THERMODYNAMICS
Introduction
• Heat is a form of energy.
• Work done gets converted into heat.
• Thermodynamics is the branch of physics
that deals with the concepts of heat and
temperature and the inter-conversion of
heat and other forms of energy
Thermal Equilibrium
• Heat is something that is transferred from a
substance at a higher temperature to that at a
lower temperature.
• When two objects are at the same
temperature, they are in thermal equilibrium
Zeroth Law of Thermodynamics
• If two systems are each in thermal equilibrium with a third
system, they are also in thermal equilibrium with each other.
• Example: If system A and B are in thermal equilibrium and system
A and C are in thermal equilibrium, then system B and C must be in
thermal equilibrium with each other.
• Then system A, B, C are at the same temperature.
Internal Energy
Thermodynamic system
Thermodynamic system is an assembly of an extremely large number of
par7cles (atoms or molecules ) having a certain value of pressure, volume and
temperature.
Thermodynamic systems and Process
Work
•
•
•
•
•
•
•
•
•
•
Work done during expansion of a gas:
Suppose at any instant during expansion the gas occupies a
volume V and exerts a uniform pressure P on the piston.
If the cross-sec?onal area of the piston is A, then force
exerted by the gas on the piston is
F = PA.
Now suppose the gas expands and moves the piston
outward through a small distance dx where dx is so small
that P can be considered to be constant.
Then work done W done by the gas on the piston is
dW = FH . dx̅ = (PA) dx = P (Adx) = P dV
The total work W done by the gas as its volume changes
from V1 to V2 is given by integral ʃ P dV
Sign conven?on: Work done by system is taken as posi?ve
Work done on system is taken as nega?ve
Difference between heat and work
• i) Heat is a mode of energy transfer due to
temperature difference between the system and the
surroundings.
• Work is the mode of energy transfer brought about by
means that do not involve temperature difference such
as moving the piston of a cylinder containing the gas,
by raising or lowering the weight connected to it.
• ii) Heat is mode of transfer of energy that produces
random mo>on while work may be regarded as the
mode of energy transfer that produces organised
mo>on.
First Law of Thermodynamics
• “The heat energy (Q) supplied to a system is
equal to the increase in the internal energy (dU)
of the system plus the work done (W) by the
system.”
•
Q = dU + W
• The first law is based on the law of conservaEon
of energy.
• Sign convenEon : When heat is added to a
system, Q is posiEve and if heat is lost by the
system, Q is negaEve.
• If work is done by the system W is posiEve and if
work is done on the system, work is negaEve.
• An increase in dU is considered posiEve while a
decrease in dU is negaEve
Isothermal Process
• An isothermal process is one in which the
pressure and volume of a system change but
temperature remains constant.
Consider an ideal gas enclosed in a cylinder
provided with a piston and having conducting
walls.
If the gas is slowly compressed, the heat
produced due the work done on the gas is
transferred to the surroundings so that
temperature of the gas remain constant.
Similarly, when the gas is allowed to expand
slowly, its temperature tends to fall but some
heat from the surroundings is conducted to the
gas, keeping the temperature constant.
Essential conditions for an isothermal
process to take place
• i) The walls of the container must be perfectly
conduc7ng to allow free exchange of heat
between the system and the surroundings.
• ii) The process of compression or expansion
should be very slow, so as to provide sufficient
7me for the exchange of heat.
Equa%on of state for an isothermal
process
• For a given mass of a gas
•
PV = constant (At fixed T)
Adiabatic Process
• An adiaba(c process is one in which the pressure, volume and
temperature of the system change but there is no exchange of heat
between the system and the surroundings.
Consider a gas enclosed in a cylinder having
perfectly insulated walls. Suppose the gas is
allowed to expand very quickly.
Work is done by the gas during its expansion,
so its internal energy decreases. As the heat
cannot enter the system from the surroundings,
so the temperature of the gas falls.
Similarly, when the gas is suddenly compressed,
work is done on the gas. This increases the
internal energy of the gas. As heat cannot
escape to the surroundings, the temperature of
the gas increases.
Essen%al condi%ons for an adiaba%c
process
• i) The walls of the container must be perfectly
insulated so that there cannot be any
exchange of heat between the gas and the
surroundings.
• ii) The process of compression or expansion
should be sudden, so that heat does not get
;me to get exchanged with the surroundings
Equa%on of state for an adiaba%c
process
• For a given mass of a gas
•
•
PVʸ = K , where K is a constant
and γ = cp/cv
Limita&ons of the first law of
thermodynamics
• i) It does not indicate the direc/on of transfer
of heat.
• ii) It does not tell us the condi/on under
which heat can be converted into mechanical
work.
• iii) It does not indicate the extend to which
heat energy can be converted into mechanical
work con/nuously.
Second Law of Thermodynamics
• i) Kelvin-Planck statement:
• “ It is impossible to construct an engine, which
will produce no effect other than extrac@ng heat
from a reservoir and performing an equivalent
amount of work.”
• This is applicable to a heat engine. It indicates
that a working substance, opera@ng in a cycle
cannot convert all the heat extracted from the
source into mechanical work. It must reject some
heat to the sink at a lower temperature.
• ii) Clausius Statement:
• “ It is impossible for a self – ac8ng machine,
unaided by any external agency to transfer heat
from a body at lower temperature to another at a
higher temperature”
• This is applicable to a refrigerator. The working
substance can absorb heat from a cold body only
if work is done on it. The work is done by an
electric compressor. If no external work is done,
the refrigerator will not work.
Significance of the second law of
thermodynamics
• The second law of thermodynamics puts a
fundamental limit to the efficiency of a heat
engine and the coefficient of performance of a
refrigerator.
Reversible Process
• Any process which can be made to proceed in
the reverse direc4on by varia4on in its
condi4ons such that any change occuring in
any part of the direct process is exactly
reversed in the corresponding part of reverse
process is called a reversible process
• Examples of Reversible process:
• i) an infinitesimally slow compression and
expansion of an ideal gas at constant
temperature
• ii) The process of gradual compression and
extension of an elas>c spring is approximately
reversible.
• Iii) The process of electrolysis is reversible if the
resistance offered by the electrolyte is negligibly
small.
• A complete reversible process is an idealised
concept as it can never be realised because
dissipa>ve forces cannot be completely
eliminated.
Necessary condition for a reversible
process
• i) The process must be quasi- sta3c. For this,
the process must be carried infinitesimally
slowly so that the system remains in thermal
and mechanical equilibrium with the
surroundings throughout.
• ii) The dissipa3ve forces such as viscosity,
fric3on, inelas3city etc should be absent.
Irreversible Process
• “ Any process which cannot be retraced in the
reverse direction exactly is called an
irreversible process.”
• Most of the process occuring in nature are
irreversible process.
• eg. Diffusion of gases
•
Rusting of iron
•
Sudden expansion or contraction of a gas