**termodinamica**

termodinamica

(Parte **1** de 2)

SOLUTION MANUAL SI UNIT PROBLEMS CHAPTER 2

FUNDAMENTALS of

Thermodynamics

Sixth Edition SONNTAG ! BORGNAKKE ! VAN WYLEN

SUBSECTION | PROB NO. |

Concept-Study Guide Problems | 1-2 |

Properties and Units | 23-26 |

Force and Energy | 27-37 |

Specific Volume | 38-43 |

Pressure | 4-57 |

Manometers and Barometers | 58-76 |

Temperature | 7-80 |

Correspondence table Review Problems 81-86

Sonntag, Borgnakke and van Wylen

Correspondence table

edition | Sonntag/Borgnakke/Wylen |

CHAPTER 2 6 th

The correspondence between the problem set in this sixth edition versus the problem set in the 5'th edition text. Problems that are new are marked new and those that are only slightly altered are marked as modified (mod).

Study guide problems 2.1-2.2 and 2.23-2.26 are all new problems.

New 5 th

Ed. New 5 th

Ed. New 5 th Ed.

English Unit Problems

New 5 th

Ed. SI New 5 th Ed. SI

96 42E 42 |

Design and Open ended problems 106-116 are from 5 th edition problems 2.50-

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Concept-Study Guide Problems

Make a control volume around the turbine in the steam power plant in Fig. 1.1 and list the flows of mass and energy that are there.

Solution: We see hot high pressure steam flowing in at state 1 from the steam drum through a flow control (not shown). The steam leaves at a lower pressure to the condenser (heat exchanger) at state 2. A rotating shaft gives a rate of energy (power) to the electric generator set.

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Make a control volume around the whole power plant in Figure 1.2 and with the help of Fig. 1.1 list what flows of mass and energy are in or out and any storage of energy. Make sure you know what is inside and what is outside your chosen C.V.

Solution:

Smoke stack

Boiler building

Coal conveyor system

Dock Turbine house

Storage gypsum

Coal storage flue gas

Underground power cable Welectrical

Hot water District heating

Coal m

Flue gas

Storage for later

Gypsum, fly ash, slag transport out:

Cold returnm

Combustion air

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Make a control volume that includes the steam flow around in the main turbine loop in the nuclear propulsion system in Fig.1.3. Identify mass flows (hot or cold) and energy transfers that enter or leave the C.V.

Solution:

Electric power gen.

6 7 Cooling by seawater

Condensate to steam gen. cold

Hot steam from generator

The electrical power also leaves the C.V. to be used for lights, instruments and to charge the batteries.

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Take a control volume around your kitchen refrigerator and indicate where the components shown in Figure 1.6 are located and show all flows of energy transfer.

Solution:

The valve and the cold line, the evaporator, is inside close to the inside wall and usually a small blower distributes cold air from the freezer box to the refrigerator room.

Q. Q leak

The black grille in the back or at the bottom is the condenser that gives heat to the room air.

The compressor sits at the bottom.

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An electric dip heater is put into a cup of water and heats it from 20oC to 80oC. Show the energy flow(s) and storage and explain what changes.

Solution:

Electric power is converted in the heater element (an electric resistor) so it becomes hot and gives energy by heat transfer to the water. The water heats up and thus stores energy and as it is warmer than the cup material it heats the cup which also stores some energy. The cup being warmer than the air gives a smaller amount of energy (a rate) to the air as a heat loss.

Welectric

Qloss C B

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Separate the list P, F, V, v, ", T, a, m, L, t and V into intensive, extensive and nonproperties.

Solution:

Extensive properties scales with mass: | V, m |

Non-properties: | F, a, L, t, V |

Intensive properties are independent upon mass: P, v, ", T

Comment: You could claim that acceleration a and velocity V are physical properties for the dynamic motion of the mass, but not thermal properties.

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An escalator brings four people of total 300 kg, 25 m up in a building. Explain what happens with respect to energy transfer and stored energy.

