Thermodynamics basics-2

Dr. Mrs. Pritee M. Raotole, MGSM’s Arts Science and Commerce,

 Thermodynamic temperature is the
absolute measure
of temperature and is one of the
principal parameters
of Thermodynamics.
 The temperature is a
thermodynamic variable which is
used to represent degree of hotness
or coldness of a body.
 Thermodynamic temperature isDr. Mrs. Pritee M. Raotole, MGSM’s Arts Science and Commerce,

 At this point, absolute zero, the particle
constituents of matter have minimal
motion.
 In the quantum mechanical description,
matter at absolute zero is in its ground
state, which is its state of lowest energy.
 Thermodynamic temperature is often
also called absolute temperature, for
two reasons:
 1.Proposed by Kelvin, that it does not
depend on the properties of a particular
material;
 2. That it refers to an absolute zero

 A number of measurement scales have been
invented to measure temperature.
Measurement
Scale
Steam Point
of Water
Ice Point of
Water
Absolute Zero
Fahrenheit 212 32 -460
Celsius 100 0 -273
Kelvin 373 273 0

 We know 1st law of thermodynamics
 𝑑𝑄 = 𝑑𝑈 + 𝑑𝑊 ---------(1)
 Now consider the Isothermal Process
 T = constant
 For ideal gas
 We know U= f(T) so U is also constant
 Hence 𝑑𝑈 = 0
 So equation (1) becomes

 𝑑𝑄 = 𝑑𝑊 ---------(2)
 This is a process that has 100%
conversion of heat into work.
 The gas enclosed in cylinder and
fitted with piston undergoes a
change in its volume from state A to
B.
 work done in this process can beDr. Mrs. Pritee M. Raotole, MGSM’s Arts Science and Commerce,

 During this process due to pressure P some
force is acting on piston is given by F= P x A
by this force piston displace by small
distance dx then work is done
 dW= force x displacement
 dW= F x dx
 dW= P A dx (Adx= dv = change in volume)
 dw=pdv
 Put this value of dW in equation 2
 𝑑𝑄 = 𝑃𝑑𝑉 ---------(3)

 The system has certain properties like
temperature, pressure, volume, etc. The
present values of the properties of the
system are called as thermodynamic
state of system.
 Thermodynamic Process
 When the system undergoes change from
one thermodynamic state to final state
due change in properties like
temperature, pressure, volume etc, the
system is said to have undergoneDr. Mrs. Pritee M. Raotole, MGSM’s Arts Science and Commerce,

Various types of thermodynamic processes
are:
 Isothermal process,
 Adiabatic process,
 Isochoric process,
 Isobaric process

 It is change in state of system during which
temperature of system remains constant is
called isothermal change
(T= constant and dT=0).
 As temperature remains constant during
this process it obeys the Boyle’s law .i.e.
 PV= constant.
 In order to perform isothermal process will
have to either add or remove heat from the
gas during the change adding heat energy if

 If the gas is compressed dW is positive and
so dQ must be negative as heat energy must
be removed from the gas to keep its
temperature constant.
 If the gas is expanded dW is negative and so
dQ must be positive as heat energy must be
supplied to the gas to keep its temperature
constant.
 Since the temperature of the gas remains
constantDr. Mrs. Pritee M. Raotole, MGSM’s Arts Science and Commerce,

 dU = 0.
 The first law of Thermodynamics for such a
change becomes
 dQ=W ------------(4)
 Hence in isothermal process heat absorbed
totally get converted to workdone.

 An adiabatic process is change in state of
system during which amount of heat
remains constant (Q = constant, dQ = 0).
 This change occurs without transfer of heat
or mass of substances between a
thermodynamic system and its
surroundings.
 Any process that occurs within a container
that is a good thermal insulator is also
adiabatic.Dr. Mrs. Pritee M. Raotole, MGSM’s Arts Science and Commerce,

 A system that expands under adiabatic
conditions does positive work, so the internal
energy decreases, and a system that contracts
under adiabatic conditions does negative work,
so the internal energy increases.
 Since the amount of heat of gas remains
constant
 dQ = 0.
 The first law of thermodynamics for such a
change becomes 0=dU+W
 Therefore
 W= -dU --------------- (5)

 An isochoric process is one where the
volume of the system stays constant.
 Again, 'iso' means the same and 'choric'
means volume.
 Volume is the amount of space the material
takes up.
 So this would be like heating a gas in a
solid, non-expandable container.
 The molecules would move faster and the
pressure would increase, but the size of theDr. Mrs. Pritee M. Raotole, MGSM’s Arts Science and Commerce,

 A quantity of heat dQ is supplied to the gas.
To keep the volume of the gas constant, a
small weight is placed over the piston.
 The pressure and the temperature of the
gas increase to P + dP and T + dT
respectively.
 This heat energy dQ is used to increase the
internal energy dU of the gas.
 If heat dQ is absorbed at constant volume
i.e.Dr. Mrs. Pritee M. Raotole, MGSM’s Arts Science and Commerce,

 Isochoric change, as dV = 0
 (but W=pdV)
 so gas does not do any work (dW = 0)
 dQ = CvdT
 for one mole of a gas
 Now, So equation 1.2 becomes
 CvdT = dU -----------

 An isobaric process is one where the
pressure of the system stays constant. Iso
means the same, and baric means pressure.
 Pressure is related to the amount of force
that the molecules apply to the walls of the
container.
 Imagine that you have a gas inside a
movable piston and you heat that gas up.

 By heating the gas up you make the
molecules move faster, which would
normally increase the pressure.
 But at the same time the piston expands,
increasing the volume and giving the
molecules more room to move.
 Since the walls of the container are now
bigger, the pressure can stay the same even
though the molecules are moving faster.
 That makes it an isobaric process.

 The additional weight is now removed from the
piston.
 The piston now moves upwards through a
distance dx, such that the pressure of the
enclosed gas is equal to the atmospheric
pressure P.
 The temperature of the gas decreases due to
the expansion of the gas.
 Now a quantity of heat ‘dQ’ is supplied to the
gas till its temperature becomes T + dT.

 This heat energy is not only used to
increase the internal energy dU of the
gas but also to do external work dW in
moving the piston upwards
 Now, heat dQ is absorbed at constant
pressure,
 Then dQ = CpdT --------------- (6)
 CpdT = dU+ PdV --------------- (7)

Thank You
Dr. Mrs. Pritee M. Raotole, MGSM’s Arts Science and

Thermodynamics basics-2

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