1. For more information about the
basics of heat
view presentation
‘Heat, Calorimetry and
Thermodynamics by Aditya
Abeysinghe’

2. Thermodynamics

3. Laws of thermodynamics
1. Zeroth law of thermodynamics
If two systems are at the same time in
equilibrium with a third system, they are in
equilibrium with each other.
Practically, this means that all three systems are at
the same temperature.
Since A and B are at
A
equilibrium and B and C
are at equilibrium, A and
C are also at equilibrium
according to the zeroth
law
B
C

4. 2. First law of thermodynamics
The change in internal energy of a system is
equal to the heat added to the system minus the
work done by the system.
dU = δQ – δW (U- internal energy of the system,
Q-heat added to the system, W- work done by the
system)
The first law of thermodynamics is the application
of the conservation of energy principle to heat and
thermodynamic processes.

5. 3. Second law of thermodynamics
When two initially isolated systems which are at
thermal equilibriums are brought into contact they
reach a common thermal equilibrium
However, the second law can also be expressed in
terms of the application in which it is used.
For example,
I. Second law in terms of heat flow:
Heat flows spontaneously from hotter to colder
objects but not vice versa.
II. Second law in terms of heat engines:
It is impossible to construct an engine which has
100% efficiency or a system in which the heat added
to the system is solely used to perform work.

6. 4. Third law of thermodynamics
The entropy of a system approaches a constant
value as the temperature approaches zero.

7. Second law: Heat engines
Kelvin-Planck statement:
It is impossible to extract an amount of heat QH from a hot reservoir and use it all to do
work W. Some amount of heat QC must be exhausted to a cold reservoir.

8. Refrigerator and Air ConditionerClausius StatementIt is impossible for heat to flow from a cold body to a warm body without any work ( or
without the aid of an external agency) having been done to accomplish the flow.
Hot Reservoir
QH
W
QC
Cold Reservoir

9. Basics of Thermodynamics
Internal energy of a gasThe internal energy of a gas is the kinetic energy of thermal
motion of its molecules.
Important:
When a cylinder of a gas is moving in a locomotive or any
other moving agent, the molecules inside the cylinder move
relative to that of the cylinder (molecules travel in the same
speed as the cylinder). Thus the kinetic energy of each
molecule is equivalent to the kinetic energy experienced by
the cylinder. Therefore, the temperature and thus the
internal energy of the molecules relative to the cylinder is
unchanged.

10. Deriving an equation for internal
energy of a gas
The internal energy of a gas is the sum of the
kinetic energy of the molecules and the potential
energy(due to intermolecular attractions)
between molecules.
But there are no intermolecular forces between
molecules in an ideal gas. Therefore, potential
energy in such an instance is zero.
Therefore, Internal Energy = Kinetic Energy of
molecules

11. But, PV = ⅓ mNc2 and E = ½mNc2
So, PV = ⅓ × 2 × ½ mNc2
E
Therefore, PV = ⅔E or, E= 3 PV
2
But, PV = nRT . Therefore, E= nRT
Now that E = ΔU,
For a difference of temperature,
Internal Energy =
nRΔT
(ΔU)

12. Work done by gasWork done by gas is the work done to increase its volume during
expansion.
A
Δ W = Fx
Δ W = PAx
Therefore, Δ W = P(ΔV)
P
F
(PA)
X

13. Specific heats of a gas
When heat is given to a substance it expands and
does external work. In the case of solids and
liquids the change in volume and hence the
external work done is negligible. Therefore, there
is only one specific heat for a substance.
Gases experience the effect of the change of
volume to a great extent. Since the volume can be
controlled, gases can be used to do variable
amounts of external work. Therefore, there are no.
of specific heats that can be defined for a gas.

14. The two most commonly used are1. Specific heat at constant volume (CV)
It is the amount of heat required to raise the temperature of
unit mass of a gas through 1°C, when volume is kept constant.
2. Specific heat at constant pressure (CP)
It is the amount of heat required to raise the temperature of
unit mass of a gas through 1°C, when the pressure of the gas is kept
constant
However, CP > CV and CP / CV = γ. γ is called the ratio of specific
heats.
At constant volume, ΔQ = n CV θ
At constant pressure, ΔQ = n CP θ

15. The features of basic thermodynamic
equations
Increase /
Decrease
Term
Positive
OR
Increasing
Negative
OR
Decreasing
ΔQ
If heat is
given
from
outside
If heat is
released
to outside
ΔU
ΔU
ΔW
If internal
energy
increases
If a gas
does work
to outside
If internal
energy
decreases
If work is
done on
the gas

16. Special Occasions
1. Isothermal Process (ΔU = 0)
This usually occurs when a system is in thermal contact with a
reservoir. The change occurs very slowly and the system will
continually adjust to the temperature of the reservoir through heat
exchange.
ΔT = 0, therefore, ΔU = 0.
Thus, ΔQ = ΔW
The graph of such a process is as follows:
P
Temperature constant.
Therefore the system obeys
Boyle’s Law
V

17. 2. Adiabatic Process (ΔQ = 0)
An expansion in which no heat energy enters or leaves
the system.
Adiabatic processes can take place if the container in
which the process takes place has thermally-insulated
walls or the process happens in an extremely short time.
Adiabatic CoolingOccurs when the pressure of a substance is decreased as it
does work on the surroundings
Adiabatic HeatingOccurs when the pressure of a gas is increased from work
done on it by the surroundings.

18. The graph of such a process is as follows:
P
A (T1)
An adiabat is similar to an isotherm,
except that during expansion an
adiabat loses more pressure than an
isotherm, so it has a steeper
inclination
B (T2)
V
Density of isotherms
stays constant but the
density of adiabats grow
Adiabatic
process
Isothermal Processes
Each adiabat intersects
each isotherm exactly
once

19. 3. Isochoric Process (ΔW = 0)
Also called constant-volume process, iso volumetric
process, and isometric process.
Is a thermodynamic process during which the volume of the
closed system stays constant.
Since the system undergoes isochoric process, the volume is
constant.
Therefore,
Q= mCv ΔT and Q = U (W=0)
The graph for such a process is as follows:
and since
V is constant
T

20. Other Processes
1. Isobaric Process (Pressure Constant)
2. Isoentropic process (Entropy of system stays constant)