ConcepTest 16.1Degrees

1
ConcepTest 16.1 Degrees
1) one Celsius degree 2) one Kelvin
degree 3) one Fahrenheit degree 4) both one
Celsius degree and one Kelvin degree 5) both
one Fahrenheit degree and one Celsius degree
Which is the largest unit one Celsius degree,
one Kelvin degree, or one Fahrenheit degree?
2
ConcepTest 16.1 Degrees
1) one Celsius degree 2) one Kelvin
degree 3) one Fahrenheit degree 4) both one
Celsius degree and one Kelvin degree 5) both
one Fahrenheit degree and one Celsius degree
Which is the largest unit one Celsius degree,
one Kelvin degree, or one Fahrenheit degree?
The Celsius degree and the Kelvin degree are the
same size. The scales only differ by an offset,
not by the size of the degree unit. For
Fahrenheit, there are 180 degrees between boiling
and freezing (212F32F). For Celsius, there
are 100 degrees between the same points, so the
Celsius (and Kelvin) degrees must be larger.
3
ConcepTest 16.2 Freezing Cold
1) yes, at 0 C 2) yes, at -273 C 3) yes,
at 0 K 4) no
It turns out that 40C is the same temperature
as 40F. Is there a temperature at which the
Kelvin and Celsius scales agree?
4
ConcepTest 16.2 Freezing Cold
1) yes, at 0 C 2) yes, at -273 C 3) yes,
at 0 K 4) no
It turns out that 40C is the same temperature
as 40F. Is there a temperature at which the
Kelvin and Celsius scales agree?
The Celsius and Kelvin scales differ only by an
offset, which is 273 degrees. Therefore, a
temperature on one scale can never match the same
numerical value on the other scale. The reason
that such agreement is possible for Celsius and
Fahrenheit is the fact that the actual degree
units have different sizes (recall the previous
question).
5
ConcepTest 16.3 Thermometers
1) the mercury contracts before the glass
contracts 2) the glass contracts before the
mercury contracts 3) the mercury contracts
before the glass expands 4) the glass expands
before the mercury expands 5) the mercury
expands before the glass contracts
You may notice that if a mercury-in-glass
thermometer is inserted into a hot liquid, the
mercury column first drops, and then later starts
to rise (as you expect). How do you explain this
drop?
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ConcepTest 16.3 Thermometers
1) the mercury contracts before the glass
contracts 2) the glass contracts before the
mercury contracts 3) the mercury contracts
before the glass expands 4) the glass expands
before the mercury expands 5) the mercury
expands before the glass contracts
You may notice that if a mercury-in-glass
thermometer is inserted into a hot liquid, the
mercury column first drops, and then later starts
to rise (as you expect). How do you explain this
drop?
The hot liquid touches the glass first, so
initially the glass expands slightly. This
increases the volume inside the glass, and so the
mercury level drops slightly. Once the mercury
heats up, it begins to expand and then the
characteristic rise in the mercury column
follows, indicating the increase in temperature
that you expected to measure.
Follow-up Is it possible to have the mercury
first rise and later drop?
7
ConcepTest 16.4 Glasses
1) run hot water over them both 2) put hot water
in the inner one 3) run hot water over the outer
one 4) run cold water over them both 5) break the
glasses

• Two drinking glasses are stuck, one inside the
  other. How would you get them unstuck?

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ConcepTest 16.4 Glasses
1) run hot water over them both 2) put hot water
in the inner one 3) run hot water over the outer
one 4) run cold water over them both 5) break the
glasses

• Two drinking glasses are stuck, one inside the
  other. How would you get them unstuck?

