The first law of thermodynamics states that ” energy can be neither created nor destroyed during a
process; it can only change forms “. The first law of thermodynamics is also known as the conservation of energy principle.
So the first law of thermodynamics can be stated as follows for a process:
If ” Q ” is the amount of heat transferred to the system and ” W ” is the amount of work transferred from the system during the process, the net energy transfer (Q-W) will be stored into the system. The energy stored into the system is neither heat nor work it is known as the internal energy of the system.
∴ Q − W = ∆E Or, Q = W + ∆E
For an isolated system, there is no interaction of the system with the surrounding.
For an isolated system dQ=0
dW=0
hence, dE=0 or, E=constant
Example:
Consider the heating of water in a pan on top of a range. If 15 kJ of heat is transferred to the water from the heating element and 3 kJ of it is lost from the water to the surrounding air, the increase in energy of the water will be equal to the net heat transfer to water, which is 12 kJ.