1. The First Law of Thermodynamics states that heat is a form of energynegativepositiveprinciple of conservation of energyQUW, and thermodynamic processes are therefore subject to the energynegativepositiveprinciple of conservation of energyQUW.

2. Δenergynegativepositiveprinciple of conservation of energyQUW = Δenergynegativepositiveprinciple of conservation of energyQUW + energynegativepositiveprinciple of conservation of energyQUW
where
Δenergynegativepositiveprinciple of conservation of energyQUW is the heat supplied to the system by the surroundings
Δenergynegativepositiveprinciple of conservation of energyQUW is the change in internal energy of the system
Δenergynegativepositiveprinciple of conservation of energyQUW is the work done by the system on the surroundings

3. Sign convention
• When work is done by the system, ΔW is energynegativepositiveprinciple of conservation of energyQUW.
• When work is done on the system, ΔW is energynegativepositiveprinciple of conservation of energyQUW.
• When heat is given to the system, ΔQ is energynegativepositiveprinciple of conservation of energyQUW.
• When heat is given by the system, ΔQ is energynegativepositiveprinciple of conservation of energyQUW.

4. Differential form of First law of Thermodynamics
δenergynegativepositiveprinciple of conservation of energyQUW = δenergynegativepositiveprinciple of conservation of energyQUW − δenergynegativepositiveprinciple of conservation of energyQUW
where
energynegativepositiveprinciple of conservation of energyQUW is the work done
energynegativepositiveprinciple of conservation of energyQUW is the internal energy of the system
energynegativepositiveprinciple of conservation of energyQUW is the heat supplied