The first law of thermodynamics is equivalent to law of conservation of energy. It deals with the transformation of heat energy into work and vice versa. For closed systems, energy can be transferred by work and heat transfer. In thermodynamics, the term work denotes a means for transferring energy. Work done by a system is considered positive: W > 0. Work done on a system is considered negative: W < 0. Heat given to a system is considered as positive: Q > 0. Heat exhaust by a system is considered as negative: Q < 0. The heat generation by the work done on the system is shown in Figure 1.2.
Figure 1.2 Mechanical Equivalent of Heat
When a small amount of work (dw) is supplied to a closed system undergoing a cycle, the work supplied will be equal to the heat transfer or heat produced (dQ) in the system.
where J is a joule constant; 1 cal = 4.18 J.
If Q amount of heat is given to a system undergoing a change of state and W is work done by the system and transferred during the process, the net energy (Q − W) will be stored in the system named as internal energy or simply energy of the system (ΔU).
Q − W = ΔU
Sign Convention: The convention is adopted that Q indicates the heat added to the system and W the work done by it. Thus,
dQ > 0, heat added to system or system absorbs heat.
dQ < 0, heat removed from system or system rejects heat.
dW > 0, work is done by system.
dW < 0, work is done on the system.
ΔU > 0, internal energy of system increases.
ΔU< 0, internal energy of system decreases.
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