The internal energy change in a system that has absorbed 5 Kcals of heat and done 1000 J of work is:

7900 J 2000 J 20000 J 3950 J

20000 J

Q = \(\Delta\) U + W \(\Delta\) U = Q - W   = 5 x 4.2 x 1000 - 1000   = 20000 J