GALAXY

FIRST LAW OF THERMODYNAMICS

STATEMENT:

"During any process total energy of a system and its surroundings is constant."
OR
"It is impossible to construct a machine which performs work
continuously with taking energy from an external source."
OR
"Energy can neither be created nor destroyed but it can be
converted from one form of energy to another form of energy."

MATHEMATICAL REPRESENTATION:

Let a system absorbs DQ amount of heat energy. Addition of heat energy increases the internal energy of system from U₁ to U₂ and some useful work is also performed by the system.
Increase in internal energy is given by:

DU = U₁ – U₂

and work done is DW
According to the first law of thermodynamics:

DQ = DU+ DW

SIGN CONVENTION:
DQ = positive if heat is added to a system
DQ = negative if heat is released from a system
DW = positive if work is done by the system
DW = negative if work is done on the system