Chapter 16 The Second Law of Thermodynamics

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Malcolm Greer
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1 Chapter 16 The Second Law of Thermodynamics To examine the directions of thermodynamic processes. To study heat engines. To understand internal combustion engines and refrigerators. To learn and apply the second law of thermodynamics To understand the Carnot engine: the most efficient heat engine. To learn the concept of entropy.

2 16.1 Directions of Thermodynamic Processes Heat flows spontaneously from a "hot" object to a "cold" object. A process can be Spontaneous Non-spontaneous Reversible Irreversible In equilibrium Overall, there can be an increase or decrease in order. Devices can interconvert order/disorder/energy. T 1 T 2 Q T 1 > T 2 a drop of ink free expansion of gas

3 16.2 Heat Engine A heat engine works in cycles. A heat engine absorbs an amount of heat energy Q H from the high-temperature reservoir. It does an amount of work Won the surrounding, and, it rejects an amount of heat energy Q C to the low-temperature reservoir. After it complete a cycle, it returns to its initial state, or, U = 0. Apply the First Law of Thermodynamics, Q W = U = 0. So, we have W = Q = Q H + Q C = Q H Q C The thermal efficiency of a heat engine Note: e = W Q H = Q H+Q C Q H = Q H Q C Q H = 1 Q C Q H (a) Q H is positive (absorbed into the system ). (b) Q C is negative (rejected from the system ). (c) W is positive (done by the system on surrounding).

4 16.3 Internal Combustion Engine Please read this section in the textbook.

5 16.4 Refrigerators---Heat Engine Running Backward A refrigerator also works in cycles. Work W is done on the refrigerator by a motor. The refrigerator absorbs an amount of heat Q C from the lowtemperature reservoir, and, it rejects an amount of heat Q H to the high-temperature reservoir. It completes a cycle and returns to its initial state, U = 0. Apply the First Law of Thermodynamics, Q W = U = 0, we have W = Q = Q c + Q H = Q C Q H, or, Q H = Q C W = Q C + W The efficiency of a refrigerator Note: K = Q C W = Q C Q H Q C (a) Q H is negative (rejected from the system ). (b) Q C is position (absorbs into the system ). (c) W is negative (done on the system by surrounding).

6 16.5 The Second Law of Thermodynamics Equivalent Statements of the Second Law of Thermodynamics The Engine Statement It is impossible for any heat engine to undergo a cyclic process in which it absorbs heat from a reservoir at a single temperature and converts the heat completely into mechanical work. The Refrigerator Statement It is impossible for any process to have as its sole result the transfer of heat from a cooler to a hotter object.

7 A workless refrigerator is impossible: It would violate the engine statement

8 A 100% efficient engine is impossible: It would violate the refrigerator statement

9 The Essence of the Second Law of Thermodynamics The one-way aspect of the processes occur in nature----irreversibility. T 1 T 2 Q a drop of ink free expansion of gas T 1 > T 2

10 Machines of Perpetual Motion that Violate the Laws of Thermodynamics Perpetual Motion of the First Type Is it possible to build a machine, which, once started its motion, could repeat its motion all by itself and carry out work perpetually? No, it is not possible. This machine violates the First Law of Thermodynamics, which dictates that, in cyclic motion with U = 0, W = Q Perpetual Motion of the Second Type Then, is it possible to build a machine that can take heat energy from a source and turn it 100% into work? No, it is not possible. This machine violates the Second Law of Thermodynamics, which dictates that part of the heat energy taken from a heat source must be rejected to a lower-temperature source, W = Q = Q H + Q C = Q H Q C

11 16.6 The Carnot Engine---The Most Efficient Engine The Carnot Cycle is an ideal model of the heat engine. If all the four processes are reversible, the efficiency is the highest possible. It can be shown that Q C Q H = T C T H therefore, the efficiency e = W Q H = Q H + Q C Q H = 1 + Q C Q H = 1 T C T H

12 16.7 Entropy----Provides a quantitative measure of disorder T 1 T 2 Q T 1 > T 2 a drop of ink free expansion of gas Consider an infinitesimal isothermal expansion of an ideal gas: or, U = 0, Q = W = p V = nrt V Q T = nr V V Since V/V is a measure of disorder, the change in entropy is define as: S = S 2 S 1 = Q T The Entropy Statement of the Second Law of Thermodynamics: V It is impossible to have a process in which the total entropy decreases when all systems taking part in the process are included. It is impossible for the entropy of a closed system to decrease.

Thermodynamic Systems, States, and Processes

Thermodynamics Thermodynamic Systems, States, and Processes A thermodynamic system is described by an equation of state, such as the ideal gas law. The location of the state can be plotted on a p V diagram,