Energy, Heat and work
What is energy-

What thermodynamics tells about
energy
• No absolute value of energy given by thermodynamics
Only deals with change of total energy
• Energy change with some reference.
• Thus total energy of a system can be assigned a value zero at
some reference point

In Thermodynamics Total energy is divided in two
groups- microscopic and macroscopic forms
1) . Microscopic form- the invisible form of energy at
atomic and molecular scales.
• This energy is defined as the energy associated with the random,
disordered motion of molecules and due to intermolecular forces OR
Potential and Kinetic energies of individual molecules/atoms.
• Popularly known as Internal Energy, U.
• Associated with molecular structure (binding forces) and molecular
activity (translation, rotation and vibration motion)
• Ideal gas -

• Sensible energy
• The portion of internal energy of a
system associated with kinetic
energies of the molecules.
• Sensible heat is related to
changes in temperature of a gas or
object with no change in phase.
• Latent Energy
• The internal energy associated
with the phase of a system
(binding forces between molecules
of substance) at constant pressure
and temperature is called the
latent energy.
• Other form of Internal energy –
chemical, nuclear etc
Latent heat of fusion- heat transferred
to melt unit mass of solid into liquid, or
liquid into solid.
Latent heat of vaporization
Quantity of heat required to vaporize
unit mass of liquid into vapor or
condense unit mass of vapor into liquid.

• Specific heat (c)
• The specific heat is the
amount of heat per unit mass
required to raise the
temperature by one degree
Celsius.
• The relationship does not
apply if a phase change is
encountered- why

2) Macroscopic form of energy
• Macroscopic Kinetic energy –
the kinetic energy of an object is
the energy that it possesses due to its
motion
• Macroscopic Potential energy -
the energy possessed by a body by
virtue of its position relative to others
In case of Closed system
K.E and Po.E of system
assumed zero.
No mass flow inside or
outside of system .
System is assumed stationary
unless stated.
Difference micro and macro potential energy
Macro Potential Energy : Gravitational Energy or
Elastic energy.
Micro Potential Energy : Chemical energy and Nuclear
energy.

• UPTO HERE ALL ENERGIES WERE STORED
ENERGIES or Static Energies.
• Stored means energy not crossing boundary of
system

DYNAMIC OR TRANSIENT FORM OF
ENERGY
• Energy interactions – crosses boundary of system
• 1) HEAT – driving force is temperature
• 2) WORK – driving force other than temperature.
• In mechanics work is done by a force as it acts upon a body moving in direction
of the force.
• In thermodynamics – work is said to be done by a system if the sole effect on
things external to the system can be reduced to raising of a weight

• For part B, as the motor turns, the weight is raised, and the sole
effect external to the system is the raising of a weight. So, for part A,
work is crossing the boundary of the system, because the sole effect
external to the system could be the raising of a weight.
• Looking at part C, the only limiting factor in having the sole external
effect be the raising of a weight is the inefficiency of the motor
• Therefore, it can be concluded that when there is a flow of electricity
across the boundary of a system, it is work.

Various types of work

Thermodynamic Processes
• Constant pressure process
(isobaric)
• Pressure constant (p1= p2)
• n = 0
• Q = ∆u + W (first law of
thermodynamics)
2
• ∆u = mcv(T2 − T1)
• Q = mcp(T2 − T1)
• W12 = ∫1 pdV = p (v2-v1)

Thermodynamic Processes
2
• Constant Volume process
(isochoric process)
• Volume constant (V1= V2)
• n = ∞
• ∆u = mcr(T2 − T1)
• W12 = ∫1 pdV = p (v1-v2) = 0
• Q = ∆u

Thermodynamic Processes
• Constant Temperature process
(isothermal
process) Process in which
pV = C • (T1= T2)
• n = 1
• ∆u = mcV
2
T2 − T1 = 0
V2
1
• W12 = ∫1 pdV = P1V1 ln
V
• Q = W12

pdV-Work - Quasi-Static Processes

OTHER TYPES OF WORK TRANSFER
• 1) SHAFT WORK
• Determine the power
transmitted through the
shaft of a car when the
torque applied is 200 N-m
and the shaft rotates at a rate
of 4000 revolutions per
minute
• (83.8 kw)

OTHER TYPES OF WORK TRANSFER
• 2) PADDLE –WHEEL WORK OR STIRRING WORK

OTHER TYPES OF WORK TRANSFER
• 3) Flow work
• Significant in open system
• It represents the energy imparted
to the fluid by a pump, blower or
compressor to make the fluid flow
across the control volume.

Free Expansion
• Expansion of gas against vacuum is called
free expansion
• W = 0
• q = 0

Heat Transfer
• Heat is defined as the form of energy that
is transferred across a boundary by virtue
of a temperature difference.
• Energy transfer by virtue of temperature difference
is called heat transfer (Q - Joules)
• Conduction : The transfer of heat between two
bodies in direct contact.
• Radiation : Heat transfer between two
bodies separated by empty space or gases
through electromagnetic waves.
• Convection: The transfer of heat between a wall
anda fluid system in motion.

Heat Transfer
• At constant
pressure
Q = mcp dT
KJ
Kg K
dT(K)Q = m Kg cp
Q = KJ
• At constant
Volume
Q = mcv dT
mcv – heat
capacity
Heat Transfer is a boundary phenomenon,
for isolated system Heat Transfer and work transfer is
zero

Heat transfer as path functions

Points to remember about work and
heat
• Both are path functions and inexact differentials
• Only observed when energy crosses boundary
(boundary phenomenon)
• Both cannot be stored ,they are transient. So
wrong to say heat or work of system.
• Closed system can interact with surrounding in
only two ways – heat and work.
• Heat transfer only by temperature difference. All
other energy transfer are work transfer
• Mutual conversion

Questions
• A rigid tank contains air at 500 kpa and 150
degree celcius. As a result of heat transfer to
the surrounding, the temperature and
pressure inside the tank drop to 65 degree
celcius and 400 kpa, respectively. Determine
the boundary work done during this process.

Questions-
• A frictionless piston cylinder device contains 5
kg of steam at 400 kpa and 200 degree celcius.
Heat is now transferred to the steam until the
temperature reaches 250 degree celcius. If the
piston is not attached to a shaft and its mass is
constant, determine the work done by the
steam during the process.
• V1= 0.53434 m3/kg
• V2= 0.5952 m3/kg