Solution:

The four people (300 kg) have their potential energy raised, which is how the energy is stored. The energy is supplied as electrical power to the motor that pulls the escalator with a cable.

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Water in nature exist in different phases like solid, liquid and vapor (gas). Indicate the relative magnitude of density and specific volume for the three phases.

Solution:

Values are indicated in Figure 2.7 as density for common substances. More accurate values are found in Tables A.3, A.4 and A.5

Water as solid (ice) has density of around 900 kg/m3 Water as liquid has density of around 1000 kg/m3 Water as vapor has density of around 1 kg/m3 (sensitive to P and T)

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Is density a unique measure of mass distribution in a volume? Does it vary? If so, on what kind of scale (distance)?

Solution:

Density is an average of mass per unit volume and we sense if it is not evenly distributed by holding a mass that is more heavy in one side than the other. Through the volume of the same substance (say air in a room) density varies only little from one location to another on scales of meter, cm or m. If the volume you look at has different substances (air and the furniture in the room) then it can change abruptly as you look at a small volume of air next to a volume of hardwood.

Finally if we look at very small scales on the order of the size of atoms the density can vary infinitely, since the mass (electrons, neutrons and positrons) occupy very little volume relative to all the empty space between them.

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2.10 Density of fibers, rock wool insulation, foams and cotton is fairly low. Why is that?

Solution:

All these materials consists of some solid substance and mainly air or other gas. The volume of fibers (clothes) and rockwool that is solid substance is low relative to the total volume that includes air. The overall density is

" = m msolid + mair

Vsolid + Vair where most of the mass is the solid and most of the volume is air. If you talk about the density of the solid only, it is high.

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2.1 How much mass is there approximately in 1 L of mercury (Hg)? Atmospheric air?

Solution:

A volume of 1 L equals 0.001 m3, see Table A.1. From Figure 2.7 the density is in the range of 10 0 kg/m3 so we get m = "V = 10 0 kg/m3 # 0.001 m3 = 10 kg A more accurate value from Table A.4 is " = 13 580 kg/m3.

For the air we see in Figure 2.7 that density is about 1 kg/m3 so we get m = "V = 1 kg/m3 # 0.001 m3 = 0.001 kg A more accurate value from Table A.5 is " = 1.17 kg/m3

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2.12 Can you carry 1 m3 of liquid water?

Solution:

The density of liquid water is about 1000 kg/m3 from Figure 2.7, see also Table A.3. Therefore the mass in one cubic meter is and we can not carry that in the standard gravitational field.

2.13 A manometer shows a pressure difference of 1 m of liquid mercury. Find $P in kPa.

Solution:

Hg : L = 1 m; " = 13 580 kg/m3 from Table A.4 (or read Fig 2.7) |

The pressure difference $P balances the column of height L so from Eq.2.2

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2.14 You dive 5 m down in the ocean. What is the absolute pressure there?

Solution:

The pressure difference for a column is from Eq.2.2 and the density of water is from Table A.4.

= 997 kg/m3 # 9.81 m/s2 # 5 m | |

= 48 903 Pa = 48.903 kPa |

$P = "gH

Pocean= P0 + $P

= 101.325 + 48.903 |

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2.15 What pressure difference does a 10 m column of atmospheric air show?

Solution: The pressure difference for a column is from Eq.2.2

$P = "gH

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2.16 The pressure at the bottom of a swimming pool is evenly distributed. Suppose we look at a cast iron plate of 7272 kg lying on the ground with an area of 100 m2. What is the average pressure below that? Is it just as evenly distributed?

Solution: The pressure is force per unit area from page 25:

P = F/A = mg/A = 7272 kg # (9.81 m/s2) / 100 m2 = 713.4 Pa |

The iron plate being cast can be reasonable plane and flat, but it is stiff and rigid. However, the ground is usually uneven so the contact between the plate and the ground is made over an area much smaller than the 100 m2. Thus the local pressure at the contact locations is much larger than the quoted value above.

The pressure at the bottom of the swimming pool is very even due to the ability of the fluid (water) to have full contact with the bottom by deforming itself. This is the main difference between a fluid behavior and a solid behavior.

