(1) It is
also known as law of energy conservation.

(2)The
first law of thermodynamics states that energy can neither be created nor
destroyed, although it can be
transformed from one form to another.

(3)
Another Statement is “total energy of universe is remain constant”

(4) In

**FLOT**we introduce the term**ENTHALY**and**INTERNAL ENERGY**.

**MATHEMATICAL EXPRESSION OF FIRST LAW**

Let U

_{A}be the energy of a system in its state A and U_{B}be the energy in its state B. Suppose the system while undergoing change from state A to state B absorbs heat q from the surroundings and also performs some work (mechanical or electrical), equal to w. The absorption of heat by the system tends to raise the energy of the system. The performance of work by the system, on the other hand, tends to lower the energy of the system because performance of work requires expenditure of energy. Hence the change of internal energy, DU, accompanying the above process will be given by**d**

**U =U**

_{B -}U_{A =}q + w
In
general, if in a given process the quantity of heat transferred from the
surrounding to the system is q and work done in the process is w, then the
change in internal energy,

**dU**

**= q + w**

This
is the mathematical statement of the first law of thermodynamics.

**ACCORDINGTO IUPAC:**

(1)
If work is done by the surroundings

**on the system**(compression of a gas), w is taken as positive so that dU = q + w.
(2)
If however work is

**done by the system**on the surroundings ( expansion of a gas), w is taken as negative so that dU = q – w.
(3) q+w it will be independent of the way the change is carried out, it only depend on initial and final state.

(4) If their is no transfer of energy as heat or work (Isolated System)

ie, if w=0 and q=0. then dU=0

**EXAMPLE:**A system gives out 25 J of heat and also does 35 J of work. What is the internal energy change ?

**SOLUTION:**According to FLOT dU = q + w

dU =-25 J+(-35 J)

dU = -60 J

**Note:**

(1) d

**U = q + w**is invalid for open system.
(2) 1st
law of T.D. is applicable for closed system in which system is at rest or
moving with constant velocity and in
absence of external fields.

(3) The
macroscopic energy changes with velocity and elevation of the system are not
considered in internal energy change of system.

EXAMPLE: The pressure of a fluid is a linear function of volume P=a+bV and internal energy of fluid is U=34+3PV in SI unit. find a ,b, W ,dU for change in state (100 Pascal ,3m cube ) to (400 Pascal ,6m cube). (given Pascal =1J) .

SOLUTION: We know by FLOT DE=q+W

**LIMITATIONS OF**

**FIRST LAW THERMODYNAMICS:**

A major limitation of the first law of thermodynamics is
that it’s merely indicates that in any process there is an exact equivalence
between the various forms of energies involved, but it provides no information
concerning the spontaneity or feasibility of the process.

For example, the first law does not indicate whether heat
can flow from a cold end to a hot end or not.

The
answers to the above questions are provided by the second law of
thermodynamics.

**PARAMETER INVOLVE IN FIRST LAW THERMODYNAMICS**

**Continue reading.........**

**(2)**

**Work done (W)**

**(3)**

**Enthalpy (H)**

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