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.

Or

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

constant.

Or

No machine can produce energy without corresponding expenditure of energy,i.e.

it is impossible to construct a PERPETUAL MOTION MACHINE OF I KIND

(PMM-I)

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 both are 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.

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

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

stored energy.

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

therefore,

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

stated as

“Energy can neither be created nor destroyed; however, it can be

converted from one from to another”.

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

as

“ A Perpetual Motion Machine of the first kind is impossible”

ENTHALPY:

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

by H.

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