Calculate the internal energy change in each of the following cases:- 

(i) A system absorbed 15 kJ of heat and does 5 kJ of work. 

(ii) 5 kJ of work is dose on the system and 15 kJ of heat is given out by the system. 

(i) Here, q = +15 kJ

w = −5 kJ

∴ According to first low of thermodynamics, ΔU = q + w = 15 + (−5) = 10 kJ

Thus, internal energy of the system increases by 10 kJ.

(ii) Here, w = +5 kJ

q = −15 kJ

∴ According to first law of thermodynamics, ΔU = q + w = −15 + (+5) = −10 kJ

Thus, the internal energy of the system