Second Law of Thermodynamics Why second law of

Second Law of Thermodynamics Why second law of thermodynamics? 1. The second law of thermodynamics helps us to predict whether the reaction is feasible or not. 2. It tells the direction of the flow of heat. 3. It also tells that energy cannot be completely converted into equivalent work.

Limitations of first law of thermodynamics 1) No restriction on the direction of the flow of heat: The first law of thermodynamics establishes equivalence between the quantity of heat used and the mechanical work but does not specify the conditions under which conversion of heat into work is possible, or the direction in which heat transfer can take place 2) Does not specify the feasibility of the reaction: First law does not specify that process is feasible or not. For example: when a rod is heated at one end then equilibrium has to be obtained which is possible only by some expenditure of energy. 3) Practically it is not possible to convert the heat energy into an equivalent amount of work

Example – 1: A cup of hot coffee left in a cooler room eventually cools off. The reverse of this process- coffee getting hotter as a result of heat transfer from a cooler room does not take place. Example – 2: Consider heating of a room by passage of electric current through an electric resistor. Transferring of heat from room will not cause electrical energy to be generated through the wire. Example – 3: Consider a paddle-wheel mechanism operated by fall of mass. Potential energy of mass decreases and internal energy of the fluid increases. Reverse process does not happen, although this would not violate first law. Example – 4: Water flows down hill where by potential energy is converted into kinetic energy. Reverse of this process does not occur in nature.

Example – 5: If two bodies having temperatures T 1 and T 2 (T 1 > T 2) come in contact with each other but are separated from their surroundings, the heat flows from the body with higher temperature to the body with lower temperature, till the temperature of both the bodies are equal. But we know that the reversal of this process in which heat flows from body with low temperature to body with high temperature is impossible. Example - 6: An automobile running at a certain speed can be stopped by applying the brakes. Here the kinetic energy of the automobile is converted into heat and the brakes get hot. If the hot brakes were to cool off and give back its internal energy to the automobile, it would cause the automobile to move on the road again. But this is impossible.

Conclusion: 1. Processes proceed in a certain direction and not in the reverse direction. 2. The first law places no restriction on direction. 3. A process will not occur unless it satisfies both the first and second laws of thermodynamics. 4. Second law not only identifies the direction of process, it also asserts that energy has quality as well as quantity.

Complete conversion? When work is converted into heat, we always have But, when heat is converted into work in a complete closed cycle process, Work High Grade Energy Heat Low Grade Energy

Thermal reservoirs 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 suffering appreciable changes in its thermodynamic properties. A Thermal source (High Temperature Reservoir – HTR) is a thermal energy reservoir which is capable of supplying heat to a heat engine operating in a cycle at constant temperature. A Thermal sink (Low Temperature Reservoir – LTR) is a thermal energy reservoir which is capable of absorbing heat rejected by a heat engine operating in a cycle at constant temperature.

Cyclic Heat Engine A cyclic heat engine is a heat engine that executes a thermodynamic cycle in which there is a net heat transfer to the system and the net work transfer from the system. Cyclic Heat Engine with Source & Sink

Thermal Efficiency of a Cyclic Heat Engine Thermal efficiency is a measure of success for a heat engine. It is defined as the rate of work developed from the heat engine operating in a cycle per unit rate of heat supplied to it. (or)

Reversed Cyclic Heat Engine Ø Reversed Cyclic Heat Engine is a cyclic heat engine which extracts heat from a cold body and reject it to a hot body by consuming external work supplied to it. Ø It may act as a heat pump or a refrigerator. Ø A heat pump is a device, operating in a cycle, maintains a body at a temperature higher than the temperature of its surroundings. (desired effect is to heat the body) Ø A refrigerator is a device, operating in a cycle, maintains a body at a temperature lower than the temperature of its surroundings. (desired effect is to cool the body)

Coefficient of Performance (COP) Ø The measure of performance of heat pump or refrigerator is called Coefficient of Performance (COP). Ø It is defined as the ratio of Desired Effect (or) Refrigerating Effect to the Net Work Done for it. Ø The desired effect (or) refrigerating effect in the case of heat pump is the amount of heat rejected to hot body, where as the desired effect for a refrigerator is the amount of heat extracted from cold body.

COP contd… Note:

Statements of II law of thermodynamics Statement – 1 (Kelvin-Plank statement): Statement – 2 (Clausius 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 It is impossible to construct a device which, operating in a cycle, will produce no effect other than the transfer of heat from a cooler to a hotter body.

Equivalence of Kelvin-Plank & Clausius statements Case – 1: Violation of Clausius statement results into violation of Kelvin-Plank statement Case – 2: Violation of Kelvin-Plank statement results into violation of Clausius statement

Causes of irreversibility 1) Lack of equilibrium during the process 2) Involvement of dissipative effects Examples of Lack of equilibrium 1) Heat transfer through temperature difference a finite 2) Lack of pressure equilibrium within the interior of the system or between the system and surroundings 3) Free expansion process

Examples of involvement of dissipative effects 1) Friction 2) Paddle wheel work transfer 3) Transfer of electricity through a resistor

Carnot cycle is an ideal hypothetical cycle in which all the processes constituting the cycle are reversible. Hence it is a reversible cycle. Pressure – volume diagram for Carnot cycle Process 1 -2: Reversible isothermal heat addition Process 2 -3: Reversible adiabatic expansion Process 3 -4: Reversible isothermal heat rejection Process 4 -1: Reversible adiabatic compression/pumping

Carnot Heat engine A cyclic heat engine operating on Carnot cycle is called Carnot Heat Engine. Reversed Carnot Heat engine A cyclic heat engine operating on reversed Carnot cycle is called Reversed Carnot Heat Engine.

Block diagrams of Carnot Heat engine and Reversed Carnot Heat engine

Carnot theorem It states that “All heat engines operating between a given temperature source and a given temperature sink, none has a higher efficiency than a reversible engine” Proof of Carnot theorem

Corollary of Carnot theorem Ø The efficiency of all heat reversible engines operating between the same temperature levels is the same. Ø The efficiency of a reversible heat engine is independent of the nature or amount of the working substance undergoing the cycle.

Absolute Thermodynamic Temperature Scale

Absolute Thermodynamic Temperature Scale