State and explain the first law of Thermodynamics.

Statement : The amount of heat given to a system is equal to sum of increase in internal energy and external work done by the system. If dQ is the Quantity of heat supplied to a system, a part of it is used to increase, Its internal energy “du” and the rest is used in doing external work dw.
Then $\therefore \mathrm{dQ}=\mathrm{du}+\mathrm{dw}$
Sign convection :
dW is +ve if the work is done by the system
dW is –ve if the work is done on the system
dQ is +ve When heat supplied to the system
dQ is –ve When heat is taken out from the system
Limitations :
i) It does not explain about the direction of heat flow
ii) It does not explain us about the efficiency with which heat can be converted into work.