First law of thermodynamics



Ø  Total energy in the universe remains constant.

Ø  Energy can neither be created nor be destroyed but can be transferred from one form to another.

When Q amount of heat is supplied to the system, some part of it is used up to increase in its internal energy and remaining some part of it is used up for work done.
i.e.          Q=∆E+W………………. (1)
If work is done by the system.
               
Q=∆E-W………………. (2)
                If work is done on the system.
                Where, Q= heat supplied
                                W=work done
                                ∆E=change in internal energy.

∆E at constant volume

We have, from first law of thermodynamics;
                Q=∆E+W
                Q=∆E+P∆V
At constant volume, ∆V=0
So,          Q=∆E
It means, at constant volume condition, the total amount of heat supplied is equal to change Internal energy of system.

∆E at adbiatic condition

We have, from first law of thermodynamics;
                Q=∆E+W
At adbiatic condition, Q=0
So,          -W=∆E
It means, at adbiatic condition, the work is done by utilizing the Internal energy of system.

Advantages of first law of thermodynamics

a.       Total energy of the universe remains constant.
b.      Different forms of energy are inter-convertible.
c.       When one form of energy disappears, an equivalent amount of energy in another form appears.

Disadvantages of first law of thermodynamics

a.       It doesn’t tell the extent and direction of the convertibility of one form of energy to another.
b.      It doesn’t tell why chemical reactions do not proceed to completion.
c.       It doesn’t tell why natural processes are unidirectional.
d.      It doesn’t explain the feasibility and spontaneity of a process.
e.      It says the equivalency of work and heat. But it has been observed that the work can be completely transformed into heat but heat cannot be transformed into work without permanent change in the system or surrounding which is not explained by the law.