40_Lessons_on_Refrigeration_and_Air_Conditioning_www.hvac-eng.ir_35

40_Lessons_on_Refrigeration_and_Air_Conditioning_www.hvac-eng.ir_35

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6 some property must be same for the three systems. This property is temperature. Thus this law is the basis for temperature measurement. Equality of temperature is a necessary and sufficient condition for thermal equilibrium, i.e. no transfer of heat. The First law of thermodynamics is a statement of law of conservation of energy. Also, according to this law, heat and work are interchangeable. Any system that violates the first law (i.e., creates or destroys energy) is known as a Perpetual Motion Machine (PMM) of first kind. For a system undergoing a cyclic process, the first law of thermodynamics is given by: = W Q δ δ (4.2) where = cycle the during transfer heat net Q δ = cycle the during transfer work net W δ Equation (4.2) can be written as: = 0 ) W Q ( δ δ (4.3) This implies that ) W Q ( δ δ must be a point function or property of the system. This property is termed as internal energy, U . Mathematically, internal energy can be written as: W Q dU δ δ = (4.4) The internal energy of a system represents a sum total of all forms of energy viz. thermal,
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