9/11/08
NOTES 9/9/08 POSTED
QUESTIONS ABOUT POWER AND ENERGY?
EFFICIENCY AND THE
SECOND LAW OF
THERMODYNAMICS
THE THREE LAWS OF THERMODYNAMICS
1ST LAW: ENERGY IS CONSERVED.
YOU CAN’T WIN.
2ND LAW: THERMAL ENERGY
CANNOT BE TURNED INTO USEFUL
WORK WITH 100% EFFICIENCY.
YOU CAN’T BREAK EVEN.
3RD LAW: AS THE ABSOLUTE ZERO
YOU CAN’T GET OUT OF
OF TEMPERATURE IS APPROACHED,
THE GAME.
ALL THERMAL MOTION CEASES. (BUT
WE CAN NEVER GET THERE!)
HEAT ENGINES AND THE RISE OF
WESTERN CIVILIZATION
~1ST CENTURY CE: HERO’S ENGINE
1769: WATT’S STEAM ENGINE
1824: CARNOT CYCLE
1877: NICOLAUS OTTO PATENTS A 4-STROKE INTERNAL
COMBUSTION ENGINE
HERO OF ALEXANDRIA’S “ENGINE”
AEOLIPILE = BALL OF AEOLUS
WATT’S STEAM ENGINE
JAMES WATT 1736-1819
OTTO AND THE GASOLINE ENGINE
NICOLAUS OTTO
1832-1891
ANIMATION
THERMAL ENERGY
THERMAL ENERGY CAN BE THOUGHT OF AS A FORM OF KINETIC
ENERGY- THE ENERGY OF MOLECULES MOVING RANDOMLY. WE
DON’T KNOW THE ENERGY OF INDIVIDUAL MOLECULES- JUST THE
TOTAL ENERGY
TEMPERATURE IS A MEASURE OF AVERAGE ENERGY: THE
AVERAGE KINETIC ENERGY OF AN ATOM OR MOLECULE IS
PROPORTIONAL TO ABSOLUTE TEMPERATURE T
ABSOLUTE TEMPERATURE IS MEASURED IN DEGREES KELVIN:
KELVIN=CELSIUS+273 (K = C + 273).
ZERO DEGREES KELVIN IS ABSOLUTE ZERO, WHERE THERMAL
MOTION STOPS.
ENERGY AND USEFUL ENERGY
ENERGY IS NEVER LOST, BECAUSE IT IS CONSERVED (FIRST
LAW OF THERMODYNAMICS).
THERMAL ENERGY CAN BE WASTED, IN THE SENSE THAT IT
MAY NOT BE AVAILABLE TO DO USEFUL WORK.
ONE FORM OF THE SECOND LAW OF THERMODYNAMICS IS
THAT IT IS IMPOSSIBLE TO CONVERT THERMAL ENERGY INTO
USEFUL WORK WITH 100% EFFICIENCY.
A THOUGHT EXPERIMENT
HEAT ENGINES USE TEMPERATURE DIFFERENCES TO
TURN THERMAL ENERGY INTO USEFUL WORK.
ONCE THE TEMPERATURE DIFFERENCE IS GONE, THE
ENERGY IS STILL THERE, BUT WE CANNOT TURN IT
INTO USEFUL WORK.
THIS IS THE DIFFERENCE BETWEEN A HOT GAS
PUSHING A PISTON (THERMAL ENERGY CONVERTED TO
MECHANICAL ENERGY) AND FREE EXPANSION OF THE
SAME HOT GAS.
ENTROPY AND ORDER
ENTROPY IS A THERMODYNAMIC QUANTITY THAT MEASURES
THE DISORDER OF A THERMAL SYSTEM
GENERALLY, THE HIGHER THE TEMPERATURE, THE MORE
DISORDERED (RANDOM) A SYSTEM IS.
ANOTHER FORM OF THE SECOND LAW OF THERMODYNAMICS IS
THAT THE ENTROPY OF A CLOSED SYSTEM NEVER DECREASES!
THE SECOND LAW ALSO STATES THAT IT IS ONLY POSSIBLE TO
PERFORM WORK BY TRANSFERRING HEAT FROM HOT BODIES TO
COLD
STEAM ENGINE
WATER IS CONVERTED TO STEAM BY BURNING WOOD OR COAL
STEAM PUSHES PISTON BACK AND FORTH, TURNING WHEEL TO DO WORK
STEAM CONDENSES BACK INTO WATER AND SO ON ...
HEAT ENGINES
HOT
A HEAT ENGINE IS AN ENGINE
POWERED BY THERMAL ENERGY.
Qh
W
EXAMPLES INCLUDE AUTO ENGINES,
COAL-FIRED POWER PLANTS, AND
NUCLEAR REACTORS.
WE REPRESENT AN ABSTRACTION OF
A HEAT ENGINE AS HAVING THREE
PARTS: A HEAT SOURCE, AN ENGINE,
AND A HEAT SINK.
Qc
COLD
HEAT ENGINES 2
1ST LAW:
HOT
Qh = W + Qc
EFFICIENCY:
Qh
W
W
Qc
ε=
= 1−
Qh
Qh
2ND LAW- NO HEAT ENGINE IS
100% EFFICIENT:
ε<1
Qc != 0
Qc
COLD
REVERSIBLE ENGINES
REVERSIBLE ENGINES CAN RUN FORWARDS
AND BACKWARDS. THEY ARE MODELS FOR
REFRIGERATORS AND HEAT PUMPS. THEY
ARE ALSO CALLED CARNOT ENGINES.
(FRICTIONLESS SO IDEALIZATION)
HOT Th
Qh
W
KEY FACT FOR REVERSIBLE ENGINES:
Qc Tc
=
Qh Th
Qc
Tc
ε = 1−
= 1−
Qh
Th
NOTE: MUST USE ABSOLUTE TEMPERATURE!
Qc
COLD Tc
NO SUPER-ENGINES EXIST!
HOT
A SUPER-ENGINE IS A HEAT ENGINE
MORE EFFICIENT THAN A REVERSIBLE
ENGINE
WE WILL PROVE THAT THE EXISTENCE
OF A SUPER-ENGINE WOULD VIOLATE
THE 2ND LAW
THEREFORE, A REVERSIBLE ENGINE IS
THE MOST EFFICIENT ENGINE
Qh
W
Qc
COLD
PROOF BY CONTRADICTION
HOT
Qh
SUPER + REVERSIBLE=
SUPER-DUPER ENGINE
Qh
W’
W
SUPER
REV
Qc’
Qc
COLD
W’>W
EFFICIENCY BOUND FOR HEAT ENGINES
Tc
ε ≤ εrev = 1 −
Th
HOT
Qh
W
THIS IS AN ULTIMATE,
IRREVOCABLE BOUND ON
HEAT ENGINE EFFICIENCY
Qc
COLD
CERN
LHC
STANDARD MODEL
THREE FORCES
12 FERMIONS
BOSONS
TRANSMIT FORCE
COST OF THE LHC $3.8X109
IS IT WORTH IT?