The Zeroth and First Laws
Dec 18, 2015
• Mechanical energy includes both kinetic and potential energy.
• Kinetic energy can be changed to potential energy and vice versa.
IntroductionIntroduction
• Total mechanical energy (E) is the sum of kinetic and potential energies.
• Changes in a system’s total mechanical energy (ΔE) are important.
IntroductionIntroduction
• due to the rapid, random motion of the molecular, atomic, and subatomic particles of matter
• can be subdivided into kinetic energy and potential energy
Thermal EnergyThermal Energy
• average kinetic energy is proportional to the temperature of a substance
• Internal energy (U): sum of the particle kinetic and potential energies
Thermal EnergyThermal Energy
• adiabatic boundary: no thermal energy can pass through
• diathermic: ideal conductor of thermal energy
Zeroth LawZeroth Law
Two systems that are in thermal equilibrium with
a third must be in thermal equilibrium with
each other
Zeroth LawZeroth Law
The heat transferred to or from a system is equal to the sum of the change of
the system’s internal energy and the work the
system does on its surroundings.
First LawFirst Law
Heat EnginesHeat Engines
• can do mechanical work by absorbing and discharging heat
• the simplest example is an expanding gas
• cylinder with piston
Heat EnginesHeat Engines
• quasi-static process: gas expands without ever being far from thermal equilibrium
• gas pressure inside cylinder is in equilibrium with external pressure
Heat EnginesHeat Engines
• work is done on the gas when it is compressed from V1 to V2 • gas warms when it is
compressed• work done by gas on
surroundings is negative
Heat EnginesHeat Engines
• work done by gas when expanding or contracting against a constant pressure:
W = P(V2 – V1)
• pressure against a gas is not always constant
• graphing pressure versus volume (P-V diagram) makes some equations easier to solve
P-V DiagramsP-V Diagrams
P-V DiagramsP-V DiagramsNotice that the area under the curve representing the process on a P-V diagram
is equal to the absolute value of the work done by
the gas during the process!
P-V DiagramsP-V DiagramsThe sign of the work
depends on whether the gas gains or loses energy.
Gas expands → does work on surroundings →
sign is positive
Gas contracts → surroundings do work on it
→ sign is negative
• If a gas is to be useful as a machine, it must be able to expand repeatedly, following a cycle.
Expansion CyclesExpansion Cycles
• For a cycle, the absolute value of the work done is equal to the area enclosed by the path of the cycle on a P-V diagram.• Clockwise path: + work• CCW path: – work
Expansion CyclesExpansion Cycles
• The work done by a gas depends on the path of the process in a P-V diagram.• Heat engines: positive• Refrigerators: negative
Expansion CyclesExpansion Cycles
• Internal energy is path-independent: its change does not depend on the way the energy is added.
• Path-independent quantities are called state variables.
State VariablesState Variables
• a piece of the universe isolated for study
• if it is not part of the system, it is part of the surroundings
Thermodynamic Systems
Thermodynamic Systems
• can exchange both matter and energy with its surroundings• Ex.: ice cube resting on
a kitchen counter
Open SystemOpen System
• can exchange energy but not matter with its surroundings• Ex.: expanding gas in a
thermally conducting cylinder with a gas-tight piston
Closed SystemClosed System
• cannot exchange energy or matter with its surroundings• Ex.: liquid in a perfectly
insulated vacuum flask
Isolated SystemIsolated System
• energy is conserved• energy may be converted
but none leaves or enters• universe is the only true
isolated system• no practical system is
isolated
Isolated SystemIsolated System
• The First Law of Thermodynamics is a conservation law
• It can be stated as...
Isolated SystemIsolated System
Isolated SystemIsolated System
In an isolated system, the total quantity of energy is
constant, neither being created nor destroyed.
• a change in the thermodynamic state of a system
• often categorized by which variables are held constant
Thermodynamic Processes
Thermodynamic Processes
• Adiabatic process: exchanges no thermal energy between system and its surroundings
• Q = 0
Thermodynamic Processes
Thermodynamic Processes
ΔU = -W
• Isothermal process: temperature of the system is constant
• no phase changes• ΔU = 0 J
Thermodynamic Processes
Thermodynamic Processes
Q = W
• Isochoric process: volume of the system is constant
• W = 0 J
Thermodynamic Processes
Thermodynamic Processes
Q = ΔU
• Isobaric process: pressure of the system is constant
• W = PΔV
Thermodynamic Processes
Thermodynamic Processes
Q = ΔU + PΔV
• A process that allows the use of ideal gas relationships is known as an ideal gas process.