Running hot water only over the outer glass will
allow the outer one to expand, while the inner
glass remains relatively unchanged. This should
loosen the outer glass and free it.
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ConcepTest 16.5a Steel Expansion I
A steel tape measure is marked such that it
gives accurate length measurements at room
temperature. If the tape measure is used outside
on a very hot day, how will its length
measurements be affected?
1) measured lengths will be too small 2)
measured lengths will still be accurate 3)
measured lengths will be too big
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ConcepTest 16.5a Steel Expansion I
A steel tape measure is marked such that it
gives accurate length measurements at room
temperature. If the tape measure is used outside
on a very hot day, how will its length
measurements be affected?
1) measured lengths will be too small 2)
measured lengths will still be accurate 3)
measured lengths will be too big
The tape measure will expand, so its markings
will spread out farther than the correct amount.
When it is laid down next to an object of fixed
length, you will read too few markings for that
given length, so the measured length will be too
small.
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ConcepTest 16.5b Steel Expansion II

• Metals such as brass expand when heated. The
  thin brass plate in the movie has a circular hole
  in its center. When the plate is heated, what
  will happen to the hole?

1) gets larger 2) gets smaller 3) stays the
same 4) vanishes
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ConcepTest 16.5b Steel Expansion II

• Metals such as brass expand when heated. The
  thin brass plate in the movie has a circular hole
  in its center. When the plate is heated, what
  will happen to the hole?

1) gets larger 2) gets smaller 3) stays the
same 4) vanishes
Imagine drawing a circle on the plate. This
circle will expand outward along with the rest of
the plate. Now replace the circle with the
hole, and you can see that the hole will expand
outward as well. Note that the material does
NOT expand inward to fill the hole!!
13
ConcepTest 16.6a Steel Ring I

• A steel ring stands on edge with a rod of some
  material inside. As this system is heated, for
  which of the following rod materials will the rod
  eventually touch the top of the ring?

1) aluminum 2) steel 3) glass 4) aluminum and
steel 5) all three
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ConcepTest 16.6a Steel Ring I

• A steel ring stands on edge with a rod of some
  material inside. As this system is heated, for
  which of the following rod materials will the rod
  eventually touch the top of the ring?

1) aluminum 2) steel 3) glass 4) aluminum and
steel 5) all three
Aluminum is the only material that has a larger
b value than the steel ring, so that means that
the aluminum rod will expand more than steel
ring. Thus, only in that case does the rod have
a chance of reaching the top of the steel ring.
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ConcepTest 16.6b Steel Ring II

• You want to take apart a couple of aluminum
  parts held together by steel screws, but the
  screws are stuck. What should you do?

1) heat the thing up 2) cool the thing down 3)
blow the thing up
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ConcepTest 16.6b Steel Ring II

• You want to take apart a couple of aluminum
  parts held together by steel screws, but the
  screws are stuck. What should you do?

1) heat the thing up 2) cool the thing down 3)
blow the thing up
Since aluminum has a larger b value, that means
aluminum expands more than steel. Thus, by
heating the part, the aluminum holes will expand
faster than the steel screws and the screws will
come loose.
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ConcepTest 16.8a Thermal Contact I
Two objects are made of the same material, but
have different masses and temperatures. If the
objects are brought into thermal contact, which
one will have the greater temperature change?
1) the one with the higher initial
temperature 2) the one with the lower initial
temperature 3) the one with the greater
mass 4) the one with the smaller mass 5) the
one with the higher specific heat
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ConcepTest 16.8a Thermal Contact I
Two objects are made of the same material, but
have different masses and temperatures. If the
objects are brought into thermal contact, which
one will have the greater temperature change?
1) the one with the higher initial
temperature 2) the one with the lower initial
temperature 3) the one with the greater
mass 4) the one with the smaller mass 5) the
one with the higher specific heat
Since the objects are made of the same material,
the only difference between them is their mass.
Clearly, the object with less mass will change
temperature more easily since not much material
is there (compared to the more massive object).
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ConcepTest 16.8b Thermal Contact II
Two different objects receive the same amount of
heat. Which of the following choices is NOT a
reason why the objects may have different
temperature changes?
1) they have different initial temperatures 2)
they have different masses 3) they have
different specific heats
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ConcepTest 16.8b Thermal Contact II
Two different objects receive the same amount of
heat. Which of the following choices is NOT a
reason why the objects may have different
temperature changes?
1) they have different initial temperatures 2)
they have different masses 3) they have
different specific heats
Since Q m c DT and the objects received the
same amount of heat, the only other factors are
the masses and the specific heats. While the
initial temperature is certainly relevant for
finding the final temperature, it does not have
any effect on the temperature change DT.
21
ConcepTest 16.9 Two Liquids

