SECOND LAW OF THERMODYNAMICS. •••••••••••••••••••••••••••••••••••••••••• Work is said to be high grade energy and heat is a low grade energy. The complete conversation of low grade energy into high grade energy in a cycle is impossible while the complete conversation of high grade energy. 1. Second Law of Thermodynamics:- ★ Kelvin Plank Statement - It is impossible for a heat engine to produce net work in a complete cycle if it exchanges heat only with bodies at a single fixed temperature. Two reversible adiabatic paths cannot intersect each other which violates the Kelvin-Planks statement. ★ Clausis Statement- It is impossible to construct a device which operating in a cycle transfers heat from cooler body to a hotter body without any work input. 2 Thermal Energy:- ★ Thermal Energy Reservoir- A thermal energy reservoir (TER) is defined as a large body of infinite heat capacity which is capable of absorbing or rejecting an unlimited quantity of heat without any appreciable changes in its thermodynamic properties. ★ Heat Engine- A heat engine works on a thermodynamic cycle in which there is net heat transfer to the sytem and a net work transfer from the sytem. Thermal Efficiency Nth = W net/Q1=Q net/Q1=1-Q2/Q1 (i ). N th (rev.cyc)=1-T2/T1. ★ Heat Pump- C.O.P =Q1/Q 1-Q2 (ii) C.O.P (rev.cyc)=T1/T1-T2. ★Refrigerator- C.O.P =Q2/Q 1-Q2 (iii ). C.O.P(rev.cyc)=T2/T1-T2. (C.O.P) H.P = 1+(C.O.P) R. The above formulae (i ) (ii) and (iii ) are applicable for both reversible and irreversible cycles. 3. Carnot Theorem:- It states that all heat engines operating between a given constant temperature source and given constant temperature sink none has higher efficiency than a reversible engine. Continual motion of a movable device in complete absence of friction is known as PMM-3. * The efficiency of all reversible heat engines operating between the same temperature levels is the same. * The effect of decreasing T2 to increase thermal efficiency of Carnot cycle is more than the effect of increasing T1 by same amount. * First law of thermodynamics gave origin to internal energy and second law leads to entropy.
Posted on: Sat, 20 Sep 2014 15:02:46 +0000