CDB1053 Introduction to Engineering ThermodynamicsBy Dr. Oh Pei Ching

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ENERGY AND ENERGY TRANSFER

• Introduce the concept of energy and define its variousforms.

• Discuss the nature of internal energy.

• Define the concept of heat and the terminologyassociated with energy transfer by heat.

• Discuss the three mechanisms of heat transfer:conduction, convection, and radiation.

• Define the concept of work

• Discuss the various forms of work: mechanical or non-mechanical

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Learning Outcome

CDB1053 Introduction to Engineering Thermodynamics

QUESTION Thermodynamics Concept

Consider a room whose door and windows are tightly closed. The walls

are well-insulated so that heat loss or gain through the walls is negligible.

Place a refrigerator in the middle of the room with its door open, and plug

it into a wall outlet. Will the average temperature of air in the room

increase or decrease? Or will it remain constant?

• If we take the entire room (including the air

and the refrigerator) as the system, which

is an adiabatic closed system since the

room is well-sealed and well-insulated, the

only energy interaction involved is the

electrical energy crossing the system

boundary and entering the room.

• As a result of the conversion of electric

energy consumed by the device to heat,

the room temperature will rise.

The first law of thermodynamics

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FORMS OF ENERGY

• Energy can exist in numerous forms such as thermal, mechanical, kinetic,

potential, electric, magnetic, chemical, and nuclear, and their sum

constitutes the total energy, E of a system.

• Thermodynamics deals only with the change of the total energy.

• Macroscopic forms of energy: Those a system possesses as a whole

with respect to some outside reference frame, such as kinetic and potential

energies.

Kinetic energy, KE: The energy that a system possesses as a result of its

motion relative to some reference frame.

Potential energy, PE: The energy that a system possesses as a result of

its elevation in a gravitational field.

The macroscopic energy of an object

changes with velocity and elevation.

• Microscopic forms of energy: Those

related to the molecular structure of a

system and the degree of the molecular

activity.

Internal energy, U: The sum of all

the microscopic forms of energy.

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Total energy of a system

Energy of a system per unit mass

Potential energy per unit mass

Kinetic energy per unit mass

Potential energy

Kinetic energy

Mass flow rate

Energy flow rate

Open system (Control volume)

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Some Physical Insight to Internal Energy

Sensible energy: The portion

of the internal energy of a

system associated with the

kinetic energies of the

molecules.

Latent energy: The internal

energy associated with the

phase of a system.

Chemical energy: The internal

energy associated with the

atomic bonds in a molecule.

Nuclear energy: The

tremendous amount of energy

associated with the strong

bonds within the nucleus of the

atom itself.

Internal = Sensible + Latent + Chemical + Nuclear

Thermal = Sensible + LatentThe various forms of

microscopic

energies that make

up sensible energy.

The internal energy of a

system is the sum of all forms

of the microscopic energies.

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• The total energy of a system, can be

contained or stored in a system, and

thus can be viewed as the static

forms of energy.

• The forms of energy not stored in a

system can be viewed as the

dynamic forms of energy or as

energy interactions.

• The dynamic forms of energy are

recognized at the system boundary

as they cross it, and they represent

the energy gained or lost by a system

during a process.

• The only two forms of energy

interactions associated with a closed

system are heat transfer and work.

• The difference between heat transfer and work: An energy interaction is

heat transfer if its driving force is a temperature difference. Otherwise it is

work.

• A control volume (open system) can also exchange energy via mass

transfer.

FIGURE 2-15

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ENERGY TRANSFER BY HEAT

Heat: The form of energy that is

transferred between two systems (or

a system and its surroundings) by

virtue of a temperature difference.

FIGURE 2-14

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Amount of heat transfer when heat transfer

rate changes with time:

During an adiabatic process, a system

exchanges no heat with its surroundings.

Adiabatic process: A process during

which there is no heat transfer.

Energy is recognized as heat transfer

only as it crosses the system boundary.

Heat transfer per unit mass:

Q = Amount of heat

transferred (kJ)

Amount of heat transfer when heat transfer

rate is constant:

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Historical Background on Heat

• Kinetic theory: Treats molecules as

tiny balls that are in motion and thus

possess kinetic energy.

• Heat: The energy associated with the

random motion of atoms and

molecules.

Heat transfer mechanisms:

• Conduction: The transfer of energy

from the more energetic particles of a

substance to the adjacent less

energetic ones as a result of interaction

between particles.

• Convection: The transfer of energy

between a solid surface and the

adjacent fluid that is in motion, and it

involves the combined effects of

conduction and fluid motion.

• Radiation: The transfer of energy due

to the emission of electromagnetic

waves (or photons).

FIGURE 2-19

ENERGY TRANSFER BY WORK• Work: The energy transfer associated with a force acting through a distance.

A rising piston, a rotating shaft, and an electric wire crossing the systemboundaries are all associated with work interactions

• Formal sign convention: Heat transfer to a system and work done by a systemare positive; heat transfer from a system and work done on a system arenegative.