• Two equal-mass liquids, initially at the same
  temperature, are heated for the same time over
  the same stove. You measure the temperatures and
  find that one liquid has a higher temperature
  than the other. Which liquid has a higher
  specific heat?

1) the cooler one 2) the hotter one 3) both
the same
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ConcepTest 16.9 Two Liquids

• Two equal-mass liquids, initially at the same
  temperature, are heated for the same time over
  the same stove. You measure the temperatures and
  find that one liquid has a higher temperature
  than the other. Which liquid has a higher
  specific heat?

1) the cooler one 2) the hotter one 3) both
the same
Both liquids had the same increase in internal
energy, because the same heat was added. But
the cooler liquid had a lower temperature change.
Since Q mcDT, if Q and m are both the same
and DT is smaller, then c (specific heat) must be
bigger.
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ConcepTest 16.10a Night on the Field
The specific heat of concrete is greater than
that of soil. A baseball field (with real soil)
and the surrounding parking lot are warmed up
during a sunny day. Which would you expect to
cool off faster in the evening when the sun goes
down?
1) the concrete parking lot 2) the baseball
field 3) both cool off equally fast
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ConcepTest 16.10a Night on the Field
The specific heat of concrete is greater than
that of soil. A baseball field (with real soil)
and the surrounding parking lot are warmed up
during a sunny day. Which would you expect to
cool off faster in the evening when the sun goes
down?
1) the concrete parking lot 2) the baseball
field 3) both cool off equally fast
The baseball field, with the lower specific
heat, will change temperature more readily, so it
will cool off faster. The high specific heat of
concrete allows it to retain heat better and so
it will not cool off so quickly it has a higher
thermal inertia.
25
ConcepTest 16.10b Night on the Beach

• Water has a higher specific heat than sand.
  Therefore, on the beach at night, breezes would
  blow

(1) from the ocean to the beach (2) from the
beach to the ocean (3) either way, makes no
difference
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ConcepTest 16.10b Night on the Beach

• Water has a higher specific heat than sand.
  Therefore, on the beach at night, breezes would
  blow

(1) from the ocean to the beach (2) from the
beach to the ocean (3) either way, makes no
difference

• Daytime
• sun heats both the beach and the water
• beach heats up faster
• warmer air above beach rises
• cooler air from ocean moves in underneath
• breeze blows ocean ? land

csand lt cwater

• Nighttime
• sun has gone to sleep
• beach cools down faster
• warmer air is now above the ocean
• cooler air from beach moves out to the ocean
• breeze blows land ? ocean

27
ConcepTest 16.11 Calorimetry
(1) 0 oC (2) 20 oC (3) 50 oC (4) 80
oC (5) 100 oC

• 1 kg of water at 100 oC is poured into a bucket
  that contains 4 kg of water at 0 oC. Find the
  equilibrium temperature (neglect the influence of
  the bucket).

28
ConcepTest 16.11 Calorimetry
(1) 0 oC (2) 20 oC (3) 50 oC (4) 80
oC (5) 100 oC

• 1 kg of water at 100 oC is poured into a bucket
  that contains 4 kg of water at 0 oC. Find the
  equilibrium temperature (neglect the influence of
  the bucket).

