Thermodynamics Chapter 4: First Law
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Questions and Answers

What does the first law of thermodynamics state?

The first law of thermodynamics states that energy can neither be created nor destroyed; it can only change forms.

How is the energy balance in a system expressed mathematically?

The energy balance is expressed as ΔE = E_in - E_out.

What concept does the conservation of energy principle encompass in thermodynamics?

The conservation of energy principle encompasses the idea that the total energy of a system remains constant, accounting for all forms of energy.

What is meant by 'specific heat' in thermodynamics?

<p>Specific heat is the amount of heat required to raise the temperature of a unit mass of a substance by one degree Celsius.</p> Signup and view all the answers

What two energy states are compared in the first law of thermodynamics for closed systems?

<p>The total energy entering the system and the total energy leaving the system are compared.</p> Signup and view all the answers

In terms of energy balance, what does ΔE represent?

<p>ΔE represents the net change in the total energy of a system during a process.</p> Signup and view all the answers

What is the primary function of a nozzle in a fluid system?

<p>A nozzle increases the velocity of a fluid while decreasing its pressure.</p> Signup and view all the answers

Why is the first law of thermodynamics important for studying energy interactions?

<p>It provides a foundational understanding of how energy is conserved and transformed across different processes.</p> Signup and view all the answers

How does a diffuser affect the pressure of a fluid?

<p>A diffuser increases the pressure of a fluid by slowing it down.</p> Signup and view all the answers

What does the term 'change in total energy of the system' refer to?

<p>It refers to the difference in energy as a result of energy entering and leaving the system.</p> Signup and view all the answers

What does the conservation of mass principle state for a control volume during a time interval Δt?

<p>The net mass transfer to or from a control volume is equal to the net change in the total mass within that control volume during Δt.</p> Signup and view all the answers

Describe the role of a turbine in power generation.

<p>A turbine drives the electric generator by converting the work done by the fluid into rotational energy.</p> Signup and view all the answers

How is flow work defined in the context of a control volume?

<p>Flow work, or flow energy, is the work required to push mass into or out of a control volume, expressed as W = PV.</p> Signup and view all the answers

What distinguishes compressors from throttling devices?

<p>Compressors require work input to increase fluid pressure, whereas throttling devices cause pressure drops without work input.</p> Signup and view all the answers

In an open system, how can the total energy change for steady flow be expressed?

<p>The total energy change can be expressed as Ein = Eout, including terms for heat, work, and flow energy.</p> Signup and view all the answers

In what application are throttling valves commonly utilized?

<p>Throttling valves are commonly used in refrigeration and air-conditioning applications.</p> Signup and view all the answers

What is a mixing chamber in fluid mechanics?

<p>A mixing chamber is the section where two streams of fluids combine.</p> Signup and view all the answers

List two examples of steady-flow engineering devices.

<p>Examples include compressors and turbines, as well as nozzles and diffusers.</p> Signup and view all the answers

What role do nozzles and diffusers play in engineering applications?

<p>Nozzles and diffusers are used to control and change the velocity and pressure of fluids in systems like jet engines and rockets.</p> Signup and view all the answers

What happens to temperature during the operation of throttling devices?

<p>There is often a large drop in temperature when a throttling device causes a pressure drop.</p> Signup and view all the answers

What equation represents the steady flow energy balance for an open system?

<p>The equation is Q˚in + W˚in + mflow = Q˚out + W˚out + m˚out.</p> Signup and view all the answers

Mention two examples of throttling devices.

<p>Ordinary adjustable valves and capillary tubes are examples of throttling devices.</p> Signup and view all the answers

Explain the significance of flow energy in a control volume.

<p>Flow energy is significant because it accounts for the work necessary to maintain mass flow through the control volume.</p> Signup and view all the answers

How does the conservation of mass principle relate to steady flow in a control volume?

<p>In steady flow, the mass flow rates in and out of the control volume remain constant, leading to Σm˚in = Σm˚out.</p> Signup and view all the answers

What is the formula for determining the energy change of a system during a process?

<p>ΔEsystem = Efinal - Einitial.</p> Signup and view all the answers

In the context of simple compressible systems, how is total energy change expressed mathematically?

<p>ΔE = ΔU + ΔK.E + ΔP.E.</p> Signup and view all the answers

For stationary systems, what are the values of ΔK.E and ΔP.E?

<p>Both ΔK.E and ΔP.E are zero.</p> Signup and view all the answers

What does it imply if a process results in changes in one form of energy in a system?

