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Thermodynamics I Chapter 3 Energy, Heat and Work Mohsin Mohd Sies Fakulti Kejuruteraan Mekanikal, Universiti Teknologi Malaysia

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  • Thermodynamics IChapter 3

    Energy, Heat and Work

    Mohsin Mohd SiesFakulti Kejuruteraan Mekanikal, Universiti Teknologi Malaysia

  • Energy, Heat and Work(Motivation)

    A system changes due to interaction with its surroundings.Energy interaction is a major factor.This chapter studies the nature of energy and its various forms and transfers so that we are able to follow its interaction with a system.

  • ENERGY & THE 1st LAW OF THERMO.

    1st Law : concerning quantity of energy

    Energy is conserved(Amount of energy is constant, but can change forms)(e.g. potential, kinetic, electrical, chemical, etc.)

  • Energy

    Macroscopic – system energy which value depends on a reference point (Kinetic Energy (KE), Potential Energy (PE)Microscopic – energy due to molecular interactions & activity (independent of any reference point) (Internal Energy, U)

    Total System Energy

    [kJ/kg]

    [kJ]

    upekee

    UPEKEE

  • Kinetic Energy (KE)

    Potential Energy (PE)

    )zg(zm

    ΔPEΔpe gz

    m

    PEpe

    )zmg(zΔPE mgzPE

    12

    12

  • Internal Energy, U

    Molecular movement (vibration, collision, etc) sensible energy (molecular activity temperature)Bond Energy between molecules (phase change) latent energy (constant temperature)Bond Energy between atoms in a moleculechemical energyBond Energy between protons & neutrons in the nucleusnuclear energy

  • 7

    Specific HeatsSpecific heat at constant volume, cv: The energy

    required to raise the temperature of the unit mass of a substance by one degree as the volume is maintained constant.

    Specific heat at constant pressure, cp: The energy required to raise the temperature of the unit mass of a substance by one degree as the pressure is maintained constant.

    Constant-volume and constant-

    pressure specific heats cv and cp(values are for

    helium gas).

  • 8

    • The equations in the figure are valid for any substance undergoing any process.

    • cv and cp are properties.

    • cv is related to the changes in internal energy and cp to the changes in enthalpy.

    • A common unit for specific heats is kJ/kg·°C or kJ/kg·K. Are these units identical?

    True or False?cp is always greater than cv

    Formal definitions of cv and cp.

  • 9

    1. By using the tabulated u and h data. This is the easiest and most accurate way when tables are readily available.

    2. By using the cv or cp relations (Table A-2c) as a function of temperature and performing the integrations. This is very inconvenient for hand calculations but quite desirable for computerized calculations. The results obtained are very accurate.

    3. By using average specific heats. This is very simple and certainly very convenient when property tables are not available. The results obtained are reasonably accurate if the temperature interval is not very large.

    Three ways of calculating u and h

    Three ways of calculating u.

  • Specific Heats for Solids & Liquids

    • For incompressible substances (solids & liquids)

    cv = cp• Thus it can just be stated as c.

  • Modes of Energy Transfer

    Energy Interaction between System and Surrounding

    Energy can cross the boundary (transferred) of a closed system by 2 methods;

    HEAT & WORK

  • HEAT, Q [J, kJ]

    Heat - Energy that is being transferred due to a temperature difference

    •Heat is a mode of energy transfer•Heat is not a property•Energy is related to states (property)•Heat is related to processes (not a property, depends on the path)

    = rate of heat transfer

    Amount of heat transferred during a process (depends on the path)

  • Heat (ctd.)

    system

    Qin= +ve Qout= -ve

    Adiabatic Process (Q = 0)insulatedTsystem = Tsurrounding

    Adiabatic ≠ Isothermal!(T can change by other methods; energy can enter system by work)

  • WORK, W [J, kJ]

    Work -–Energy that is crossing the boundary other than heat(electrical, stirrer, shaft, moving piston, etc.)Also stated as the energy transfer associated with a force acting for a distance.

