Thermodynamics: Systems and Control Volumes

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Questions and Answers

What defines the specific volume of a substance?

  • The volume per unit mass (correct)
  • The total mass of the substance
  • The mass per unit volume
  • The density of the substance

Which of the following units is used for density in the SI system?

  • cm3/g
  • kg/m3 (correct)
  • lb/ft3
  • g/cm3

How is mass associated with a particular volume V determined?

  • By integrating density over the volume (correct)
  • By subtracting the density from the volume
  • By multiplying density with volume
  • By taking the density as a constant value

Why is the volume V9 considered the smallest volume for which matter can be a continuum?

<p>It has a large number of particles (A)</p> Signup and view all the answers

Which property varies from point to point within a system and is classified as intensive?

<p>Specific volume (B)</p> Signup and view all the answers

What is the SI unit for specific volume?

<p>m3/kg (C)</p> Signup and view all the answers

In certain applications, how might properties like specific volume be expressed?

<p>On a molar basis (A)</p> Signup and view all the answers

What is the significance of the mole in the context of specific volume?

<p>It equals the molecular weight of a substance (A)</p> Signup and view all the answers

What happens to the composition of the system during combustion?

<p>It becomes products of combustion. (C)</p> Signup and view all the answers

What is the area called where mass flows across a boundary?

<p>Control volume (A)</p> Signup and view all the answers

In thermodynamic analyses, why is it simpler to use a control volume instead of a closed system?

<p>Mass flows are easier to visualize within a region of space. (B)</p> Signup and view all the answers

Which components typically cross the boundary of a control volume in an automobile engine?

<p>Air, fuel, and exhaust gases (D)</p> Signup and view all the answers

What is often depicted in a schematic for engineering analysis of a control volume?

<p>The boundary surrounding the volume and mass flows (B)</p> Signup and view all the answers

What type of system is defined by a fixed quantity of matter passing through a device?

<p>Closed system (B)</p> Signup and view all the answers

How do engineers often analyze devices like turbines and pumps?

<p>Through studying mass flow across control volumes (D)</p> Signup and view all the answers

What is the primary reason for using control volumes in thermodynamics?

<p>To facilitate the study of mass flow and energy transfer (D)</p> Signup and view all the answers

What is the name of the stress component that is normal to the area in a fluid?

<p>Normal stress (A)</p> Signup and view all the answers

How is the pressure typically defined when referring to a fluid in motion?

<p>Absolute pressure (C)</p> Signup and view all the answers

In fluid mechanics, what varies with the orientation of the area?

<p>Shear stresses (A)</p> Signup and view all the answers

What does a manometer measure?

<p>Pressure (D)</p> Signup and view all the answers

What happens to normal stress when a fluid is in motion compared to when it is at rest?

<p>It remains the same. (C)</p> Signup and view all the answers

When measuring pressure in a fluid, what does the term 'equal elevations in a continuous mass' imply?

<p>The pressures are equal. (B)</p> Signup and view all the answers

What is the lowest possible value of absolute pressure?

<p>0 kPa (C)</p> Signup and view all the answers

Which statement best describes the relationship between pressure and depth in fluids?

<p>Pressure increases with depth. (C)</p> Signup and view all the answers

What is the primary function of a diaphragm in a pressure sensor?

<p>To alter an inductance when force is applied. (A)</p> Signup and view all the answers

According to Archimedes' principle, what does the buoyant force acting on a submerged body equal?

<p>The weight of the displaced liquid. (B)</p> Signup and view all the answers

What does the equation for buoyant force involve in terms of fluid properties?

<p>The density of the fluid and the volume of the object. (D)</p> Signup and view all the answers

What is the relationship between Celsius and Kelvin temperatures?

<p>T1⬚C = T1K - 273.15 (A)</p> Signup and view all the answers

How does pressure change with depth in a liquid according to the content?

<p>Pressure increases as depth increases. (A)</p> Signup and view all the answers

What temperature corresponds to the triple point of water on the Celsius scale?

<p>0.018°C (D)</p> Signup and view all the answers

In the context of a rectangular block submerged in a liquid, which variable represents the volume of the block?

<p>V (A)</p> Signup and view all the answers

What is the line that connects Kelvin and Celsius with respect to the ice point?

<p>273.15 K to 0°C (A)</p> Signup and view all the answers

What effect does an increase in liquid density have on the buoyant force acting on a submerged object?

