Question : A system is provided with 200 cal of heat and the work done by the system on the surrounding is 40 J. Then its internal energy : [Use 1 cal = 4.2 J]
(a) increases by 600 J
(b) decreases by 800 J
(c) increases by 800 J
(d) decreases by 600 J
Doubt by Manav
Solution :
Amount of Heat Provided (ΔQ)
= +200 Cal
= +200×4.2 [∵1 cal = 4.2 J]
= +840 J
Work done by the system (ΔW) = +40 J
Internal Energy (ΔU)=?
According to first law of thermodynamics
ΔQ=ΔU+ΔW
ΔQ-ΔW=ΔW
ΔW=ΔQ-ΔW
ΔW=+840-[40]
ΔW=840-40
ΔW=+800 J
Clearly, the internal energy of the system increases by 800 J
Hence, (c) increases by 800 J, would be the correct option.