Question

1. A system releases 82 J of heat. And the same time 4 J of work is done on the system by the surroundings. What is the ∆U?

   		-53 J
   		-3 J
   		8 J
   		-78 J
   		-6 J
   		-15 J

Answer 1

Answer:-

As we know that according to the first law of thermodynamics energy can neither be created nor destroyed but one form of energy can be changed into equivalent amount of the other form.The mathematical form of first law of thermodynamics is as follows:-

change in internal energy (U) = q + w --------------------------------------------------(1)

where

q = heat energy

w = work

Also we know that sign convention of heat energy and work are as follows:-

heat energy released by the system (q) = -ve

heat energy absorbed by the system (q) = + ve

and

work done by the sysytem (w) = - ve

work done on the sysytem (w) = + ve

therefore in a given question the sign convention of heat energy and work are as follows:-

Heat energy released by the system (q) = - 82 J

work is done on the system (w) = +  4 J

So

change in internal energy (U) = q + w

change in internal energy (U) = - 82 J +  4 J

change in internal energy (U) = - 78 J (i.e the answer)