The Zeroth and First Laws. Mechanical energy includes both kinetic and potential energy. Kinetic...

The Zeroth and First Laws

The Zeroth and First Laws

• 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

• thermal equilibrium: objects have reached the same temperature


Two systems that are in thermal equilibrium with

a third must be in thermal equilibrium with

each other


• If no net energy exchange occurs in (a), then none will occur in (b).


• The General Law of Conservation of Energy

• in general:

First LawFirst Law

Q + Wncf = ΔU + ΔE

• mathematical statement of the first law of thermodynamics:

First LawFirst Law

Q = ΔU + W

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


• quasi-static process: gas expands without ever being far from thermal equilibrium

• gas pressure inside cylinder is in equilibrium with external pressure


• 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


• 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


• The work done by a gas depends on the path of the process in a P-V diagram.• Heat engines: positive• Refrigerators: negative


• 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


• The First Law of Thermodynamics is a conservation law

• It can be stated as...



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


• Adiabatic process: exchanges no thermal energy between system and its surroundings

• Q = 0



ΔU = -W

• Isothermal process: temperature of the system is constant

• no phase changes• ΔU = 0 J



Q = W

• Isochoric process: volume of the system is constant

• W = 0 J



Q = ΔU

• Isobaric process: pressure of the system is constant

• W = PΔV



Q = ΔU + PΔV

• A process that allows the use of ideal gas relationships is known as an ideal gas process.



The Second and Third Laws

The Second and Third Laws

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


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


• Thomas Newcomen• James Watt

• used separate chambers to heat and cool steam

• helped begin the Industrial Revolution


• 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

The Carnot Cycle The Carnot Cycle • P-V diagram:

Thermal EfficiencyThermal Efficiency

• For Carnot engine, thermal efficiency (ε) is defined as:

ε = × 100%TH – TC

TH


• To increase thermal efficiency (ε):• raise temperature of hot

reservoir• lower temperature of cold

reservoir


• 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



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.

Third Law of Thermodynamics

Third Law of Thermodynamics

Absolute zero is unattainable.

Entropy and its ConsequencesEntropy and its Consequences

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?



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.


For a reversible process:


ΔS ≡T

ΔQ

Units: J/K

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.


In natural (irreversible) processes, the entropy of the

universe increases.ΔS is positive.


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


• The first law does not support nor refute the theory of evolution.

• Naturalistic evolutionary cosmology is thermo-dynamically impossible.



• The second law would need to be almost constantly violated in order for evolution to occur.

• This has never been observed.



• The heat death of the universe is hypothetical.

• The universe will not be left to itself by God.



The Zeroth and First Laws. Mechanical energy includes both kinetic and potential energy. Kinetic...

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