<p>It increases the buoyant force. (C)</p> Signup and view all the answers

How much does a temperature increase of 100 K correspond to in Fahrenheit degrees?

<p>180°F (D)</p> Signup and view all the answers

What happens to the buoyant force if the volume of the displaced liquid increases?

<p>It increases proportionally. (B)</p> Signup and view all the answers

What direction does the buoyant force act on a submerged body?

<p>Vertically upward. (B)</p> Signup and view all the answers

What is the formula to convert degrees Celsius to degrees Fahrenheit?

<p>T1⬚F = 1.8T1⬚C + 32 (B)</p> Signup and view all the answers

What is the Celsius temperature at absolute zero?

<p>-273.15°C (C)</p> Signup and view all the answers

What is the relationship between the degrees Fahrenheit and Rankine?

<p>T1⬚F = T1⬚R - 459.67 (B)</p> Signup and view all the answers

How many Celsius degrees correspond to the temperature interval between the ice point and steam point?

<p>100 degrees (A)</p> Signup and view all the answers

How is the number of kilomoles of a substance calculated?

<p>By dividing the mass in kilograms by the molecular weight (A)</p> Signup and view all the answers

What signifies that a property is on a molar basis?

<p>A bar positioned over the symbol (A)</p> Signup and view all the answers

What is the value of Avogadro's number?

<p>6.022 × 10^23 (C)</p> Signup and view all the answers

What units are typically used for the variable y on a molar basis?

<p>m3/kmol and ft3/lbmol (D)</p> Signup and view all the answers

In the context of pressure, what happens at a small area A in a fluid at rest?

<p>A compressive force is exerted on area A (C)</p> Signup and view all the answers

Which of the following principles helps to understand the behavior of fluids at rest?

<p>The continuum viewpoint of pressure (D)</p> Signup and view all the answers

What is the formula for the relationship between y and its molar counterpart?

<p>y = My (C)</p> Signup and view all the answers

What is the molecular weight in the equation used to find kilomoles when represented in kg/kmol?

<p>Weight in kg per kmol (B)</p> Signup and view all the answers

Flashcards

Combustion

A process where a combustible substance reacts with an oxidant, typically oxygen, to produce heat and light.

Composition change

When a substance reacts, the initial material transforms into different products.

Closed system

A fixed quantity of matter that does not exchange mass with its surroundings.

Control volume

A specific region in space chosen for analysis, through which mass can flow.

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Open system

A system that allows for the exchange of mass with its surroundings.

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Thermodynamic analysis

Study of energy and its transformations in a system.

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Kilomoles (kmol)

The number of kilomoles of a substance is calculated by dividing the mass (in kg) by the molecular weight (in kg/kmol).

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Molecular Weight

The mass of one mole of a substance, expressed in kg/kmol or lb/lbmol. It determines the number of kilomoles/pound moles in a given mass.

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Molar Volume (y)

The volume occupied by one kilomole or pound mole of a substance. It's expressed as m³/kmol or ft³/lbmol, giving volume per mole.

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Relationship between y and y

The relationship between molar volume (y) and basic volume (y) is y = My, where M is the molecular weight

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Pressure (Fluid at rest)

The force exerted per unit area by a fluid on a surface normal to it when at rest.

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Density

Local mass per unit volume. It's an intensive property that can change from place to place within a system.

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Specific Volume

The reciprocal of density; volume per unit mass. It's also an intensive property.

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Intensive property

A property that does not depend on the amount of substance.

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Extensive property

A property that depends on the amount of substance.

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Volume V9

The smallest volume where matter can be considered continuous; often a 'point'.

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Mole

Amount of substance numerically equal to its molecular weight.

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Molar Basis

Expressing a property (like specific volume) based on the number of moles rather than mass.

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Kilomole (kmol)

A unit of amount of substance (number of moles) in the SI system.

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Pound-mole (lbmol)

A unit of amount of substance (number of moles) in the English system.

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Equation 1.7

Formula for calculating mass (m) from density and volume (V).

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Fluid pressure variation

Pressure within a fluid at rest can vary based on factors like elevation and depth.

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Fluid stress components

Forces in a moving fluid can be broken down into normal stress (perpendicular to the area) and shear stresses (in the plane of the area).

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Normal stress and pressure

The normal stress in a moving fluid is closely related to the pressure if the fluid were at rest.

