The first law of thermodynamics is stated as dQ=dU−dW. Since dQ is heat supplied to the system and positive thus, dW is work done on the system, i.e., negative work.
We know that the first law of thermodynamics deals with work done, and heat energy supplied or removed from a system. Therefore, it is based on the conservation of energy. The law states that energy cannot be created or destroyed; it can only be converted from one form to another.