• Alternative to sign convention is to use the subscripts in and out to indicatedirection. This is the primary approach in this text.

Specifying the directions of heat and work.

Work done per unit mass:

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Heat vs. Work• Both are recognized at the boundaries of

a system as they cross the boundaries.

That is, both heat and work are boundary

phenomena.

• Systems possess energy, but not heat or

work.

• Both are associated with a process, not a

state. Unlike properties, heat or work

has no meaning at a state.

• Both are path functions (i.e., their

magnitudes depend on the path followed

during a process as well as the end

states).

Properties are point functions and have exact

differentials (d ).

Path functions have inexact differentials ( ).

FIGURE 2-22

Example: The total volume change during a process

between states 1 and 2 is:

Example: The total work done during process 1-2 is:

EXAMPLE 2-3 Burning of a Candle in an Insulated Room

A candle is burning in a well-insulated room. Taking the room (the air plus the

candle) as the system, determine

(a) If there is any heat transfer during this burning process

(b) If there is any change in the internal energy of the system

The interior surfaces of the room form the system boundary. Heat is recognized as it

crosses the boundaries. Since the room is well insulated, we have an adiabatic system and

no heat will pass through the boundaries. Therefore, Q = 0 for this process.

EXAMPLE 2-4 Heating of a Potato in an Oven

A potato initially at room temperature (25oC) is being baked in an oven

that is maintained at 200oC. Taking the potato as the system, is there any

heat transfer during this baking process?

Since the potato is the system, the outer surface of the skin of the potato can be

viewed as the system boundary. Part of the energy in the oven will pass through

the skin to the potato. Since the driving force for this energy transfer is a

temperature difference, this is a heat transfer process.

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Electrical Work

Electrical work

Electrical power

When potential difference

and current change with time

When potential difference

and current remain constantFIGURE 2-27

V = potential difference

N = coulombs of electrical charge

I = current or

number of electrical charges

flowing per unit time

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MECHANICAL FORMS OF WORK

• There are two requirements for a work interaction between a system and its surroundings to exist:

there must be a force acting on the boundary.

the boundary must move.

Work = Force Distance

When force is not constantFIGURE 2-28

• In many thermodynamic problems, mechanical work is the only form of work involved.

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MOVING BOUNDARY WORK

Moving boundary work (P dV work): The expansion and compression work

in a piston-cylinder device.

Quasi-equilibrium process: A process during which the system remains

nearly in equilibrium at all times.

The work associated with a moving boundary

(expansion and compression) is called

boundary work.

For quasi-equilibrium

process

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Consider a gas enclosed in a piston-cylinder device.If the piston is allowed to move a distance ds in a quasi-equilibrium

manner, the differential work (boundary work) done during this process is:

Where

P = initial pressure

dV = volume change

P is the absolute pressure and is always positive.

When dV is positive, Wb is positive for expansion

When dV is negative, Wb is negative for compression

A gas does a differential

amount of work Wb as it

forces the piston to move

by a differential amount ds.

Total boundary work from initial state to final state:

To calculate total boundary work, the process by

which the system changed states must be known,

i.e. the functional relationship between P and V

during the process.

P=f (V ) should be available: equation of the process

path on a P-V diagram.

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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.

P-V diagram of quasi-equilibrium

expansion process:

The boundary work = Area under the process

curve plotted on a P-V

diagram

Differential area is equal to differential work:

dA = PdV

Total area under the process curve:

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• A gas can follow several different paths

(with different area underneath) as it

expands from state 1 to state 2.

• Each process path gives a different value

for boundary work.

• The net work output is produced during

a cycle if the work done by the system

during the expansion process (area

under path A) is greater than the work

done on the system during the

compression part of the cycle (area

under path B).

The boundary work done during a process depends

on the path followed as well as the end states.

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Some typical processes

P

V

1

2

P-V diagram for V = Constant

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(b) Boundary work for a constant-pressure process

If the pressure is held constant, the boundary work equation becomes

P

V

2 1

P-V diagram for P = Constant

For the constant pressure process shown above, is the boundary work

positive or negative and why?

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The above equation is the result of applying the ideal gas assumption for

the equation of state. For real gas undergoing an isothermal process, the

integral in the boundary work equation would be done numerically.

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Process Exponent n

Constant pressure 0

Constant volume ∞

Isothermal & ideal gas 1

Adiabatic & ideal gas k = Cp/Cv

Where

Cp= specific heat at constant pressure;

Cv= specific heat at constant volume

For an ideal gas (PV=mRT), this equation can also be written as

For special case of n=1 the boundary work becomes

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2

1

1dV

VC

n 1

1

1

1

2

n

VVC

nn

n

VPVP

1

1122 since C = P1V1n =P2V2

n

1,

1

12

n

n

TTmRWb

1

2

1

22

1

2

1

1 lnlnV

VmRT

V

VPVdVCVPdVWb

2

1PdVWb

nCVP

dVV

Cn

2

1

How to determine the boundary work for polytropic process?

