Short Version : 18. Heat, Work, & First Law of Thermodynamics
18.1. The 1st Law of Thermodynamics
Either heating or stirring can raise T of the water.
Joule’s apparatus
1st Law of Thermodynamics:Increase in internal energy = Heat added Work done
U Q W
Thermodynamic state variable = variable independent of history.
e.g., U, T, P, V, …
Not Q, W, …
dU dQ dWdt dt dt
PE of falling weight
KE of paddle
Heat in water
18.2. Thermodynamic Processes
Quasi-static process: Arbitrarily slow process such that system always stays arbitrarily close to thermodynamic equilibrium.
Reversible process:Any changes induced by the process in the universe (system + environment) can be removed by retracing its path.
Reversible processes must be quasi-static.
Irreversible process:Part or whole of process is not reversible.
e.g., any processes involving friction, free expansion of gas ….
Twater = Tgas & rises slowly
system always in thermodynamic equilibrium
Work & Volume Changes
W F x p A x p V
W d W2
1
V
Vp dV
面積
Work done by gas on piston
extW F x
Isothermal Processes
Isothermal process: T = constant.
2
1
V
VW p dV 2
1
V
V
n R T dVV
2
1ln V
Vn R T V
2
1
ln VW n R TV
32
U N k T 0U Q W
2
1
ln VQ W n R TV
Isothermal processes on ideal gas
Constant-Volume Processes & Specific Heat
Constant-volume process ( isometric, isochoric, isovolumic ) : V = constant
0V 0W p V
U Q
VU Q n C T
CV = molar specific heat at constant volume
Ideal gas: U = U(T) ideal gas VU n C T for all processes
isometric processes
VU n C T only for isometric processes
1V
V
dQCn dT
Non-ideal gas:
Isobaric Processes & Specific Heat
Isobaric Process : constant P
2 1W p V V p V
Q U W U p V
isobaric processesPQ n C T
CP = molar specific heat at constant pressure
P Vn C T n C T p V Ideal gas, isobaric :
Vn C T n R T
P VC C R Ideal gas
Isotherms
1P
P
dQCn dT
Adiabatic Processes
Adiabatic process: Q = constant
e.g., insulated system, quick changes like combustion, …
U W
Tactics 18.1. p V const adiabat, ideal gas 1P
V
CC
1T V const Prob. 66
1 1 2 2
1p V p VW
Prob. 62
Adiabatic: larger p
cdf
TACTIC 18.1. Adiabatic Equation
Ideal gas, any process: VdU n C dT
p dV
p V n R T
Adiabatic process: dU dW Vn C dT
p dV V d p n R dT
V
p p dV V dpdVC R
0V VR C p dV C V d p
0p VC p dV C V d p
0dV d pV p
p
V
CC
ln lnV p const ln p V p V const
Example 18.3. Diesel Power
Fuel ignites in a diesel engine from the heat of compression (no spark plug needed).
Compression is fast enough to be adiabatic.
If the ignit temperature is 500C, what compression ratio Vmax / Vmin is needed?
Air’s specific heat ratio is = 1.4, & before the compression the air is at 20 C.
1T V const
1 / 1.4 1273 500273 20
K KK K
1 / 1
max min
min max
V TV T
11
Ideal Gas Processes
Cyclic Processes
Cyclic Process : system returns to same thermodynamic state periodically.
Example 18.4. Finding the Work
An ideal gas with = 1.4 occupies 4.0 L at 300 K & 100 kPa
pressure.
It’s compressed adiabatically to ¼ of original volume,
then cooled at constant V back to 300 K,
& finally allowed to expand isothermally to its original V.
How much work is done on the gas?
1A A B B
ABp V p VW
741 J
AB (adiabatic):
0BCW BC (isometric):
ln ACA
C
VW n R TV
CA (isothermal):
1.4 1100 4.0 1 4
1.4 1
kPa L
AB A
B
Vp pV
1
11
A A AAB
B
p V VWV
ln 4A Ap V 555 J
work done by gas: ABCA AB BC CAW W W W 186 J
18.3. Specific Heats of an Ideal Gas
32ideal gasU N k T
1V
UCn T
Ideal gas: 212
K m v32
k T
32
n R T32
R
52
RP VC C R P
V
CC
53
1.67
Experimental values ( room T ):
For monatomic gases, 5/3, e.g., He, Ne, Ar, ….
For diatomic gases, 7/5 = 1.4, CV = 5R/2, e.g., H2 , O2 , N2 ,
….
For tri-atomic gases, 1.3, CV = 3.4R, e.g., SO2 , NO2 , ….
Degrees of freedom (DoF) = number of independent
coordinates required to describe the system
Single atom: DoF = 3 (transl)
For low T ( vib modes not active ) :
Rigid diatomic molecule : DoF = 5 (3 transl + 2 rot)
Rigid triatomic molecule : DoF = 6 (3 transl + 3 rot)
The Equipartition Theorem
Equipartition theorem ( kinetic energy version):
For a system in thermodynamic equilibrium, each degree of freedom of a
rigid molecule contributes ½ kT to its average energy.
Equipartition theorem ( general version):
For a system in thermodynamic equilibrium, each degree of freedom described
by a quadratic term in the energy contributes ½ kT to its average energy.
2AfU n N k T
2VfC R
2f n R T
2ff
1
2PfC R
DoF ( f ) CV CP
Monatomic 3 3/2 5/2 5/3Diatomic 5 5/2 7/2 7/5Triatomic 6 3 4 4/3
Example 18.5. Gas Mixture
A gas mixture consists of 2.0 mol of oxygen (O2) & 1.0 mol of Argon (Ar).
Find the volume specific heat of the mixture.
2.2 R
2 2
52O OU n R T
32Ar ArU n R T
2
5 32 2mix O ArU n n R T
1 mixmix
UCn T
5 32.0 1.02 2
2.0 1.0
mol molR
mol mol
2
2
5 32 2O Ar
O Ar
n nR
n n
Quantum Effects
CV of H2 gas as function of T.
Below 20 K hydrogen is liquid,
above 3200 K it dissociates into individual atoms.
Quantum effect:
Each mechanism has a threshold energy.
Etransl < Erot < Evib
Translation
rotation+Translation
rotation+Translation+vibration
RepriseQuasi-static process : Arbitrarily slow process such that system always stays arbitrarily close to thermodynamic equilibrium.
Reversible process: Any changes induced by the process in the universe (system + environment) can be removed by retracing its path.
a c : Free expansion with no dissipative work.c b : Adiabatic.
a d : Adiabatic.d b : Free expansion with no dissipative work.
a e : Adiabatic.e b : Adiabatic dissipative work.
Insulated gas
1st law: The net adiabatic work done in all 3 processes are equal (shaded areas are equal).
Dissipative work: Work done on system without changing its configuration, irreversible.