The document discusses the first law of thermodynamics which states that energy cannot be created or destroyed, only changed from one form to another. It also discusses different types of thermodynamic systems and processes, including open, closed, and isolated systems, as well as isothermal, adiabatic, isobaric, and isochoric processes. Specific heat capacities at constant volume and pressure are also defined for gases on both a specific and molar basis.

1. The First Law of
Thermodynamics
PREPARED BY:Adip Rijal
Anish Sharma
Aashish Poudel
Bishal Kandel

2. The First Law of Thermodynamics
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FIRST LAW OFTHERMODYNAMICS
→ CONSERVATION OF ENERGY:
Energy Can Be Changed From One Form ToAnother,
But It Cannot Be Created Or Destroyed.
The TotalAmount Of EnergyAnd Matter In The
Universe Remains Constant, Merely Changing From One
Form ToAnother.
Energy Exists In Many Forms, Such As Mechanical
Energy, Heat, Light, Chemical Energy,And Electrical
Energy.
Energy Is TheAbility To BringAbout Change Or To Do
Work.
Thermodynamics Is The Study Of Energy.

3. Surroundings
System
The Boundary Of The System Is Arbitrarily Chosen
 The System Can Exchange Mass And Energy Through The
Boundary With The Environment.
 Thermodynamic system is defined as an assembly of
an extremely large number of particles having a certain pressure,
volume and temperature. It can exchange energy with
its surrounding by heat transfer or by mechanical work.

4. Types of System
 Open System
In this system both mass and energy of a system are
exchanged to its surrounding. Ex; boiling water in open vessel
 Closed System
In this system, energy is exchanged but mass cannot be exchanged
with the surrounding. An example of 'Closed System' is :- gas
confined in a cylinder. The Boundary in this case real. Made by the
cylinder and the piston.
 Isolated System
In this system, both mass and energy are not exchanged with its
surrounding. Example: Thermos

5. Thermodynamic variables
The physical parameters which can be varied in a
thermodynamic system are called thermodynamic variables. They
are also called thermodynamic co-ordinates which are used to
represent the state of a system. For example: temperature,
pressure, volume, internal energy, total mass, specific heat
capacity etc.

6. Consider a thermodynamic system having certain gas in a cylinder. A cylinder is fitted with movable,
frictionless piston. Let P be the pressure exerted by the gas, V be its volume and A be the cross-sectional area of
the piston as shown in the figure.
Now, force exerted by the gas on the piston is,
F = P×A ……………………. (i)
If dw be the infinitesimal work done by the gas during expansion dx then,
dw = Fdx = PAdx
But Adx = dV is small change in volume of gas due to expansion.
>>> dw = PdV ……………… (ii)
When the volume of the gas changes from V1 to V2, the total work done W is obtained by integrating equation
(ii) from V1 to V2 i.e,
W = …………… (iii)
dx
When V2> V1, then V2 – V1 is Positive. Hence, during expansion of the gas, work done by the gas or system
is positive.
When gas is compressed, V2< V1, then V2 – V1 is negative. Hence, during compression of the gas, work
done by the gas or system is negative
Work done in thermodynamic process

7. Indicator Diagram
 In general, Pressure of a thermodynamic
system changes with change in volume. In such
case, the work done by a system is determined by
the graph of P as a function of V. This graph is
called P-V diagram or indicator diagram. It is
shown in the figure below.
 The area under the curve AB gives the work
done by the system.

8. Let P and V be the pressure and volume of gas at point
E. Let the volume increases by small amount dV at
constant pressure to a point F very close to E.
Now, from diagram,
Area of the small strip EFGH = EG×GH
= PdV
= Work done during a small change of volume dV
Therefore, total work done W during the expansion can
be obtained by adding the area of such small strips
from A (P1, V1) to B (P2, V2).
W = area of ABCD = area under P-V diagram
Indicator Diagram

9. FIRST LAW OF THERMODYNAMICS
It states that “The total amount of heat supplied to the system is equal to
the sum of increase in internal energy of the system and external work done by
the system”.
Let dQ be the heat supplied to the system, dU be the change in internal
energy of the system and dW be the work done by the system.
Then, first law of thermodynamics is expressed mathematically as,
dQ = dU + dW …................. ( i )
Note: dQ = +ve If heat is given to the system
dQ = -ve If heat is lost by the system
dW = +ve If work is done by the system
dW = -ve If work is done on the system

10. Specific heat capacities of a gas
A gas has two specific heat capacities and two molar heat capacities.
They are discussed below.
A.Specific heat capacity
a.Specific heat capacity at constant volume
The amount of heat required to raise the temperature of 1Kg of a gas through 1K or 10C at
constant volume.
It is denoted by cv and has SI unit JKg-1K-1 or JKg-1 0C-1.
cv = (dQ)v ………………… (i) Where m is total mass of a gas
mdT
b. Specific heat capacity at constant pressure
The amount of heat required to raise the temperature of 1Kg of a gas through 1K or 10C at
constant pressure.
It is denoted by cp and has SI unit JKg-1K-1 or JKg-1 0C-1.
cp = (dQ)p ………………… (ii) Where m is total mass of a gas
mdT

11. n (no of moles) = m (mass of gas)
M ( molecular weight)
B. Molar heat capacity
a.Molar heat capacity at constant volume
The amount of heat required to raise the temperature of 1 mole of a gas through 1K or
10C at constant volume.
It is denoted by Cv and has SI unit Jmol-1K-1 or Jmol-1 0C-1.
Cv = (dQ)v = (dQ)v M = cvM…… (iii) Where n is the mole of a gas
ndT mdT
b. Molar heat capacity at constant pressure
The amount of heat required to raise the temperature of 1 mole of a gas through 1K or
1oC at constant pressure.
It is denoted by Cp and has SI unit Jmol-1K-1 or Jmol-1 0C-1.
Cp = (dQ)p = (dQ)p M = cpM…… (iv) Where n is the mole of a gas
ndT mdT
We have relation, Cp- Cv = R
Mcp- Mcv = R
cp - cv = R / M = r ( r is gas constant per unit molar mass )

12. ▶ An Isothermal Process Is One Where The Temperature
Does Not Change.

13. ▶In Order For An Isothermal Process ToTake Place, We
Assume The System Is In Contact WithAHeat Reservoir.
▶In General, WeAssume That The System Remains In
Equilibrium ThroughoutAll Processes.
 AnAdiabatic Process Is One Where There Is No Heat Flow
Into Or Out Of The System.

14.  An Isobaric Process OccursAt Constant Pressure;An
Isovolumetric One At Constant Volume.
▶If The Pressure Is Constant, The Work Done Is The
Pressure Multiplied By The Change In Volume:

15. ▶For Processes Where The Pressure Varies, The Work Done
Is TheArea Under The P-V Curve.

16.  If WeApply The First Law Of
Thermodynamics To The Human Body:
We Know That The Body Can Do Work. If The Internal
Energy Is Not To Drop, There Must Be Energy Coming
In. It Isn’t In The Form Of Heat; The Body Loses Heat
Rather ThanAbsorbing It.
Rather, It Is The Chemical Potential Energy Stored In
Foods.

17. ☺THANK YOU☺