Isothermal

process

EXAMPLE 4-1 Boundary Work for a Constant-Volume Process

A rigid tank contains air at 500 kPa and 150oC. As a result of heat transfer to the

surroundings, the temperature and pressure inside the tank drop to 65oC and 400

kPa, respectively. Determine the boundary work done during this process.

Solution:

The boundary work can be determined to be

EXAMPLE 4-2 Boundary Work for a Constant-Pressure Process

A frictionless piston-cylinder device contains 5 kg of steam at 400 kPa

and 200oC. Heat is now transferred to the steam until the temperature

reaches 250oC. If the piston is not attached to a shaft and its mass is

constant, determine the work done by the steam during this process.

m = 5 kg

P = 400 kPa

P, kPa

P0 = 400 kPa400

v1 = 0.53434 v2 = 0.59520 v, m3/kg

EXAMPLE 4-2 Boundary Work for a Constant-Pressure Process

Solution:Assumption: The expansion process is quasi-equilibrium.

Analysis: Even though it is not explicitly stated, the pressure of the steam within the cylinder

remains constant during this process since both the atmospheric pressure and the weight of the

piston remain constant. Therefore, this is a constant-pressure process, and

or

since V = mv. From the superheated vapor table, the specific volumes are determined to be

v1 = 0.53434 m3/kg at state 1 (400 kPa, 200oC) and v2 = 0.59520 m3/kg at state 2 (400 kPa, 250oC).

Substituting these values yields

Discussion: The positive sign indicates that the work is done by the system. That is, the steam

used 122 kJ of its energy to do this work. The magnitude of this work could also be determined by

calculating the area under the process curve on the P-V diagram, which is simply P0∆V for this case.

EXAMPLE 4-3 Isothermal Compression of an Ideal Gas

A piston-cylinder device initially contains 0.4 m3 of air at 100 kPa and

80oC. The air is now compressed to 0.1 m3 in such a way that the

temperature inside the cylinder remains constant. Determine the work

done during this process.

EXAMPLE 4-3 Isothermal Compression of an Ideal Gas (Cont.)

Solution:Assumption: 1 The compression process is quasi-equilibrium. 2 At specified condition, air can be

considered to be an ideal gas since it is at a high temperature and low pressure relative to its critical-

point values.

Analysis: For an ideal gas at constant temperature T0,

where C is a constant. Substituting this into boundary work equation, we have

P1V1 can be replaced by P2V2 or mRT0. Also, V2 / V1 can be replaced by P2 / P1 for this case since

P1V1= P2V2.

Substituting the numerical values yields

Discussion: The negative sign indicates that this work is done on the system (a work input), which

is always the case for compression processes.

1

211

1

22

1

2

1

2

1lnln

V

VVP

V

VC

V

dVCdV

V

CPdVWb

V

CPorCmRTPV 0

kJmkPa

kJmkPaWb 5.55

.1

1

4.0

1.0ln)4.0)(100(

3

3

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Shaft Work

The power transmitted through the shaft is

the shaft work done per unit time

Shaft work

A force F acting through a moment arm r

generates a torque T

This force acts through a distance s

• Often the torque T applied to a rotating shaft isconstant (Force F applied is also constant).

• For a specified constant torque, the work doneduring n revolutions is determined as follows:

EXAMPLE 2-7 Power Transmission by the Shaft of a Car

Determine the power transmitted through the shaft (kW) of a car when the

torque applied is 200 N.m and the shaft rotates at a rate of 4000

revolutions per minute (rpm).

Solution:

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Spring Work

Elongation of a spring under

the influence of a force.

• When a force is applied on a spring, the

length of the spring changes.

• When the length of the spring changes

by a differential amount dx under the

influence of a force F, the work done is

For linear elastic springs, the displacement

x is proportional to the force applied

k: spring constant (kN/m)

Substituting and integrating yield

x1 and x2: the initial and the final

displacements

The

displacement

of a linear

spring doubles

when the force

is doubled.

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Work Done to Raise or to Accelerate a Body

1. The work transfer needed to raise a body is equal to

the change in the potential energy of the body.

2. The work transfer needed to accelerate a body is

equal to the change in the kinetic energy of the body.

Electrical work: The generalized force is the

voltage (the electrical potential) and the

generalized displacement is the electrical charge.

Magnetic work: The generalized force is the

magnetic field strength and the generalized

displacement is the total magnetic dipole

moment.

Electrical polarization work: The generalized

force is the electric field strength and the

generalized displacement is the polarization of

the medium.

Nonmechanical Forms of Work

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Summary• Forms of energy

Macroscopic = kinetic + potential

Microscopic = Internal energy (sensible + latent + chemical + nuclear)

• Energy transfer by heat

• Energy transfer by work

• Mechanical forms of work

Moving boundary work

Constant-volume process

Constant-pressure process

Isothermal process

Polytropic process

Shaft work

Spring work

• Non-mechanical forms of work