ENERGY, ENERGY TRANSFER

& GENERAL ENERGY

ANALYSIS

Open
CHAPTER 2a
1
 CONTENTS

Forms of Energy

Energy Transfer by Heat

Energy Transfer by Work

Mechanical Forms of Work

The First Law of Thermodynamics

Energy Conversion Efficiencies
Open
2
 LESSON OBJECTIVES

At the end of this lesson, you should be able

to:
State the various forms of energy
Describe the nature of internal energy
Describe the energy transfer by heat and work
Define the concept of work and several forms of
mechanical work

Open
3
 FORMS OF ENERGY

Open
 FORMS OF ENERGY

Energy exists in numerous forms (thermal, mechanical, chemical, kinetic,
potential, electric, magnetic & nuclear)

The sum of the energies is the total energy, E (kJ)

Or for a unit mass, E
e= (kJ/kg)
m
Grouping of
Energy forms
energy of a system as a whole with respect to some
macroscopic outside reference frames, e.g. KE, PE

• related to molecular structure of a system and the

microscopic degree of molecular activity
• independent of outside reference frames
Open
• the sum is the Internal Energy, U 5
 FORMS OF ENERGY (cont’d)
Macroscopic forms of energy

Kinetic energy (KE) Potential energy (PE)

- result of motion relative to some - due to elevation in a gravitational
field (unit J)
reference frame (unit J)
1 PE = mgz
KE = mυ 2
2 ∆PE = PE2 − PE1 = mg (z 2 − z1 )
1
∆KE = KE2 − KE1 =
2
(
m υ 22 − υ12 )
where
g = gravitational acceleration, 9.81 m/s2
where h = elevation of center of gravity of a
υ = velocity of the system relative to system relative to some arbitrarily
some fixed reference frame (m/s) plane (m)
Open m = mass of an object (kg) 6
 FORMS OF ENERGY (cont’d)
Due to internal
Microscopic forms of energy
structure of the matter

Sensible Energy Chemical energy: The

Latent Energy internal energy associated
- Kinetic energy of molecules with the atomic bonds in a
- Associated with
phase of a molecule.
system Nuclear energy: The
tremendous amount of
energy associated with the
strong bonds within the
nucleus of the atom itself.

The internal energy of a system is the sum of all forms of

the microscopic energies.
Thermal = Sensible + Latent
Open
7
Internal = Sensible + Latent + Chemical + Nuclear
 FORMS OF ENERGY (cont’d)

Total Energy of a system

1
E = U + KE + PE = U + mυ 2 + mgz UNIT ?
2

Total Energy of a system per unit mass

1 2
e = u + ke + pe = u + υ + gz UNIT ?
2

Open
8
 Energy in Closed System

The macroscopic kinetic energy is an organized form of energy and is much more useful than the
disorganized microscopic kinetic energies of the molecules.

Most of the closed system remains

stationary, so for that system;
Open
∆E = ∆U + ∆KE + ∆PE
 FORMS OF ENERGY (cont’d)

Energy Interaction

The only two forms of energy interactions
associated with a closed system are heat
transfer and work.

Recognized as it crosses boundary,

represent energy lost or gain

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.
Energy can cross the
boundaries of a closed system
in the form of heat and work.
Open
 ENERGY TRANSFER BY HEAT
Means of energy transfer caused by temperature
Heat Transfer difference between the system and the surroundings

Direction of heat transfer: Higher T

to Lower T

Energy is recognized as heat

transfer only as it crosses the
system boundary

Temperature difference is the driving force

Open
for heat transfer. The larger the temperature
difference, the higher is the rate of heat11
transfer.
 ENERGY TRANSFER BY HEAT
(cont’d)

Adiabatic process is a process during
which there is no heat transfer, Q=0

How a process can be adiabatic?

