L43-s1,12

Physics 114 – Lecture 43• §15.4 The Second Law of Thermodynamics –

Introduction• First Law of Thermodynamics – energy is conserved• BUT – some processes, which conserve energy, occur

naturally, e.g., hot tea cooling, a glass breaking, an object falling and striking the ground, where its PE → KE → thermal energy, whereas others, which also conserve energy, do not occur naturally

• The first law of thermodynamics holds in each case• Second Law of Thermodynamics addresses why some

processes occur naturally, whereas others do not

L43-s2,12

Physics 114 – Lecture 43• The Second Law of Thermodynamics states which

processes can occur in nature• This law can be and has been stated in several ways, all

of which have been shown to be equivalent• Clausius’ statement:• Heat can flow spontaneously from a hot body to a cold

body but heat will not flow spontaneously from a cold body to a hot body

• A more general statement is needed• This more general statement was developed from studies

of heat engines, which are devices that convert thermal energy into mechanical work

L43-s3,12

Physics 114 – Lecture 43• §15.5 Heat Engines• Examples of Heat Engines − Steam Engines• Schematic Diagram

L43-s4,12

Physics 114 – Lecture 43• Four Stroke Internal Combustion Engine

L43-s5,12

Physics 114 – Lecture 43• ΔT is needed to drive a heat engine• At higher T the pressure, P, is higher than at lower T,

so W = P ΔV is higher on expansion than on compression, where T and hence P is lower

• Efficiency

• Efficiency, e = W/QH

• From the First Law, QH = W + QL

• → e = W/QH = (QH – QL)/ QH = 1 – (QL/QH)

• Study example 15.9

L43-s6,12

Physics 114 – Lecture 43• Carnot Engine• This is an ideal heat engine: it is reversible, as

opposed to real processes which are irreversible

L43-s7,12

Physics 114 – Lecture 43• Carnot proved that, for this ideal heat engine,

• eideal = (TH - TL)/TH = 1 – (TL/TH)

• Study examples 15.10 and 15.11• Kelvin-Planck statement of the 2nd Law of

Thermodynamics• No device is possible the sole

effect of which is to transform

a given amount of heat

completely into work

L43-s8,12

Physics 114 – Lecture 43• §15.6 Refrigerators, Air Conditioners and Heat Pumps• These devices operate in the opposite way to the heat

engine

L43-s9,12

Physics 114 – Lecture 43• Clausius’ Statement of the 2nd Law of Thermodynamics• No device is possible, the sole effect of which is to

transfer heat from a system at temperature, TL, to another system at temperature, TH

• It can be shown that all

statements of the 2nd Law

of Thermodynamics are

equivalent• Heat Pump

L43-s10,12

Physics 114 – Lecture 43• §15.7 Entropy and the Second Law of Thermodynamics• The concept of entropy was introduced by Clausius• The change in entropy in any process is the important

parameter• When an amount of heat, Q, is added to a system by a

reversible process at constant temperature, T, the change in entropy is defined to be

• ΔS = Q/T

L43-s11,12

Physics 114 – Lecture 43• General Statement of the 2nd Law of Thermodynamics• The total entropy of any system plus that of its

environment increases as a result of any natural process

L43-s12,12

Physics 114 – Lecture 43

• Read• §15.8 Order to Disorder• §15.9 Unavailability of Energy; Heat Death• §15.10 Evolution and Growth; “Time’s Arrow”• §15.11 Statistical Interpretation of Entropy and the

Second Law• §15.12 Thermal Pollution and Global Warming