# Exercises – Chapter 8

Mechanics

Oct 27, 2013 (4 years and 8 months ago)

102 views

Exercises

Chapter 8

1.

Drinking fountains that actively chill the water they serve can’t work without ventilation. They usually have louvers
on their sides so that air can flow through them. Why do they need this airflow?

E.1

They are heat pumps and tra
nsfer heat to the surrounding air.

2.

If you open the door of your refrigerator with the hope of cooling your room, you will find that the room’s
temperature actually increases somewhat. Why doesn’t the refrigerator remove heat from the room?

E.2

The refr
igerator doesn't eliminate thermal energy; it merely pumps it from one place to the other and
consumes ordered energy in the process. If you open the refrigerator, it will just move heat around the
room and produce additional thermal energy in the process.

The room will actually become warmer.

3.

The outdoor portion of a central air
-
conditioning unit has a fan that blows air across the condenser coils. If this fan
breaks, why won’t the air conditioner cool the house properly?

E.3

Without the fan, the air c
onditioner won’t be able to transfer its waste heat to the surrounding air. It will
stop pumping heat.

4.

If you block the outlet of a hand bicycle pump and push the handle inward to compress the air inside the pump, the
pump will become warmer. Why?

E.4

As you push the handle inward to compress the air, you do work on the handle and air. The air receives
this work and converts it into thermal energy within the air. The air becomes hotter.

5.

When the gas that now makes up the sun was compressed together
by gravity, what happened to the temperature of
that gas? Why?

E.5

Its temperature increased. Gravity did work on the gas, and this work appeared in the gas as thermal
energy. The gas became hot.

6.

Why is a car more likely to knock on a hot day than on a

cold day?

E.6

When a car engine compresses air that was already hot, the air's peak temperature is higher than it would
have been if it had started cold. Since knocking occurs when the compression process heats the fuel and air
mixture until it ignites, t
he higher the mixture's peak temperature, the more likely it is to knock.

7.

A soda siphon carbonates water by injecting carbon dioxide gas into it. The gas comes compressed in a small steel
container. As the gas leaves the container and pushes its way in
to the water, why does the container become cold?

E.7

The gas still in the container does work pushing the other gas into the siphon and uses up some of its
thermal energy. It cools.

8.

A high
-
flying airplane must compress the cold, rarefied outside air b
efore delivering it to the cabin. Why must this
air be air conditioned after the compression?

E.8

Compressing the outside air involves work and the air's temperature rises as a result of this work. The
temperature becomes high enough that the compressed ai
r must air
-
conditioned before it can be delivered
to the cabin.

9.

If you drop a glass vase on the floor, it will become fragments. If you drop those fragments on the floor, however,
they will not become a glass vase. Why not?

E.9

Though not forbidden by
the laws of motion, it’s extraordinarily unlikely for the vase fragments to
reassemble themselves.

10.

When you throw a hot rock into a cold puddle, what happens to the overall entropy of the system?

E.10

The entropy of the system goes up.

E.10

By letting
heat flow freely from a hot object to a cold object, you lower the entropy of the hot object but
raise the entropy of the cold object even more. The total entropy of the system increases.

11.

What prevents the bottom half of a glass of water from spontaneo
usly freezing while the top half becomes boiling
hot?

E.11

This uneven distribution of thermal energies is extraordinarily unlikely. It would violate the second law of
thermodynamics.

12.

Suppose someone claimed to have a device that could convert heat fro
m the room into electric power continuously.
You would know that this device was a fraud because it would violate the second law of thermodynamics. Explain.

E.12

Converting thermal energy continuously into electric energy involves an overall decrease in en
tropy (the
disorder thermal energy becomes ordered electric energy) and thus violates the second law of
thermodynamics.

13.

Why does snow blanket the ground almost uniformly rather than creating tall piles in certain areas and bare spots in
others?

E.13

Th
ough not forbidden by the laws of motion, such uneven distributions of snow are remarkably unlikely.

14.

If you transfer a glass baking dish from a hot oven to a cold basin of water, that dish will probably shatter. What
produces the ordered mechanical ene
rgy needed to tear the glass apart?

E.14

Heat flowing from the hot dish to the cold water provides the necessary entropy to allow some of that heat
to become mechanical work.

15.

Freezing and thawing cycles tend to damage road pavement during the winter, c
reating potholes. What provides the
mechanical work that breaks up the pavement?

E.15

Heat flowing into or out of the pavement during weather changes allows the pavement to do work as it
tears itself apart.

16.

The air near a woodstove circulates throughou
t the room. What provides the energy needed to keep the air moving?

E.16

Heat flowing from the hot stove to the cold room provides the necessary entropy to allow some of that heat
to become mechanical work.

17.

Winds are driven by differences in temperatur
e at the earth’s surface. Air rises over hot spots and descends over
cold spots, forming giant convection cells of circulating air. Near the ground, winds blow from the cold spots toward
the hot spots. Explain how the atmosphere is acting as a heat engine.

E.17

As heat flows from the hot spots to the cold spots, by way of huge convection cells, some heat is becoming
ordered kinetic energy.

18.

Hurricanes are giant heat engines powered by the thermal energy in warm ocean regions and the order in colder
surro
unding areas. Why are hurricanes most violent when they form over regions of unusually hot water at the end of
summer?

E.18

The hotter the water, the more thermal energy can be harnessed and the greater the temperature difference
between the hot water and
the colder surrounding, the more efficient the hurricane is at converting that
thermal energy into mechanical work.

19.

On a clear sunny day, the ground is heated uniformly and there is very little wind. Use the second law of
thermodynamics to explain this

absence of wind.

E.19

Without temperature differences, heat won’t naturally flow. Such heat flow is what powers a heat engine
such as the winds.

20.

A plant is a heat engine that operates on sunlight flowing from the hot sun to the cold earth. The plant i
s a highly
ordered system with relatively low entropy. Why doesn’t the plant’s growth violate the second law of thermodynamics?

E.20

Heat flowing from the hot sun to the colder earth provides the necessary entropy to allow the plant to
harness some of that

heat to make it grow.

21.

A diesel engine burns its fuel at a higher temperature than a gasoline engine. Why does this difference allow the
diesel engine to be more efficient at converting the fuel’s energy into work?

E.21

The hotter the burned gas, the l
arger the fraction of heat that can be converted to work as it flows to the
outdoor air.

22.

A chemical rocket is a heat engine, propelled forward by its hot exhaust plume. The hotter the fire inside the
chemical rocket, the more efficient the rocket can b
e. Explain this fact in terms of the second law of thermodynamics.

E.22

The hotter the hot region and the colder the cold region, the larger the fraction of heat flowing from the hot
region to the cold region that can be diverted and converted into useful
work.

23.

An acquaintance claims to have built a gasoline
-
burning car that doesn’t release any heat to its surroundings. Use
the second law of thermodynamics to show that this claim is impossible.

E.23

Converting burned fuel entirely into work would violat
e the second law of thermodynamics.