Topic 5 Flashcards
The most fundamental law of
science
Second Law of Thermodynamics
Charles Percy Snow in his Two Cultures wrote
“Not knowing the Second Law of Thermodynamics is like never having read a work of Shakespeare”
CBE was a novelist and English physical chemist who also served in several important positions in the British Civil Service and briefly in the UK government.
Charles Percy Snow, Baron Snow
How highly Einstein regarded thermodynamics can be appreciated in the following quote
“A law is more impressive the greater the simplicity of its premises, the more different are the kinds of things it relates, and the more extended its range of applicability. It is the only physical theory of universal content, which I am convinced, that within the framework of applicability of its basic concepts will never be overthrown”
The second law of Thermodynamics simply means
“How things work, and why things work”
In fact in modern times the second law of
thermodynamics is
second law of thermodynamics is being used to even explain popular theories like big bang, expansion of the cosmos and how the time is fast tickling away towards the “heat death” of the universe
Aspects of the Second Law
- Predicting the direction of processes
- Establishing conditions for equilibrium
- Determining the best theoretical performance of cycles, engines, and other devices.
- Evaluating the quantitatively the factors that preclude the attainment of the best theoretical performance.
- Defining a temperature scale
- Developing means for evaluating properties
Three alternative statements of the second law of thermodynamics:
Traditional formulations of the second law
1. Clausius statement
2. Kelvin-Planck statement
3. Entropy statement
The Clausius statement of the second law asserts that:
“It is impossible for any system to operate in such a way that the sole result would be an energy transfer by heat from a cooler to a hotter body”
- contributed to the theory of electrolysis (the
breaking down of a compound by electricity)
GERMAN MATHEMATICIAN AND PHYSICIST
one of Europe’s elite theoretical physicists, was born in Köslin, Poland, in 1822.
Rudolf Julius Emanuel Clausius
Kelvin-Planck statement of the second law
“It is impossible for any system to operate in a thermodynamic cycle and deliver a net amount of energy by work to its surroundings while receiving energy by heat transfer from a single thermal reservoir”
a special kind of system that always remains at constant temperature even though energy is added or removed by heat transfer.
Thermal reservoir / reservoir
sequence of processes that begins and ends at the same state.
Thermodynamic cycle
Analytical form of the Kelvin–Planck statement
W(cycle) ≤ 0 (single reservoir)
- Devising the absolute
temperature scale, now called
the ‘Kelvin scale’ - Formulating the second law of
thermodynamics - Working to install telegraph
cables under the Atlantic.
Lord Kelvin
He was able to deduce the relationship between the energy and the frequency of radiation.
Max Karl Ernst Ludwig Planck
the equivalence of the Clausius and Kelvin – Planck statements of the second law
W(cycle) = Q(H) - Q(C)
Entropy Statement of the Second Law
Mass and energy are familiar examples of extensive properties of systems.
Entropy is another important extensive property.
Entropy balance
Entropy Balance
(change in the amount of entropy contained within the system during some time interval)
=
(net amount of entropy transferred in across the system boundary during the time interval)
+
(amount of entropy produced within the system during the time interval)
Unlike mass and energy, which are conserved
entropy is produced (or generated) within systems whenever nonidealities (called irreversibilities) such as friction are present.
Entropy statement of the second law
“It is impossible for any system to operate in a way that entropy is destroyed”
a process that can be reversed without
leaving any trace on the surroundings. It means both system and surroundings are returned to their initial states at the end of the reverse process.
do not occur
idealizations of actual processes.
things happen very slowly, without any resisting force, without any space limitation
everything happens in a highly organized way
Reversible process
one in which heat is transferred through a finite temperature.
Irreversible process
if no irreversibilities occur within the boundaries of the system
system undergoes through a series of equilibrium states, and when the process is reversed, the system passes through exactly the same equilibrium states while returning to its initial state.
Internally reversible process:
if no irreversibilities occur outside the system
boundaries during the process.
Heat transfer between a reservoir and a system is an externally reversible process if the surface of contact between the system and reservoir is at the same temperature.
Externally reversible process:
Totally reversible (reversible):
both externally and internally reversible processes.
Two types of Irreversabilities
External irreversibilities
Internal irreversibilities
These are associated with dissipating effects outside the working fluid.
ex. Mechanical friction occurring during a
process due to some external source.
External irreversibilities
These are associated with dissipating effects within the working fluid.
ex. Unrestricted expansion of gas, viscosity and
inertia of the gas.
Internal irreversibilities
Commonly encountered causes of irreversibilities
Friction
Mixing of two fluids
electric resistance
Thermal Efficiency of the cycle is
n = W(cycle) / Q(H) = 1 - Q(C) / Q(H)
refrigeration cycle the coefficient of performance (COP) is
β = Q(C) / W(cycle) = Q(C) / Q(H) - Q(C)
The coefficient of performance (COP) for a
heat pump cycle is
γ = Q(H) / W(cycle) = Q(H) / Q(H) - Q(C)
Corollaries of the Second Law for Refrigeration and Heat Pump Cycles
- The coefficient of performance of an irreversible refrigeration cycle is always less than the coefficient of performance of a reversible refrigeration cycle when each operates between the same two thermal reservoirs.
- All reversible refrigeration cycles operating between the same two thermal reservoirs have the same coefficient of performance.
Note: By replacing the term refrigeration with heat pump, we obtain counterpart corollaries for heat pump cycles.
Carnot Efficiency
n(max) = 1 - T(C) / T(H)
The value of the Carnot efficiency increases as 𝑻𝑯 increases and/or 𝑻𝑪 decreases.
Thermal efficiency of a system undergoing a
reversible power cycle while operating between
thermal reservoirs at temperatures 𝑇𝐻 and 𝑇𝐶 .
Carnot Efficiency
Coefficient of performance of any system undergoing a reversible refrigeration cycle while operating between the two reservoirs:
β(max) = T(C) / T(H) - T(C)
Coefficient of performance of any system undergoing a reversible heat pump cycle while operating between the two reservoirs:
γ(max) = T(H) / T(H) - T(C)
n < n(max)
cycle operates Irrevesibly
n > n(max)
the power cycle is impossible
n = n(max)
the cycle operates reversibly
the temperatures used to evaluate βmax and γmax
It must be absolute temperatures on the Kelvin or Rankine scale.
the system executing the cycle undergoes a series of four internally reversible processes
Carnot Cycle
two adiabatic processes
alternated with two isothermal processes.
an ideal reversible closed thermodynamic cycle
Carnot cycle
The four processes of the cycle are:
Process 1–2:
The gas is compressed adiabatically to state 2, where the temperature is T(H)
Process 2–3:
The assembly is placed in contact with the reservoir at T(H). The gas expands isothermally while receiving energy Q(H) from the hot reservoir by heat transfer.
Process 3–4:
The assembly is again placed on the insulating stand and the gas is allowed to continue to expand adiabatically until the temperature drops to T(C)
Process 4–1:
The assembly is placed in contact with the reservoir at TC. The gas is compressed isothermally to its initial state while it discharges energy QC to the cold reservoir by heat transfer.
Second law of Thermodynamics states that
The Second Law of Thermodynamics states that in any natural process, the total entropy of an isolated system always increases or remains constant but never decreases.