Iron plate |

Ground

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A laboratory room keeps a vacuum of 0.1 kPa. What net force does that put on the door of size 2 m by 1 m?

Solution:

The net force on the door is the difference between the forces on the two sides as the pressure times the area

F = Poutside A – Pinside A = $P A = 0.1 kPa # 2 m # 1 m = 200 N

Remember that kPa is kN/m2 |

Pabs = Po - $P

$P = 0.1 kPa

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A tornado rips off a 100 m2 roof with a mass of 1000 kg. What is the minimum vacuum pressure needed to do that if we neglect the anchoring forces?

Solution:

The net force on the roof is the difference between the forces on the two sides as the pressure times the area

F = Pinside A – PoutsideA = $P A That force must overcome the gravitation mg, so the balance is

$P A = mg

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2.19 What is a temperature of –5oC in degrees Kelvin?

Solution:

The offset from Celsius to Kelvin is 273.15 K, so we get

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2.20 What is the smallest temperature in degrees Celsuis you can have? Kelvin?

Solution:

The lowest temperature is absolute zero which is at zero degrees Kelvin at which point the temperature in Celsius is negative

TK = 0 K = %273.15 oC

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Density of liquid water is " = 1008 – T/2 [kg/m3] with T in oC. If the temperature increases 10oC how much deeper does a 1 m layer of water become?

Solution:

The density change for a change in temperature of 10oC becomes

$" = – $T/2 = –5 kg/m3 from an ambient density of

Assume the area is the same and the mass is the same | m = "V = "AH, then we |

have

$m = 0 = V$" + "$V | & $V = - V$"/" |

and the change in the height is

barely measurable.

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2.2 Convert the formula for water density in problem 21 to be for T in degrees Kelvin.

" = 1008 – TC/2 | [kg/m3] |

Solution:

We need to express degrees Celsius in degrees Kelvin

TC = TK – 273.15 and substitute into formula

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Properties and units

A steel cylinder of mass 2 kg contains 4 L of liquid water at 25oC at 200 kPa. Find the total mass and volume of the system. List two extensive and three intensive properties of the water

Solution:

Density of steel in Table A.3: | " = 7820 kg/m3 |

Volume of steel: | V = m/" = |

2 kg

Mass of water: | m = "V = 997 kg/m3 #0.004 m3 = 3.988 kg |

Total mass: | m = msteel + mwater = 2 + 3.988 = 5.988 kg |

Total volume: | V = Vsteel + Vwater = 0.0 256 + 0.004 |

Density of water in Table A.4: " = 997 kg/m3 = 0.004 256 m3 = 4.26 L

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An apple “weighs” 80 g and has a volume of 100 cm3 in a refrigerator at 8oC. What is the apple density? List three intensive and two extensive properties of the apple.

Solution:

" = m

0.0001 kg

Intensive

m3 ; | v = |

T = 8'C; P = 101 kPa

Extensive m = 80 g = 0.08 kg

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One kilopond (1 kp) is the weight of 1 kg in the standard gravitational field. How many Newtons (N) is that?

F = ma = mg 1 kp = 1 kg # 9.807 m/s2 = 9.807 N

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A pressurized steel bottle is charged with 5 kg of oxygen gas and 7 kg of nitrogen gas. How many kmoles are in the bottle?

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Force and Energy

2.27 The “standard” acceleration (at sea level and 45° latitude) due to gravity is

9.80665 m/s2. What is the force needed to hold a mass of 2 kg at rest in this gravitational field ? How much mass can a force of 1 N support ?

Solution:

ma = 0 = ( F = F - mg

F = mg => |

F g

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A force of 125 N is applied to a mass of 12 kg in addition to the standard gravitation. If the direction of the force is vertical up find the acceleration of the mass.

Solution:

Fup = ma = F – mg a = F – mg m = m – g =

12 – 9.807

= 0.61 ms-2 |

F g

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A model car rolls down an incline with a slope so the gravitational “pull” in the direction of motion is one third of the standard gravitational force (see Problem 2.1). If the car has a mass of 0.45 kg find the acceleration.

