# First law of thermodynamics

intrapreneur um engineeringmasters
9. Feb 2015
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### First law of thermodynamics

• 1. FIRST LAW OF THERMODYNAMICS Statement of First law: When a system undergoes a thermodynamic cycle then the net heat supplied to the system from the surrounding is equal to the net work done by the system on its surrounding. Or Heat and work are mutually convertible but, since energy can neither be created nor be destroyed the total energy associated with an energy conversation remains constant. Or No machine can produceenergy without corresponding expenditure of energy,i.e. it is impossible to constructa PERPETUALMOTION MACHINE OF I KIND (PMM-I) Mathematically it is expressed as Where the symbol represents the cycle integral of closed path. This is the first law of thermodynamics for a closed system undergoing a cycle. Work and heat bothare Path functions but there difference is a point function and this difference is called Internal Energy. δQ - δW = dE E is known as Internal energy which is a Property of a system that’s why cycle integral of internal energy is Zero.
• 2. Above Equation can be written as δQ = dE + δW Above equation is valid for both open and closed system. If we want to written the first law for a process (open or closed system) then it can be written like this Q1-2 = (E2 – E1)+ W1-2 Internal Energy is the summation of following energy E = U + K.E +P.E + other Energy Where U = Intermolecular Energy K.E = Kinetic Energy P.E = Potential Energy Internal in differential form dE = dU + d(KE) +d(PE) + d(other Energy) First law for Closed System: Kinetic energy, Potential energy is very small for closed system therefore they are considered as zero Hence E = U δQ = dU + δW This equation is only valid for closed system
• 3. it can also be written as Q1-2 = U2- U1 + ∫ Pdv Corollaries of First law: Corollary 1. (First law for a process): When a system executes a process, the net heat equals the net work interaction plus change in stored energy. That is, δQ = dU + δW Above aspect is referred to as corollary 1 which may be stated as: There exists a property of a closed system such that a change in its value during any change of state is given by the difference between the heat supplied and work done. Corollary 2.(First law for an isolated system) : For an isolated system, both heat and work interactions are absent ( δQ = 0, δW =0) and therefore, dE = 0 or E = constant The above identity indicates that the energy of a system remains unchanged if the system is isolated from its surroundings as regards heat and work interactions. This fact is known as principle of conservation of energy and may be stated as “Energy can neither be created nor destroyed; however, it can be converted from one from to another”.
• 4. Corollary 3.(Perpetual Motion Machine of First Kind): A perpetual motion of first kind (PMM1) is an imaginary device which produces a continuous supply of work without absorbing any energy from the surroundings or from other systems. Corollary 3 may then be stated as “ A Perpetual Motion Machine of the first kind is impossible” ENTHALPY: Enthalpy is a property which is defined as the sum of intermolecular energy and product of pressure and volume of a system. It is denoted by H. H =U +PV Unit of every term is kJ Enthalpy (H) is an Extensive property. Specific enthalpy is defined as h = H/m or h = u + pv unit KJ/Kg