First law of thermodynamics;
Energy cannot be created or destroyed.
Energy can neither be created or nor destroyed although it can be converted from one form into another.
The energy of a system that isolated from its surroundings is constant.
Mathematical expresson for the first law
ΔU = q + w
q = heat added to the sytem
w = work done on the system
Sign conventions for heat and work
When w and q are positive, the internal energy increases. It means that energy is supplied to the system.
When w and q are negative, the internal energy decreases. It means that energy has left the system.