Solution:

ma = ( F = mg / 3 a = mg / 3m = g/3

= 9.80665 / 3 = 3.27 m/s2 |

This acceleration does not depend on the mass of the model car.

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2.30 When you move up from the surface of the earth the gravitation is reduced as g =

9.807 % 3.32 # 10-6 z, with z as the elevation in meters. How many percent is the weight of an airplane reduced when it cruises at 1 0 m?

Solution:

Wo = m go ; WH = m gH |

WH/Wo = gH/go = 9.7705

Reduction = 1 – 0.9963 = 0.0037 | or 0.37% |

i.e. we can neglect that for most application

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A car drives at 60 km/h and is brought to a full stop with constant deceleration in 5 seconds. If the total car and driver mass is 1075 kg find the necessary force.

Solution:

Acceleration is the time rate of change of velocity.

a = dV

dt = ma = ( F ; Fnet = ma = 1075 kg # 3.3 m/s2 = 3583 N

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A car of mass 1775 kg travels with a velocity of 100 km/h. Find the kinetic energy. How high should it be lifted in the standard gravitational field to have a potential energy that equals the kinetic energy?

Solution: Standard kinetic energy of the mass is

Standard potential energy is POT = mgh h = ½ m V2 / mg = 684 800

1775 # 9.807 = 39.3 m

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A 1200-kg car moving at 20 km/h is accelerated at a constant rate of 4 m/s2 up to a speed of 75 km/h. What are the force and total time required?

Solution:

a = dV dt =

F = ma = 1200 kg # 4 m/s2 = 4800 N

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A steel plate of 950 kg accelerates from rest with 3 m/s2 for a period of 10s. What force is needed and what is the final velocity?

Solution:

Constant acceleration can be integrated to get velocity.

a = dV

=> / dV = / a dt | => $V = a $t |

=> | V = 30 m/s |

F = ma = 950 kg # 3 m/s2 = 2850 N |

$V = a $t = 3 m/s2 # 10 s = 30 m/s F

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A 15 kg steel container has 1.75 kilomoles of liquid propane inside. A force of 2 kN now accelerates this system. What is the acceleration?

Solution:

The molecular weight for propane is M = 4.094 from Table A.2. The force must accelerate both the container mass and the propane mass.

ma = ( F | & a = ( F / m |

a = 2000 N

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2.36 A bucket of concrete of total mass 200 kg is raised by a crane with an acceleration of 2 m/s2 relative to the ground at a location where the local gravitational acceleration is 9.5 m/s2. Find the required force.

Solution:

F = ma = Fup % mg

Fup = ma + mg = 200 ( 2 + 9.5 ) = 2300 N g

F up

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On the moon the gravitational acceleration is approximately one-sixth that on the surface of the earth. A 5-kg mass is “weighed” with a beam balance on the surface on the moon. What is the expected reading? If this mass is weighed with a spring scale that reads correctly for standard gravity on earth (see Problem 2.1), what is the reading?

Solution: Moon gravitation is: g = gearth/6 m m

Beam Balance Reading is 5 kg Spring Balance Reading is in kg units This is mass comparison Force comparison length 0 F 0 g

Reading will be 5

6 kg

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Specific Volume

A 5 m3 container is filled with 900 kg of granite (density 2400 kg/m3 ) and the rest of the volume is air with density 1.15 kg/m3. Find the mass of air and the overall (average) specific volume.

Solution:

mair = " V = "air ( Vtot % mgranite

= 1.15 [ 5 - |

v = V

m =

Comment: Because the air and the granite are not mixed or evenly distributed in the container the overall specific volume or density does not have much meaning.

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2.39 A tank has two rooms separated by a membrane. Room A has 1 kg air and volume

0.5 m3, room B has 0.75 m3 air with density 0.8 kg/m3. The membrane is broken and the air comes to a uniform state. Find the final density of the air.

(Parte **1** de 2)