Well insulated system

No temperature difference

Adiabatic is not necessarily means
isothermal process. Temperature of the
system can still be changed by other
means

Open
12
 ENERGY TRANSFER BY HEAT
(cont’d)

Amount of heat transferred is denoted by Q (kJ)

For a unit mass: Q
q= (kJ/kg)
m

Sign Convention for Heat:
Q positive indicates heat input
Q negative indicates heat lost

Specifying the directions

of heat using in and out

Open
13
 ENERGY TRANSFER BY HEAT
(cont’d)
MECHANISMS OF HEAT TRANSFER
Conduction Convection Radiation

The transfer of energy from the The transfer of energy between

more energetic particles of a a solid surface and the adjacent The transfer of energy due to the
substance to the adjacent less fluid that is in motion, and it emission of electromagnetic waves
Open
energetic ones as a result of involves the combined effects of 14
(or photons).
interaction between particles. conduction and fluid motion.
 ENERGY TRANSFER BY WORK

In mechanics, work is defined as the product of the force

and the displacement in the direction of the force.

WORK Energy transfer associated with a force acting

through a distance, e.g. rotating shaft, rising piston

The work done by, or on, a system is defined as (unit kJ):

s2
W =∫ F .ds or Work done per unit mass w=
W
s1
m

The work done per unit time is Power (unit kJ/s or kW):
Open
15
 ENERGY TRANSFER BY WORK
(cont’d)

Sign convention for work:
W positive indicates work done by system (work output)
W negative indicates work done on the system (work input)

Specifying the directions

of work using in and out

The work done by, or on, a
system is defined as (unit
kJ):

Open
16
 ENERGY TRANSFER BY WORK
(cont’d)
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 Properties are point functions; but heat
during a process as well as the end and work are path functions (their
Open
states). magnitudes depend on the path 17
followed).
 Moving Boundary Work

If the piston is allowed to move a distance dx in a quasi-equilibrium manner, the

differential work done by the system is

δW = F dx = pA dx = p dV
2
The total work done by the system is W = ∫ p dV (kJ)
1

Work done by the system can be interpreted as the area under

Open
the process path on a p-V diagram. 18
 Moving Boundary Work

The total area under the curve 1-2

is: 2 2
Area = A = ∫ dA = ∫ pdV
1 1

Note:
P is the absolute pressure and is always positive.
When dV is positive, Wb is positive.
When dV is negative, Wb is negative

A gas can follow several different paths

as it expands from state 1 to state 2.
Each process path gives a different value
for the boundary work
Wb depends on the path (thus process)
and the end states
Open
Work is not a property 19
19
 ENERGY TRANSFER BY WORK
(cont’d)
Electrical work

NN= Coulombs of electric charge

Electrical power

When potential difference and

current change with time

Electrical power in terms of

When potential difference resistance R, current I, and
and current remain constant potential difference V.

Open
20
 EXAMPLE 2

Consider an electric
refrigerator located in a
room. Determine the
DIRECTION of work and
heat interactions (in our out)
for:
(a) Contents of the refrigerator

(b) All parts of refrigerators,

including the contents

(c) Everything contained within room

Open
21
during winter day
 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 constant

In thermo, most of the work is mechanical work, which

associated with moving boundary work

Other common forms of work

Shaft work
If there is no movement, no
Open

Spring work 22
work is done.
 SHAFT WORK
A force F acting through a moment arm r
generates a torque T

This force acts through a distance s for

n revolution:

Shaft work is
Shaft work: proportional to the
torque applied and the
number of revolutions
of the shaft.

The power transmitted through the shaft is the shaft work done per unit time

Open
23
 SPRING WORK
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

The displacement of a linear

spring doubles when the force
x1 and x2: the initial and the final displacements
is doubled.
Open
24
 NET WORK DONE BY A
SYSTEM
Different forms of work transfer could occur in a system simultaneously
during a process.

The total or net work done by the system = algebraic sum of all work

Wtotal = Wb + We + Wsh + Ws + ...

Open
25
 Class Takeaway

1. How can a closed system and its surroundings interact?

2. What are the sign conventions used for energy transfer?

3. Express total energy and for each of the terms, indicate the unit.

Open
26