Transcript First law of thermodynamics
The Laws of Thermodynamics
Physics Lecture Notes
The Laws of Thermodynamics (01 of 38)
Photo © Vol. 05 Photodisk/Getty FOUNDRY: It requires about 289 Joules of heat to melt one gram of steel. In this chapter, we will define the quantity of heat to raise the temperature and to change the phase of a substance.
• • • • •
Sadi Carnot 1796 – 1832 French Engineer Founder of the science of thermodynamics First to recognize the relationship between work and heat
Thermodynamics
is the study of processes in which energy is transferred as heat and as work .
IThe internal energy
is the sum of all the energy of all the
molecules in an object:
random translational kinetic energy
rotational kinetic energy vibrational energy intermolecular energy associated with their bonding.
ZEROTH LAW The
Zeroth law
states that "If bodies A and B are each separately in thermal equilibrium with body C , then A and B are in thermal equilibrium with each other." The common property between A and B is called
temperature.
Topics 1) The First Law of Thermodynamics 2) Work Done on a Gas 3) Pressure - Volume Graph 4) Thermodynamic Processes 5) The Second Law of Thermodynamics 6) Heat Engines 7) Carnot cycle 8) Entropy
The Laws of Thermodynamics (02 of 38)
First Law of Thermodynamics Q System U
Environment
D
U = Q
-
W
W
The Laws of Thermodynamics (03 of 38)
Pressure (Pa)
The Ideal Gas Law
Volume (m 3 )
PV = NkT
Absolute Temperature (K) Number of Molecules Boltzmann’s Constant (1.38 x 10 -23 J/K) Temperature and kinetic Theory13
Thermodynamic Processes
A. Isobaric Constant Pressure B. Iso-volumetric Constant Volume C. Isothermal Constant Temp.
D. Adiabatic No Heat Transfer between systems
The Laws of Thermodynamics (06 of 38)
Pressure
P
Pressure - Volume Graph Volume Isotherms (lines of constant temperature) T T 4 Area under curve represents work T 3 T 1 2 Internal energy is proportional to temperature
V
The Laws of Thermodynamics (05 of 38)
HEAT ENGINES
Hot Res. T H Q hot W out A heat engine is any device which through a cyclic process : Engine
• Absorbs heat Q
hot Q cold Cold Res. T C
• Performs work W
out
• Rejects heat Q
cold
THE SECOND LAW OF THERMODYNAMICS
Hot Res. T H Q hot Q Engine cold W out It is impossible to construct an engine that, operating in a cycle, produces no effect other than the extraction of heat from a reservoir and the performance of an equivalent amount of work.
Cold Res. T C Not only can you not win (1st law); you can’t even break even (2nd law)!
THE SECOND LAW OF THERMODYNAMICS
Hot Res. T H
400 J 100 J
Engine
300 J
Cold Res. T C • A possible engine.
Hot Res. T H
400 J 400 J
Engine Cold Res. T C • An IMPOSSIBLE engine.
EFFICIENCY OF AN
Hot Res. T H
Q H W
ENGINE
The efficiency of a heat engine is the ratio of the net work done W to the heat input Q H .
Q C
Engine W e = = Q H Q H - Q C Q H Cold Res. T C Q C e = 1 Q H
EFFICIENCY EXAMPLE
Hot Res. T H
800 J W 600 J
Engine Cold Res. T C An engine absorbs 800 J and wastes 600 J every cycle. What is the efficiency?
Q C e = 1 ---- Q H 600 J e = 1 800 J e = 25% Question: How many joules of work is done?
EFFICIENCY OF AN IDEAL
Hot Res. T H
Q
H
Q
C
ENGINE (Carnot Engine)
Engine
W
maember perfect engine, the quantities Q of heat gained and lost are proportional to the absolute temperatures T.
e = T H
Cold Res. T C T C e = 1 T H
For the engine The Carnot Cycle T h = 550 K Work done by engine each cycle W
Q h
-
Q c W
890 J
-
470 J
420 J Q h Engine = 890 J Q c T c = 470 J The efficiency of the engine e
W Q h
420 J 8 90 J
0 .
472
47 .
2 % W
The Laws of Thermodynamics (26 of 38)
The Carnot Cycle T h = 550 K Temperature of the cool reservoir Q c Q h
T c T h
T c
T h Q c Q h T c
550 K 470 J 5 50 J
290 K Q h Engine = 890 J W = 420 J Q c T c = 470 J The engine undergoes 22 cycles per second, its mechanical power output P
W t
Wf
420 J cycle
22 cycles s
9 .
24 kW
The Laws of Thermodynamics (27 of 38)
The Carnot Cycle A carnot engine absorbs 900 J of heat each cycle and provides 350 J of work T h Q h Engine = 900 J W = 350 J The efficiency of the engine e
W Q h
3 50 J 9 00 J
0 .
389
38 .
9 % The heat ejected each cycle W
Q h
-
Q c Q c
Q h
-
W
900 J
-
350 J
550 J Q c T c
The Laws of Thermodynamics (28 of 38)
The Carnot Cycle A carnot engine absorbs 900 J of heat each cycle and provides 350 J of work T h Q h Engine = 900 J W = 350 J The engine ejects heat at 10 o C The temperature of the hot reservoir Q c Q h
T c T h
T h
T c Q h Q c T c
283 K 900 J 5 50 J
463 K
190 o C Q c =550 J T c = 283 K
The Laws of Thermodynamics (29 of 38)
The Carnot Cycle T h = 650 K A carnot engine operates between a hot reservoir at 650 K and a cold reservoir at 300 K. If it absorbs 400 J of heat at the hot reservoir, how much work does it deliver?
Q h Engine = 400 J W = ?
Q c T c = 300 K e
W Q h
T h
-
T h T c W
Q h
T h
-
T h T c
W
400 J 650 K
-
300 K 650 K
215 J
The Laws of Thermodynamics (30 of 38)
Entropy Entropy is a measure of the disorder of a system. This gives us yet another statement of the second law: Natural processes tend to move toward a state of greater disorder.
Example: If you put milk and sugar in your coffee and stir it, you wind up with coffee that is uniformly milky and sweet. No amount of stirring will get the milk and sugar to come back out of solution.
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Entropy Another example: when a tornado hits a building, there is major damage. You never see a tornado approach a pile of rubble and leave a building behind when it passes.
Thermal equilibrium is a similar process – the uniform final state has more disorder than the separate temperatures in the initial state.
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Entropy Another consequence of the second law: In any natural process, some energy becomes unavailable to do useful work.
If we look at the universe as a whole, it seems inevitable that, as more and more energy is converted to unavailable forms, the ability to do work anywhere will gradually vanish. This is called the heat death of the universe.
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Summary First law of thermodynamics: Δ U
Q
-
W Isothermal process: temperature is constant.
Adiabatic process: no heat is exchanged.
Work done by gas at constant pressure: W
P Δ U Heat engine changes heat into useful work (requires temperature difference).
Efficiency of a heat engine: Carnot efficiency: e
W Q H e c
1
-
T L T H
1
-
Q L Q H
The Laws of Thermodynamics (36 of 38)