Lecture 3.5

Question 1

2nd law of thermodynamics

Answer 1

ΔS ≥ 0

Question 2

Carnot cycles

Answer 2

an idealised, reversible heat engine of maximal efficiency operating on a working substance between two thermal reservoirs to convert heat Q into mechanical energy W.

1-2 isothermal
2-3 adiabatic
3-4 isothermal
4-1 adiabatic

Question 3

cycle

Answer 3

a series of processes that return a system to its initial state

Question 4

heat engine

Answer 4

a device operating on a working substance between two thermal reservoirs to convert heat, Q, into mechanical energy, W.

Question 5

Efficiency

Answer 5

η = W/Q = Q1-Q2/Q1 = 1 - Q2/Q1

Question 6

Kelvin-Planck statement

Answer 6

it is impossible to construct a device that operating in a cycle will produce no other effect than the conversion of heat into work

Question 7

Clausius statement

Answer 7

It is impossible to construct a device that, operating in a cycle, produces no other effect than the transfer of heat from a colder body to a hotter body.

Question 8

The Kelvin-Planck and Clausius statement of the second law are

Answer 8

equivalent

Question 9

the Kelvin-Planck and Clausius forms of the 2nd law proof of equivalence

Answer 9

Consider now a heat heat engine that violates the Kelvin-Planck statement, extracting heat Q1 from a hot body at T1 to perform work W = Q1.

Let this engine run in parallel with a normal heat engine Q2 from the cold body and transfers Q3 to the hot body.

Energy conservation gives Q3 = Q2 + W = Q2 + Q1, so the net result is a transfer of heat from the cold body to the warm body.

This violates Clausius’s statement.