Since the cold water mass is greater, it will
have a smaller temperature change! The masses
of cold/hot have a ratio of 41, so the
temperature change must have a ratio of 14
(cold/hot).
Q1 Q2 m1cDT1 m2cDT2 DT1 / DT2 m2 / m1
29
ConcepTest 17.1a Nitrogen and Oxygen I
Which has more molecules a mole of nitrogen
(N2) gas or a mole of oxygen (O2) gas?
1) oxygen 2) nitrogen 3) both the same
30
ConcepTest 17.1a Nitrogen and Oxygen I
Which has more molecules a mole of nitrogen
(N2) gas or a mole of oxygen (O2) gas?
1) oxygen 2) nitrogen 3) both the same
A mole is defined as a quantity of gas molecules
equal to Avogadros number (6.02 ? 1023). This
value is independent of the type of gas.
31
ConcepTest 17.1b Nitrogen and Oxygen II
Which weighs more a mole of nitrogen (N2) gas
or a mole of oxygen (O2) gas?
1) oxygen 2) nitrogen 3) both the same
32
ConcepTest 17.1b Nitrogen and Oxygen II
Which weighs more a mole of nitrogen (N2) gas
or a mole of oxygen (O2) gas?
1) oxygen 2) nitrogen 3) both the same
The oxygen molecules have a molecular mass of
32, while the nitrogen molecules have a molecular
mass of 28.
Follow-up Which one will take up more space?
33
ConcepTest 17.2a Ideal Gas Law I

• Two identical cylinders at the same temperature
  contain the same gas. If A contains three times
  as much gas as B, which cylinder has the higher
  pressure?

1) cylinder A 2) cylinder B 3) both the same 4)
it depends on temp. T
34
ConcepTest 17.2a Ideal Gas Law I

• Two identical cylinders at the same temperature
  contain the same gas. If A contains three times
  as much gas as B, which cylinder has the higher
  pressure?

1) cylinder A 2) cylinder B 3) both the same 4)
it depends on temp. T
Ideal gas law PV nRT Solve for pressure
P nRT / V For constant V and T, the one with
more gas (the larger value of n) has the
higher pressure P.
35
ConcepTest 17.2b Ideal Gas Law II

• Two identical cylinders at the same pressure
  contain the same gas. If A contains three times
  as much gas as B, which cylinder has the higher
  temperature?

1) cylinder A 2) cylinder B 3) both the same 4)
it depends on the pressure P
36
ConcepTest 17.2b Ideal Gas Law II

• Two identical cylinders at the same pressure
  contain the same gas. If A contains three times
  as much gas as B, which cylinder has the higher
  temperature?

1) cylinder A 2) cylinder B 3) both the same 4)
it depends on the pressure P
Ideal gas law PV nRT Solve for
temperature T PV / nR For constant V and P,
the one with less gas (the smaller value of n)
has the higher temperature T.
37
ConcepTest 17.2c Ideal Gas Law III
Two identical cylinders at the same temperature
contain the same gas. If B has twice the volume
and half the number of moles as A, how does the
pressure in B compare with the pressure in A?
1) PB 1/2 PA 2) PB 2 PA 3) PB 1/4
PA 4) PB 4 PA 5) PB PA
38
ConcepTest 17.2c Ideal Gas Law III
Two identical cylinders at the same temperature
contain the same gas. If B has twice the volume
and half the number of moles as A, how does the
pressure in B compare with the pressure in A?
1) PB 1/2 PA 2) PB 2 PA 3) PB 1/4
PA 4) PB 4 PA 5) PB PA
Ideal gas law PV nRT Since B has a
factor of twice the volume, it has a factor of
two less the pressure. But B also has half the
amount of gas, so that is another factor of two
reduction in pressure. Thus, B must have only
1/4 the pressure of A.
39
ConcepTest 17.3 Soda Bottle

• A plastic soda bottle is empty and sits out in
  the sun, heating the air inside. Now you put the
  cap on tightly and put the bottle in the fridge.
  What happens to the bottle as it cools?

