the “mile high city”) than it does in
New York City?
E.25 Water boils at a lower temperature in Denver.
26. You’re floating outside the space station when your fellow astronaut tosses you a cold bottle of mineral water. It’s
outside your space suit but you open it anyway. It immediately begins to boil. Why?
E.26 With no atmospheric pressure to crush them, any bubbles of steam that form in the cold water grow by
evaporation and expand. The water boils even though it’s cold.
27. The molecules of antifreeze dissolve easily in water. Why does adding antifreeze to the water in a car radiator keep
the water from freezing in the winter and boiling in the summer?
E.27 Anything dissolved in water stabilizes the water so that it has trouble freezing or evaporating.
28. When the outside temperature is –2 °C (29 °F), you can melt the ice on your front walk by sprinkling salt on it. But
if you are out of salt, will baking soda do?
E.28 Yes, anything that dissolves in water will do.
29. A quality perfume is a mixture of essential oils and has a scent that changes with time and skin temperature.
Explain the gradual change of the perfume’s scent with time in terms of evaporation.
E.29 Different chemicals leave the liquid perfume at different rates, so some evaporate away earlier than others.
Higher skin temperature promotes evaporation, even in the slower leaving chemicals.
30. An icy sidewalk gradually loses its ice even when the weather stays cold. Where does the ice go?
E.30 It sublimes and the resulting water vapor is carried off by the air.
31. How would you estimate the temperature of a glowing coal in a fireplace?
E.31 From the color of its light; whiter is hotter.
32. A lightbulb burns out when a small portion of its thinning filament overheats and vaporizes. Why is this event
accompanied by a bright flash of blue-white light?
E.32 As the thin spot in the filament overheats and burns out, its blackbody spectrum shifts more and more
toward the blue end of the spectrum. At its highest temperature, the filament emits white or even bluewhite light.
33. The strongest evidence for the Big Bang theory of the origin of the universe is the thermal radiation emitted by that
explosion. This radiation has cooled over the years to only 3 K and is now mostly microwaves. Why should 3 K
thermal radiation be microwaves?
E.33 The cooler the object, the longer the wavelengths of its blackbody spectrum. The spectrum of a very cold
object peaks in the microwave portion of the electromagnetic spectrum.
34. You have a table lamp with a dimmer switch. The dimmer allows you to adjust the temperature and brightness of
the lamp’s incandescent bulb from very dim red up to brilliant yellow-white. How does the bulb’s energy efficiency,
the amount of visible light it produces per unit of power consumed, depend on the dimmer’s setting?
E.34 The lower the dimmer setting, the smaller the fraction of the electric power consumed by the bulb that is
converted into visible light.
E.34 As you turn down the dimmer, the filament becomes cooler and the fraction of its thermal radiation that is
visible grows smaller. At the lowest setting, the bulb emits mostly invisible infrared light.
35. When you operate a 50-100-150 W three-way bulb at its 50-W setting, it emits yellow-white light. When you use a
dimmer to operate a regular 150-W bulb on only 50 W of electric power, it emits orangish light. Explain the difference.
E.35 In a three-way bulb, any filament that’s on is at full temperature. A dimmed bulb’s filament operates below
full temperature.
36. Astronomers can tell the surface temperature of a distant star without visiting it. How is this done?
E.36 Since stars emit primarily blackbody radiation, the astronomers can simply observe the spectrum of light
coming from a star and know how hot its surface is.
37. A doctor can study a patient’s circulation by imaging the infrared light emitted by the patient’s skin. Tissue with
poor blood flow is relatively cool. What changes in the infrared emissions would indicate such a cool spot?
E.37 Cooler skin emits dimmer infrared with longer wavelengths.
38. The filament of an incandescent bulb is quite small, yet it emits as much as 100 W of thermal radiation. That’s
almost as much as your whole body emits. What accounts for the strength of the filament’s thermal radiation?
E.38 The temperature of the filament is roughly 10 times that of your body (on an absolute scale), so it radiates
approximately 10,000 times as much thermal radiation per unit of surface area. That amount is the
prediction of the Stefan-Boltzmann law.
39. Why are concrete sidewalks divided into individual squares rather than being left as continuous concrete strips?
E.39 The sidewalk has a different coefficient of volume expansion than the ground and would break when the
temperature changed.
40. If you were laying steel track for a railroad, what influence would thermal expansion have on your work?
E.40 You would have to accommodate any differences in thermal expansion between the steel track and the
ground. If you forgot to do this, the track would buckle or break when the temperature changed and two
changed lengths by different amounts.
41. A difficult-to-open jar may open easily after being run under hot water for a moment. Explain.
E.41 The metal lid has a larger coefficient of volume expansion than the glass jar, and it pulls away from the jar
when you heat them both.
42. Why do bridges have special gaps at their ends to allow them to change lengths?
E.42 Changes in temperature cause a bridges length to increase or decrease and its mountings must allow it to
change length without buckling or breaking.