1. FIRST LAW OF THERMODYNAMICS
Statement of First law: When a system undergoes a thermodynamic cycle then the
net heat supplied to the system from the surrounding is equal to the net work done
by the system on its surrounding.
Heat and work are mutually convertible but, since energy can neither be created
nor be destroyed the total energy associated with an energy conversation remains
No machine can produceenergy without corresponding expenditure of energy,i.e.
it is impossible to constructa PERPETUALMOTION MACHINE OF I KIND
Mathematically it is expressed as
Where the symbol represents the cycle integral of closed path. This is the first
law of thermodynamics for a closed system undergoing a cycle.
Work and heat bothare Path functions but there difference is a point function and
this difference is called Internal Energy.
δQ - δW = dE
E is known as Internal energy which is a Property of a system that’s why cycle
integral of internal energy is Zero.
2. Above Equation can be written as
δQ = dE + δW
Above equation is valid for both open and closed system.
If we want to written the first law for a process (open or closed system)
then it can be written like this
Q1-2 = (E2 – E1)+ W1-2
Internal Energy is the summation of following energy
E = U + K.E +P.E + other Energy
Where U = Intermolecular Energy
K.E = Kinetic Energy
P.E = Potential Energy
Internal in differential form
dE = dU + d(KE) +d(PE) + d(other Energy)
First law for Closed System: Kinetic energy, Potential energy is very
small for closed system therefore they are considered as zero
Hence E = U
δQ = dU + δW
This equation is only valid for closed system
3. it can also be written as
Q1-2 = U2- U1 + ∫ Pdv
Corollaries of First law:
Corollary 1. (First law for a process): When a system executes a
process, the net heat equals the net work interaction plus change in
That is, δQ = dU + δW
Above aspect is referred to as corollary 1 which may be stated as:
There exists a property of a closed system such that a change in its
value during any change of state is given by the difference between
the heat supplied and work done.
Corollary 2.(First law for an isolated system) : For an isolated system,
both heat and work interactions are absent ( δQ = 0, δW =0) and
dE = 0 or E = constant
The above identity indicates that the energy of a system remains
unchanged if the system is isolated from its surroundings as regards
heat and work interactions.
This fact is known as principle of conservation of energy and may be
“Energy can neither be created nor destroyed; however, it can be
converted from one from to another”.
4. Corollary 3.(Perpetual Motion Machine of First Kind): A perpetual
motion of first kind (PMM1) is an imaginary device which produces a
continuous supply of work without absorbing any energy from the
surroundings or from other systems. Corollary 3 may then be stated
“ A Perpetual Motion Machine of the first kind is impossible”
Enthalpy is a property which is defined as the sum of intermolecular
energy and product of pressure and volume of a system. It is denoted
H =U +PV
Unit of every term is kJ
Enthalpy (H) is an Extensive property. Specific enthalpy is defined as
h = H/m or h = u + pv unit KJ/Kg