1) it expands and may burst 2) it does not change
3) it contracts and the sides collapse inward 4)
it is too dark in the fridge to tell
40
ConcepTest 17.3 Soda Bottle

• A plastic soda bottle is empty and sits out in
  the sun, heating the air inside. Now you put the
  cap on tightly and put the bottle in the fridge.
  What happens to the bottle as it cools?

1) it expands and may burst 2) it does not change
3) it contracts and the sides collapse inward 4)
it is too dark in the fridge to tell
The air inside the bottle is warm, due to
heating by the sun. When the bottle is in the
fridge, the air cools. As the temperature
drops, the pressure in the bottle also drops.
Eventually, the pressure inside is sufficiently
lower than the pressure outside (atmosphere) to
begin to collapse the bottle.
41
ConcepTest 17.4 Balloon in Freezer
1) it increases 2) it does not change 3) it
decreases

• What happens to the volume of a balloon if you
  put it in the freezer?

42
ConcepTest 17.4 Balloon in Freezer
1) it increases 2) it does not change 3) it
decreases

• What happens to the volume of a balloon if you
  put it in the freezer?

According to the Ideal Gas Law, when the
temperature is reduced at constant pressure, the
volume is reduced as well. The volume of the
balloon therefore decreases.
Follow-up What happens to the volume when the
balloon rises in the air?
43
ConcepTest 17.5 Adding Heat
If you add some heat to a substance, is it
possible for the temperature of the substance to
remain unchanged?
1) yes 2) no
44
ConcepTest 17.5 Adding Heat
If you add some heat to a substance, is it
possible for the temperature of the substance to
remain unchanged?
1) yes 2) no
Yes, it is indeed possible for the temperature
to stay the same. This is precisely what occurs
during a phase change the added heat goes into
changing the state of the substance (from solid
to liquid or from liquid to gas) and does not go
into changing the temperature! Once the phase
change has been accomplished, then the
temperature of the substance will rise with more
added heat.
Follow-up Does that depend on the substance?
45
ConcepTest 17.6 Hot Potato
Will potatoes cook faster if the water is
boiling faster?
1) yes 2) no
46
ConcepTest 17.6 Hot Potato
Will potatoes cook faster if the water is
boiling faster?
1) yes 2) no
The water boils at 100 C and remains at that
temperature until all of the water has been
changed into steam. Only then will the steam
increase in temperature. Since the water stays
at the same temperature, regardless of how fast
it is boiling, the potatoes will not cook any
faster.
Follow-up How can you cook the potatoes faster?
47
ConcepTest 17.7 Water and Ice

• You put 1 kg of ice at 0oC together with 1 kg of
  water at 50oC. What is the final temperature?
• LF 80 cal/g
• cwater 1 cal/g oC

1) 0oC 2) between 0oC and 50oC 3) 50oC 4)
greater than 50oC
48
ConcepTest 17.7 Water and Ice

• You put 1 kg of ice at 0oC together with 1 kg of
  water at 50oC. What is the final temperature?
• LF 80 cal/g
• cwater 1 cal/g oC

1) 0oC 2) between 0oC and 50oC 3) 50oC 4)
greater than 50oC
How much heat is needed to melt the ice? Q
m Lf (1000g) ? (80 cal/g) 80,000 cal How
much heat can the water deliver by cooling from
50oC to 0oC? Q cwater m DT (1 cal/g oC) ?
(1000g) ? (50oC) 50,000 cal Thus, there is
not enough heat available to melt all the ice!!
Follow-up How much more water at 50oC would you
need?
49
ConcepTest 17.8 Ice and Steam

• You put 1 kg of ice at 0oC together with 1 kg of
  steam at 100oC. What is the final temperature?
• LF 80 cal/g, Lv 540 cal/g
• cwater 1 cal/g oC

1) between 0oC and 50oC 2) 50oC 3) between
50oC and 100oC 4) 100oC 5) greater than 100oC
50
ConcepTest 17.8 Ice and Steam

• You put 1 kg of ice at 0oC together with 1 kg of
  steam at 100oC. What is the final temperature?
• LF 80 cal/g, Lv 540 cal/g
• cwater 1 cal/g oC

