Question

Heat gain by system in phy is neagtive or positive ? Pls tell according to physics convention ....

Answer 1

Heat given to system means heat gain is always positive in physics

Answer 2

Volume increase in the system is due to work done by the system. Therefore W is negative using your notation. Think of it this way, work done on the system would push the system inwards, decreasing volume. Therefore a volume increase is work done by the system.

Alternatively you could reason using the formula: dU=dQ−dW (using your notation conventions, were U is internal energy, W is work and Q is heat added to the system)

dW=PdV.

Therefore

dU=dQ−PdV.

Therefore if dV (change in volume) is positive, dU (change in internal energy) is negative.