Thermodynamic Processes
Thermodynamic Processes
Heat EnginesHeat Engines• The surroundings must
contain either a source for thermal energy, a sink (receiver) for thermal energy, or both.
• Heat reservoir—temperature cannot be changed significantly
Heat EnginesHeat Engines• Hot reservoir
• higher temperature than the system
• source of thermal energy for the system
Heat EnginesHeat Engines• Cold reservoir
• lower temperature than the system
• thermal energy sink for the system
• Both types are used to operate a heat engine.
Second Law of Thermodynamics
Second Law of Thermodynamics
Energy flows from an area of higher concentration to
an area of lower concentration.
Heat EnginesHeat Engines• Requirements:
• hot reservoir• cold reservoir• working fluid (liquid or
gas)
Heat EnginesHeat Engines• Overview:
• thermal energy absorbed from hot reservoir causes fluid to expand
• expansion causes mechanical work
Heat EnginesHeat Engines• Overview:
• fluid gives up thermal energy to cold reservoir and contracts
• fluid is heated to expand again
Early Steam EnginesEarly Steam Engines
• aeolipile• Hero of Alexandria• not cyclic
• Thomas Savery• first practical steam
engine—water pump
Early Steam EnginesEarly Steam Engines
• Thomas Newcomen• James Watt
• used separate chambers to heat and cool steam
• helped begin the Industrial Revolution
Early Steam EnginesEarly Steam Engines
• James Watt• double-acting piston• additional mechanical
improvements
The Carnot Cycle The Carnot Cycle • Reversible process: quasi-
static process that leaves the system in exactly the same state after occurring twice, once normally and once in reverse
The Carnot Cycle The Carnot Cycle • Reversible cycle: leaves the
system in the same state as it was before the entire process occurred
• most efficient means of converting thermal energy to mechanical work
The Carnot Cycle The Carnot Cycle • Carnot cycle is the most
efficient cycle that can operate between two temperatures
• four-step, reversible cycle
The Carnot Cycle The Carnot Cycle • Step 1: isothermal
expansion from V1 to V2 at temperature TH
• Step 2: adiabatic expansion from V2 to V3; temperature changes from TH to TC
The Carnot Cycle The Carnot Cycle • Step 3: isothermal
compression from V3 to V4 at temperature TC
• Step 4: adiabatic compression from V4 to V1; temperature returns to TH
Thermal EfficiencyThermal Efficiency
• For Carnot engine, thermal efficiency (ε) is defined as:
ε = × 100%TH – TC
TH
Thermal EfficiencyThermal Efficiency
• To increase thermal efficiency (ε):• raise temperature of hot
reservoir• lower temperature of cold
reservoir
Thermal EfficiencyThermal Efficiency
• To increase thermal efficiency (ε):• both• efficiency can never
reach 100%
Heat PumpsHeat Pumps• can be used to move
thermal energy from a cold reservoir to a hot reservoir• air conditioning• refrigeration
Second Law of Thermodynamics
Second Law of Thermodynamics
Energy flows from an area of higher concentration to
an area of lower concentration.
Thermal energy naturally flows from hot bodies to
cold bodies.
Energy cannot be completed converted to work in a cyclic process.
You cannot get as much work out of a machine as
you put into it.
Perpetual motion machines are an impossibility.
Entropy (S) is a measurement of the
randomness, or disorder, of the particles in a specific
part of the universe.
What is Entropy?What is Entropy?
Second Law of Thermodynamics
Second Law of Thermodynamics
You cannot get as much work out of a machine as
you put into it.
Perpetual motion machines are an impossibility.
Entropy increases in all natural processes.
All natural processes make the universe more disorderly.
Disorder implies unusable energy; the energy still
exists but can no longer do useful work.
What is Entropy?What is Entropy?
In reversible processes, the entropy of the universe
remains constant.The change in a system’s entropy is balanced by the
change in the entropy of the surroundings.
What is Entropy?What is Entropy?
In natural (irreversible) processes, the entropy of the
universe increases.ΔS is positive.
What is Entropy?What is Entropy?
The most likely state of a system is one of disorder.
Entropy is also related to the encoding of information.
EntropyEntropy
• God created the universe with an immense supply of usable energy.
• Only God can create or destroy.
Conservation and Degeneration in Nature
Conservation and Degeneration in Nature
• The first law does not support nor refute the theory of evolution.
• Naturalistic evolutionary cosmology is thermo-dynamically impossible.
Conservation and Degeneration in Nature
Conservation and Degeneration in Nature
• The second law would need to be almost constantly violated in order for evolution to occur.
• This has never been observed.
Conservation and Degeneration in Nature
Conservation and Degeneration in Nature