First law of thermodynamics

Ø   The first law of thermodynamics is a version of the law of conservation of energy, adapted for thermodynamic systems.

Ø   According to this, the energy can neither be created nor destroyed but just can be converted from one form to another.

    In thermodynamic way we can say that

Ø   “The law of conservation of energy states that the total energy of an isolated system is constant.”

Ø   The first law is often formulated by stating that the change in the internal energy of a closed system is equal to the amount of heat supplied to the system.

Ø   When we supply heat to any thermodynamic system, a part of it is used to change its internal energy while rest is converted into work.

Ø   Suppose U_i and U_f are the initial and final energy of the system, W the work done and Q is the heat energy absorbed, then the first law can be represented as under

Q = ( U_f  - U_i ) + W

Ø   Above eqn. is the mathematical representation of this law.

Ø   For a process taking place at constant pressure, if the volume change is dV, then W = PdV

dQ = dU + PdV

Ø   It should be noted that

1.   If the system gained heat, then dQ is +Ve and if lost then –Ve.

2.   Work done by a system is taken as positive, while work done on the system is taken as negative.