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Absolute pressure

Pressure measured with respect to a complete vacuum (zero pressure).

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Manometer

Instrument using a liquid column (mercury, water, etc.) to measure pressure differential.

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Pressure at equal liquid elevations

In a stationary liquid or gas, pressure at the same height within a connected system is equal.

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Buoyant Force

The upward force exerted on an object submerged in a fluid.

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Archimedes' Principle

The buoyant force on an object is equal to the weight of the fluid displaced by the object.

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Pressure Sensor (Diaphragm)

A sensor that measures pressure by detecting the deflection of a diaphragm, which changes an electrical property (inductance, resistance, or capacitance).

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Pressure Increase with Depth

Pressure in a fluid increases as the depth increases.

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Piezoelectric Pressure Sensor

A pressure sensor that uses piezoelectric materials to produce an electrical signal in response to pressure changes.

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Automatic Data Acquisition System

A system that automatically collects and processes sensor data.

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Celsius scale

Temperature scale using degree Celsius (°C) unit, with the same magnitude as Kelvin (K), and 0.01°C at the triple point of water.

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Kelvin temperature

Temperature scale with its zero at absolute zero and uses the unit K.

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Fahrenheit scale

Temperature scale using degree Fahrenheit (°F), with the freezing point of water at 32°F and the boiling point at 212°F.

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Relationship: Celsius to Kelvin

T(°C) = T(K) - 273.15

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Relationship: Fahrenheit to Rankine

T(°F) = T(°R) - 459.67

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Triple point of water

A fixed point in temperature and pressure where water exists as solid, liquid, and gas simultaneously.

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Standard fixed point

Temperature at 273.16 Kelvin, defining the temperature of a standard point.

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Study Notes

Combustion and System Composition

  • Combustion alters the composition of a system, transforming the initial combustible mixture into products of combustion.

Control Volumes

  • Thermodynamic analyses of devices with mass flow (e.g., turbines, pumps) can be performed using control volumes.
  • Control volumes are defined regions in space, through which mass flows.
  • Mass crosses the boundary of a control volume.
  • A control volume can be used to model an engine, where air, fuel, and exhaust gases cross the boundary.

Systems and Control Volumes

  • Systems are a specific quantity of matter.
  • Control volumes are a region of space.

Density and Specific Volume

  • Density is the mass per unit volume, an intensive property that can vary within a system.
  • Mass of a volume is determined by integration of density over the volume.
  • Specific volume is the volume per unit mass (reciprocal of density).
  • Specific volume, like density, is an intensive property.
  • Units for density are kg/m3 (or g/cm3)
  • Units for specific volume are m3/kg (or cm3/g)

Molar Basis

  • Properties can be expressed on a molar basis, using kilomoles (kmol) or pound moles (lbmol).
  • Number of kilomoles (n) = mass (m) / molecular weight (M). A similar equation exists for lbmol.
  • Molar specific volume (y) is volume per kmol or lbmol.
  • Relationship between molar and mass-based specific volume: y = My

Pressure

  • Pressure is normal force per unit area exerted by a fluid.
  • Pressure is a point property, and can vary locally within a fluid at rest. Examples: Atmospheric pressure variation with elevation. Pressure variation with depth in water.
  • In a moving fluid, forces on an area passing through a point can be resolved into normal and shear stresses.
  • Normal stress at a point in a fluid is assumed to be equal to pressure, except when otherwise stated.
  • Unless explicitly stated otherwise, pressure refers to absolute pressure (pressure relative to a vacuum).

Pressure Measurement

  • Manometers and barometers measure pressure using liquid columns (mercury, water, oil).
  • Equal elevations in a continuous mass of liquid or gas at rest have equal pressures.
  • Pressure sensors (e.g., piezoelectric sensors) use a diaphragm and a change in inductance, resistance, or capacitance to measure pressure.

Buoyancy

  • Buoyant force is the resultant pressure force on a body submerged in a liquid.
  • Buoyant force acts vertically upward.
  • Buoyant force equals the weight of the displaced liquid, by Archimedes’ principle.

Temperature Scales

  • Temperature scales like Celsius and Fahrenheit are related to the Kelvin scale.
  • Celsius temperature = Kelvin temperature - 273.15
  • Fahrenheit temperature = 1.8 * Celsius temperature + 32

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