    -Not a property (related to process)-Mode of energy transfer

    Win= -ve Wout= +vesystem

    = rate of work = power

    Depends on path

  • Types of Work

    WorkElectrical

    Mechanical

    Boundary

    Gravitational

    Acceleration

    Shaft

    Spring

    2

    1VIdtW el

    2

    1dsFW

    2

    1dVpW b

    )( 12 zzmgW g

    nW 2

    )(2

    1 21

    22 xxkW

  • 16

    MOVING BOUNDARY WORKMoving boundary work (P dV work): The expansion and compression work in a piston-cylinder device.

    The work associated with a moving

    boundary is called boundary work.

    A gas does a differential amount of work Wb as it forces the piston to move by a differential amount ds.

    Quasi-equilibrium process: A process during which the system remains nearly in equilibrium at all times.

    Wb is positive for expansionWb is negative for compression

  • 17

    The boundary work done during

    a process depends on the path

    followed as well as the end states.

    The area under the process curve on a P-V diagram is equal, in magnitude, to the work done during a quasi-equilibrium expansion or compression process of a closed system.

  • Boundary Work for Polytropic ProcessesBoundary Work

    General Polytropic Work

    n

    VPVPW

    1

    1122 ( n 1 )

    Const. Pressure Work (n=0)

    )( 12 VVpW

    Isothermal Work (n=1) ideal gas

    Const. Volume Work (dv=0)

    Area under P-v graph

  • 19

    ENERGY CONVERSION & TRANSMISSION EFFICIENCIESEfficiency is one of the most frequently used terms in thermodynamics, and it

    indicates how well an energy conversion or transfer process is accomplished.

    Efficiency of a water heater: The ratio of the energy delivered to the house by hot water to the energy supplied to the water heater.

  • 20

    Heating value of the fuel: The amount of heat released when a unit amount of fuel at room temperature is completely burned and the combustion products are cooled to the room temperature.Lower heating value (LHV): When the water leaves as a vapor.Higher heating value (HHV): When the water in the combustion gases is completely condensed and thus the heat of vaporization is also recovered.

    The efficiency of space heating systems of residential and commercial buildings is usually expressed in terms of the annual fuel utilization efficiency (AFUE), which accounts for the combustion efficiency as well as other losses such as heat losses to unheated areas and start-up and cool down losses.

  • 21

    • Generator: A device that converts mechanical energy to electrical energy.• Generator efficiency: The ratio of the electrical power output to the mechanical

    power input. • Thermal efficiency of a power plant: The ratio of the net electrical power output

    to the rate of fuel energy input.

    Lighting efficacy: The amount of light output in lumens per W of electricity consumed.

    Overall efficiency of a power plant

  • 22

    • Using energy-efficient appliances conserve energy.

    • It helps the environment by reducing the amount of pollutants emitted to the atmosphere during the combustion of fuel.

    • The combustion of fuel produces • carbon dioxide, causes global warming• nitrogen oxides and hydrocarbons,

    cause smog• carbon monoxide, toxic• sulfur dioxide, causes acid rain.

  • 23

    Efficiencies of Mechanical and Electrical Devices

    The effectiveness of the conversion process between the mechanical work supplied or extracted and the mechanical energy of the fluid is expressed by the pump efficiency and turbine efficiency,

    Mechanical efficiency

  • 24

    Generator efficiency

    Pump-Motor overall efficiency

    Turbine-Generator overall efficiency

    Pump efficiency

  • 1st Law Closed System

    Qin

    QoutWin

    Wout

    Net total energy transfer in the form of heat and work

    Net change of system energy

  • Q – W = U + KE + PE

    Net transfer of energy Energy storage in different modes

    Esystem= U + KE + PE

    Esystem= U + KE + PE- Recall that System Energy consists of:

    or

    thus;

    Q – W = Esystem= U + KE + PEOr can be called as the 1st Law/Energy Balance of a Closed System;

  • Q – W = U + KE + PE [kJ]1st Law for Closed System in different forms;

    q – w = u + ke + pe [kJ/kg]per unit mass;

    Rate form;

    [kW]

    Cyclic Process E =0

  • summary;

    Q – W = U + KE + PEmg(z2-z1)

    2

    )( 212

    2 VVm

    m(u2-u1) water (Table)

    mCv(T2-T1) ideal gas

    Electrical

    Mechanical

    Boundary

    etc…

    2

    1VIdtW el

    2

    1dsFW

    2

    1dVpW b


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