<p>The total energy of the system will change even if the other forms of energy remain unchanged.</p> Signup and view all the answers

What is the condition for energy change in a closed system undergoing a cycle?

<p>ΔEsystem = 0, which implies Ein = Eout.</p> Signup and view all the answers

How do you interpret the energy balance equation E in - Eout = 0?

<p>It indicates that all energy entering the system must equal the energy leaving the system.</p> Signup and view all the answers

What components contribute to the expression for internal energy change ΔU?

<p>ΔU = m(u2 - u1).</p> Signup and view all the answers

What key assumption is made for stationary systems regarding energy changes?

<p>Only the change in internal energy contributes to the total energy change.</p> Signup and view all the answers

What does the energy balance equation Qnet = Wnet signify in the context of a closed system?

<p>It indicates that the net heat added to the system is equal to the net work done by the system.</p> Signup and view all the answers

Under what condition is the equation ΔE = 0 used when analyzing heat and work interactions?

<p>It is used when there is no change in the internal energy of the system.</p> Signup and view all the answers

What does efficiency (ɳ) signify in energy conversion processes?

<p>Efficiency (ɳ) signifies how well an energy conversion or transfer process is accomplished.</p> Signup and view all the answers

What is the relationship between specific heat at constant pressure (cp) and specific heat at constant volume (cv)?

<p>Specific heat at constant pressure (cp) is always greater than specific heat at constant volume (cv).</p> Signup and view all the answers

How is mechanical efficiency (ɳ) defined in terms of energy input and output?

<p>Mechanical efficiency (ɳ) is defined as the ratio of mechanical energy output to mechanical energy input.</p> Signup and view all the answers

In the first law of thermodynamics for closed systems, what does the term ΔU represent?

<p>ΔU represents the change in internal energy of the system.</p> Signup and view all the answers

What must be considered when dealing with unknown heat or work interactions in thermodynamic problems?

<p>One must assume a direction for the heat or work interactions.</p> Signup and view all the answers

What is the formula for combustion efficiency (ɳcombustion)?

<p>The formula for combustion efficiency (ɳcombustion) is ɳcombustion = Q / HV.</p> Signup and view all the answers

In the context of electric appliances, what does electric efficiency (ɳelectric app) represent?

<p>Electric efficiency (ɳelectric app) represents the ratio of useful energy transferred to the energy consumed by the appliance.</p> Signup and view all the answers

How does the concept of a closed system in thermodynamics restrict mass flow?

<p>A closed system does not allow any mass to enter or leave its boundaries.</p> Signup and view all the answers

What is the physical significance of the term '(u2 - u1)' in the equation q - w = Δu?

<p>'(u2 - u1)' represents the change in internal energy resulting from the process.</p> Signup and view all the answers

What factors can lead to irreversibilities affecting efficiency in energy systems?

<p>Friction and mechanical energy loss can lead to irreversibilities affecting efficiency.</p> Signup and view all the answers

How is pump efficiency (ɳpump) calculated?

<p>Pump efficiency (ɳpump) is calculated as the mechanical energy increase of fluid divided by the mechanical energy input.</p> Signup and view all the answers

Why is the first law of thermodynamics considered universally valid, despite not being mathematically provable?

<p>It is supported by the observation that no natural process has been found to violate it.</p> Signup and view all the answers

Describe turbine efficiency (ɳturbine) and its formula.

<p>Turbine efficiency (ɳturbine) is the ratio of mechanical energy output to mechanical energy decrease of fluid.</p> Signup and view all the answers

What role does the heating value (HV) play in the calculation of combustion efficiency?

<p>The heating value (HV) serves as the reference point for the total energy available from the fuel.</p> Signup and view all the answers

Flashcards

Nozzle

A device that increases the velocity of a fluid at the expense of pressure.

Diffuser

A device that increases the pressure of a fluid by slowing it down.

Turbines

Devices that drive the electric generator in power plants.

Compressors, Pumps, Fans

Devices that increase the pressure of a fluid, such as pumps, fans, and compressors. They require work input from an external source.

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Throttling Valves

Devices that cause a significant pressure drop in a fluid, such as adjustable valves, capillary tubes, and porous plugs.

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Mixing Chamber

A section where two streams of fluids are mixed.

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Throttling

A process involving throttling valves where a pressure drop is accompanied by a significant temperature drop.

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Electric Generator

A device that converts mechanical energy into electricity in a power plant.

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Efficiency (ɳ)

A measure of how effectively an energy conversion or transfer process is completed. It's the ratio of desired output to required input.

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Combustion Efficiency

The efficiency of a combustion process. Calculated as the ratio of heat energy released (Q) to the heating value (HV) of the fuel.