1) between 0oC and 50oC 2) 50oC 3) between
50oC and 100oC 4) 100oC 5) greater than 100oC
How much heat is needed to melt the ice? Q
m Lf (1000g) ? (80 cal/g) 80,000 cal How
much heat is needed to raise the water
temperature to 100oC? Q cwater m DT (1
cal/g oC)?(1000g)?(100oC) 100,000 cal But if
all of the steam turns into water, that would
release 540,000 cal. Thus, some steam is left
over, and the whole mixture stays at 100oC.
Follow-up How much more ice would you need?
51
ConcepTest 17.9 Youre in Hot Water!
1) water 2) steam 3) both the same 4) it
depends...

• Which will cause more severe burns to your skin
  100 C water or 100 C steam?

52
ConcepTest 17.9 Youre in Hot Water!
1) water 2) steam 3) both the same 4) it
depends...

• Which will cause more severe burns to your skin
  100 C water or 100 C steam?

While the water is indeed hot, it releases only
1 cal/g of heat as it cools. The steam, however,
first has to undergo a phase change into water
and that process releases 540 cal/g, which is a
very large amount of heat. That immense release
of heat is what makes steam burns so dangerous.
53
ConcepTest 17.10 Spring Break

• You step out of a swimming pool on a hot day,
  where the air temperature is 90 F. Where will
  you feel cooler, in Phoenix (dry) or in
  Philadelphia (humid)?

1) equally cool in both places 2) Philadelphia
3) Phoenix
54
ConcepTest 17.10 Spring Break

• You step out of a swimming pool on a hot day,
  where the air temperature is 90 F. Where will
  you feel cooler, in Phoenix (dry) or in
  Philadelphia (humid)?

1) equally cool in both places 2) Philadelphia
3) Phoenix
In Phoenix, where the air is dry, more of the
water will evaporate from your skin. This is a
phase change, where the water must absorb the
heat of vaporization, which it takes from your
skin. That is why you feel cool as the water
evaporates.
55
ConcepTest 18.1 Free Expansion
1) increase 2) decrease 3) stay the same

• A free expansion occurs when a valve is opened
  allowing a gas to expand into a bigger container.
  In such an expansion the temperature of the gas
  will

56
ConcepTest 18.1 Free Expansion
1) increase 2) decrease 3) stay the same

• A free expansion occurs when a valve is opened
  allowing a gas to expand into a bigger container.
  In such an expansion the temperature of the gas
  will

In such a process W 0 there is no object to
move, Q 0 there is no heat exchange,
therefore ?U 0 by the First Law of
Thermodynamics. Thus, there is no change in the
temperature.
Free expansion is an irreversible process --- the
gas molecules have virtually no chance of
returning to the original state.
Free expansion is neither adiabatic nor
isothermal expansion, even though ?T and Q are
zero.
57
ConcepTest 18.2 Work

• In the closed thermodynamic cycle shown in the
  P-V diagram, the work done by the gas is

1) positive 2) zero 3) negative
58
ConcepTest 18.2 Work

• In the closed thermodynamic cycle shown in the
  P-V diagram, the work done by the gas is

1) positive 2) zero 3) negative
The gas expands at a higher pressure and
compresses at a lower pressure. In general,
clockwise positive work counter-clockwise
negative work.
59
ConcepTest 18.3 Heat Engine

• The heat engine below is

1) a reversible (Carnot) heat engine 2) an
irreversible heat engine 3) a hoax 4) none of
the above
60
ConcepTest 18.3 Heat Engine

• The heat engine below is

1) a reversible (Carnot) heat engine 2) an
irreversible heat engine 3) a hoax 4) none of
the above
Carnot e 1-TC/TH1-270/6000.55. But by
definition e 1-QL/QH 1 - 4000/80000.5,
smaller than Carnot e, thus irreversible.
Follow-up What would you need to change to make
it a Carnot engine?