First Law of Thermodynamics
First Law of Thermodynamics
First Law of Thermodynamics
The first law of thermodynamics Haga clic para modificar el estilo de subttulo del patrn By: Mariam Pitt
Essential UNIVERSITY PHYSICS, Volume 1 Richard Wolfson 5/28/12
U = Q W
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Q
Haga clic para modificar el estilo de subttulo del to the patrn system
Heat added
U
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U
U = (UU)
U = Q W
Haga clic para modificar el estilo de subttulo del patrn dU/dt is the rate of change of a systems internal energy. dQ/dt is the rate of heat transfer to the system. dW/dt is the rate at which the system does work. 5/28/12
EXAMPLE 18.1
The reactor in a nuclear power plant supplies energy at the rate of 3.0 GW, boiling water to produce steam that turns a turbine-generator. The spent steam is then condensed through thermal contact with water taken from a river. If the power plant produces electrical energy at the rate of 1.0 GW, at what rate is heat transferred to the river? The entire power plant is our system, comprising the nuclear reactor, including its fuel, and the turbine-generator. We identify U as the internal energy stored in the fuel, W as the mechanical work that ends up as electrical energy, and Q as the heat transferred to the river. We knowpatrn that: dU/dt = dQ/dt dW/dt. The reactor extracts internal energy from its fuel, so the rate dU/dt is negative; the power plant delivers electrical energy to the outside world, so dW/dt is positive. We need to solve for dQ/dt.
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EXAMPLE 18.1
-3.0 GW
1.0 GW
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pV = nRt
The thermodynamic state is completely determined by any two of this quantities p, V, or T.
p p,V, T V
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patrn
del
Wat er
Ga s
Temperature control
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er
Temperature control
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Haga clic para modificar el estilo de subttulo del patrn (work done during volume change)
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Haga clic para modificar el estilo constant, giving For an isothermal process, the temperature T is de subttulo del patrn
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Q = U W=0
Q = nCvT
V
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W = p(V-V) = pV
(isobaric Haga clic para modificar el estilo de subttulo del process) patrn
Cp is the molar specific heat at constant pressure.
Q = U + W W = p(V-V) Q = nCpT Cp = Cv + R
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U = -W
(adiabatic process)
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(adiabatic process)
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EXAMPLE 18.3
Fuel ignites in a diesel engine from the heat of compression as the piston moves toward the top of the cylinder; theres no spark plug as in a gasoline engine. Compression is fast enough that the process is essentially adiabatic. If the ignition temperature is 500C, WHAT COMPRESSION RATIO Vmax/Vmin is needed? Airs specific-heat ratio is =1.4, and before compression the air is at 20C. The problem involves temperature and volume, we apply the formula TminVmin (-1) = TmaxVmax (-1). Solving for the compression ratio Vmax/Vmin gives
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Monatomic Cv = (3/2)R
Diatomic
3 degrees of freedom
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THANK YOU!
Haga clic para modificar el estilo de subttulo del patrn
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