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Electric Appliance Efficiency

The efficiency of an electric appliance. Calculated as the ratio of energy utilized to the energy transferred to the appliance.

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Mechanical Efficiency

The efficiency of a mechanical process. Calculated as the ratio of mechanical energy output to mechanical energy input. It represents how much energy is lost due to factors like friction.

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Pump, Compressor, or Fan Efficiency

The efficiency of a pump, compressor, or fan. Calculated as the ratio of the mechanical energy increase of the fluid to the mechanical energy input into the system.

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Turbine Efficiency

The efficiency of a turbine. Calculated as the ratio of mechanical energy output from the turbine to the mechanical energy decrease in the fluid passing through it.

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Conservation of Mass Principle (Control Volume)

The net mass transfer into or out of a control volume during a time interval is equal to the net change in the total mass within the control volume during that same time interval.

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Flow Work (Flow Energy)

The work required to push mass into or out of a control volume, due to the pressure difference and volume change.

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Total Energy of Flowing Fluids

The total energy of a flowing fluid, including internal, kinetic, and potential energies.

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Compressor

A device that increases the pressure of a fluid by adding work to the system.

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What is the first law of thermodynamics?

The first law of thermodynamics states that energy cannot be created or destroyed, only converted from one form to another.

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What is the energy balance equation?

The total energy entering a system minus the total energy leaving the system equals the change in the system's total energy (ΔEsystem).

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What is a closed system?

A closed system is a system where no mass crosses the boundaries, but energy can be exchanged in the form of heat or work.

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What is specific heat at constant volume (Cv)?

The specific heat at constant volume (Cv) is the amount of heat required to raise the temperature of one unit mass of a substance by one degree Celsius, assuming the volume is constant.

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What is specific heat at constant pressure (Cp)?

The specific heat at constant pressure (Cp) is the amount of heat required to raise the temperature of one unit mass of a substance by one degree Celsius, assuming the pressure is constant.

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What is an open system?

An open system is a system where both mass and energy can cross the boundaries.

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What is efficiency?

Efficiency is the ratio of useful work output to the total energy input.

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How does the first law of thermodynamics apply to open systems?

The first law of thermodynamics can be applied to open systems by considering the energy flow across the system boundaries.

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First Law of Thermodynamics for Closed Systems

The law states that energy cannot be created or destroyed, only transformed from one form to another. In a closed system, the total energy remains constant, with no exchange of mass with the surroundings.

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Energy Balance for a Closed System Cycle

The energy balance for a closed system during a cycle is defined as the net heat transfer (Qnet) being equal to the net work done (Wnet) by the system.

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First Law of Thermodynamics: Validity

The first law of thermodynamics cannot be proven mathematically, but its validity is supported by the fact that no known process in nature has violated it.

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First Law Equation: Closed System

The first law dictates that the change in internal energy (ΔU) of a closed system equals the net heat transfer (Q) minus the net work done (W) by the system.

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Specific Heat

The specific heat of a substance is defined as the amount of energy required to increase the temperature of a unit mass of the substance by one degree.

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Specific Heat at Constant Volume (cv)

Specific heat at constant volume (cv) is the amount of energy required to raise the temperature of a unit mass of a substance by one degree while maintaining constant volume.

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Specific Heat at Constant Pressure (cp)

Specific heat at constant pressure (cp) is the amount of energy required to raise the temperature of a unit mass of a substance by one degree at constant pressure.

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Relationship Between cv and cp

Specific heat at constant pressure (cp) is always greater than specific heat at constant volume (cv) because at constant pressure the system is allowed to expand, requiring additional energy for expansion work.

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Energy Change of a System

The energy change of a system during a process is determined by comparing its energy at the beginning and end of the process. Energy change = Final Energy - Initial Energy

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Total Energy Change in Simple Systems

For simple compressible systems, the total energy change during a process is the sum of changes in internal, kinetic, and potential energies. ΔE = ΔU + ΔK.E + ΔP.E

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Internal Energy Change (ΔU)

The change in internal energy (ΔU) is the change in the energy stored within the system. It is dependent on mass and the change in specific internal energy (u). ΔU = m(u2 - u1)

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Kinetic Energy Change (ΔK.E)

The change in kinetic energy (ΔK.E) is the change in energy due to the system's motion. It depends on mass and the change in velocity. ΔK.E = ½m(v2² - v1²)

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Potential Energy Change (ΔP.E)

The change in potential energy (ΔP.E) is the change in energy due to the system's position in a gravitational field. It depends on mass, gravity, and the change in height. ΔP.E = mg(z2 - z1)

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Energy Balance in Stationary Systems

Stationary systems do not move or change their elevation during a process. Therefore, their kinetic and potential energy changes are zero, leaving the total energy change equal to the internal energy change. ΔE = ΔU

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Energy Balance for Closed Systems in a Cycle

For a closed system undergoing a cycle, the initial and final states are identical, meaning the total energy change is zero. This leads to the energy balance for a cycle: E in - E out = 0 or E in = E out.

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Study Notes

Thermodynamics - Chapter 4: First Law of Thermodynamics

  • Objectives of Chapter 3: Introduce energy balance, identify the first law of thermodynamics for closed systems, develop the general energy balance applied to closed systems, define the specific heat at constant volume and the specific heat at constant pressure
  • Definitions: The first law of thermodynamics, also known as the conservation of energy principle, provides a sound basis for studying the relationships among various forms of energy and energy interactions. Energy cannot be created or destroyed during a process; it only changes forms.
  • Energy Balance: The net change in the total energy of a system during a process is equal to the difference between the total energy entering and the total energy leaving the system during that process. ΔEsystem = Ein - Eout
  • Energy Change of a system: The energy change of a system during a process involves evaluating the energy at the beginning and end of the process. ΔEsystem = Efinal - Einitial= E2 - E1
  • For simple compressible systems, the change in total energy during a process is the sum of the changes in internal, kinetic, and potential energies. ΔE = ΔU + ΔKE + ΔPE = m(u2 - u1) + ½m(v22 - v12) + mg(z2 - z1)

First Law for Closed Systems

  • Energy Balance for Closed Systems: For a closed system undergoing a cycle, the initial and final states are identical, therefore ΔEsystem = 0. Thus, Ein - Eout = 0 or Ein = Eout
  • General Formula for Closed Systems: (Q - W) = ΔU = m(u2 - u1) or q - w = Δu = u2 - u1

Specific Heats

  • Specific Heat: The energy required to raise the temperature of a unit mass of a substance by one degree. In thermodynamics, two kinds are considered, specific heat at constant volume (cv) and specific heat at constant pressure (cp).

  • Specific heat at constant pressure (cp) is always greater than cv because at constant pressure the system is allowed to expand, and the energy for this expansion work must also be supplied to the system. cv = (∂u/∂T)v cp = (∂h/∂T)p

  • Internal Energy Change (Ideal Gas): The change in internal energy of an ideal gas during a process from state 1 to state 2 is given by: Δu = u2 - u1 = ∫cv(T) dT

  • Enthalpy Change (Ideal Gas): The change in enthalpy of an ideal gas during a process from state 1 to state 2 is given by: Δh = h2 - h1 = ∫cp(T)dT

  • Specific Heats of Solids and Liquids: Δu = cavg (T2 -T1) Δh = Δu + v ΔP = cavg ΔT + v ΔP

First Law for Open Systems

  • Mass and Volume Flow: The amount of mass flowing through a cross-section per unit time is called mass flow rate (ṁ). ṁ = ρAcVavg (kg/s). ρ is density, A is the cross-sectional area, and Vavg is the average velocity.

  • Conservation of Mass: The net mass transfer into or out of a control volume during a time interval is equal to the net change in the total mass within the control volume during that time. ṁin - ṁout = Δmcv

  • Flow Work: Work required to push mass into or out of a control volume. It's a necessary part of maintaining continuous flow.

  • Total Energy of a Flowing Fluid: Includes internal, kinetic, and potential energies. Ein = Qin + Win + ṁ(hin +V2in/2 + gzin) Eout = Qout + Wout + ṁ(hout +V2out/2 + gzout)

  • Typical Engineering Devices (Steady Flow): Examples of devices that use the first law for open systems include nozzles, diffusers, compressors, turbines, throttling valves, and mixing chambers.

Efficiency

  • Efficiency (η): Measures how effectively an energy conversion or transfer takes place, expressed as the ratio of desired output to required input.
  • Electrical Appliances: Efficiency is the ratio of useful energy transferred to the energy consumed. ηelectric equipment = energy utilised / energy transferred
  • Mechanical Efficiency: Mechanical energy output/ Mechanical energy input. ηmechanical = Wout/Win -Turbine efficiency, ηturbine = Mechanical energy output/ Mechanical energy decrease
    • Pump Efficiency, ηpump = Mechanical energy increase / Mechanical energy input
  • Combustion Efficiency: Efficiency calculated as the ratio of output energy to the heat energy. ηcomb = Q/H.V

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