Review for Final

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# Review for Final - PowerPoint PPT Presentation

Review for Final. Physics 313 Professor Lee Carkner Lecture 25. Final Exam. Final is Tuesday, May 18, 9am 75 minutes worth of chapters 9-12 45 minutes worth of chapters 1-8 Same format as other tests (multiple choice and short answer) Worth 20% of grade

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Review for Final

Physics 313

Professor Lee Carkner

Lecture 25

Final Exam
• Final is Tuesday, May 18, 9am
• 75 minutes worth of chapters 9-12
• 45 minutes worth of chapters 1-8
• Same format as other tests (multiple choice and short answer)
• Three formula sheets given on test (one for Ch 9-12 and previous two)
• Bring pencil and calculator
Exercise #24 Maxwell
• Set escape velocity equal to maximum Maxwell velocity
• (2GM/R)½ = 10(3kT/m) ½
• m = (150KTR/GM)
• Planetary atmospheres
• Earth: m > 9.5X10-27 kg (NH3, O2)
• Jupiter: m > 1.4X10-28 kg (He, NH3, O2)
• Titan: m > 5.6X10-26 kg (None)
• Moon: m > 2.2X10-25 kg (None)
Thermal Equilibrium

Two identical metal blocks, one at 100 C and one at 120 C, are placed together. Which transfers the most heat?

• Two objects at different temperatures will exchange heat until they are at the same temperature
• Zeroth Law: Two systems in thermal equilibrium with a third are in thermal equilibrium with each other
Heat Transfer
• Heat:

Q = mcDT = mc(Tf-Ti)

• Conduction:

dQ/dt = -KA(dT/dx)

Q/t = -KA(T1-T2)/x

dQ/dt = Aes(Tenv4-T4)

Temperature

How would you make a tube of mercury into a Celsius thermometer? A Kelvin thermometer?

• Thermometers defined by the triple point of water
• A system at constant temperature can have a range of values for the other variables
• Isotherm
Measuring Temperature
• Thermometers

T (X) = 273.16 (X/XTP)

• Temperature scales

T (R) = T (F) + 459.67

T (K) = T (C) + 273.15

T (R) = (9/5) T (K)

T (F) = (9/5) T (C) +32

Equations of State

If the temperature of an ideal gas is doubled while the volume stays the same, what happens to the pressure?

• Equation of state detail how properties change with temperature
• Increasing T will generally increase the force and displacement terms
Mathematical Relations
• General Relations:

dx = ( x/y)zdy + (x/z)ydz

(x/y)z = 1/(y/x)z

(x/y)z(y/z)x(z/x)y = -1

• Specific Relations:
• Volume Expansivity: b = (1/V)(dV/dT)P
• Isothermal Compressibility: k=-(1/V)(dV/dP)T
• Linear Expansivity: a = (1/L)(dL/dT)t
• Young’s modulus: Y = (L/A)(dt/dL)T
Work

How much work is done in an isobaric compression of a gas at 1 Pa from 2 to 1 m3?

• The work done a system is the product of a force term and a displacement term
• No displacement, no work
• Compression is positive, expansion is negative
• Work is area under PV (or XY) curve
• Work is path dependant
Calculating Work

dW = -PdV

W = - PdV

• For ideal gas P = nRT/V
• Examples:
• Isothermal ideal gas:

W = -nRT  (1/V) dV = -nRT ln (Vf/Vi)

• Isobaric ideal gas:

W = -P  dV = -P(Vf-Vi)

First Law

Rank the following processes in order of increasing internal energy:

Isothermal expansion

Isochoric cooling

• Energy is conserved
• Internal energy is a state function, work and heat are not
First Law Equations

DU = Uf-Ui = Q+W

dU = dQ +dW

dU = CdT - PdV

Ideal Gas
• If the volume of an ideal gas is doubled and the pressure is tripled isothermally, how does the internal energy change?

lim (PV) = nRT

(dU/dP)T = (dU/dV)T = 0

(dU/dT)V = CV

CP = CV + nR

dQ = CVdT+PdV = CPdT-VdP

• Can an adiabatic process keep constant P, V, or T?

PVg = const

TVg-1 = const

T/P(g-1)/g = const

W = (PfVf - PiVi)/g-1

Kinetic Theory
• If the rms velocity of gas molecules doubles what happens to the temperature and internal energy

(1/2)mv2 = (3/2)kT

U = (3/2)NkT

T = mv2/3k

Engines
• If the heat entering an engine is doubled and the work stays the same what happens to the efficiency?
• Engines are cycles
• Change in internal energy is zero
• Composed of 4 processes

h = W/QH = (QH-QL)/QH = 1 - QL/QH

QH = W + QL

Types of Engines
• Otto

h = 1 - (T1/T2)

• Diesel

h = 1 - (1/g)(T4-T1)/(T3-T2)

• Rankine (steam)
• Stirling
• Isothermal, isochoric
Refrigerators
• Transfer heat from low to high T with the addition of work
• Operates in cycle
• Transfers heat with evaporation and condensation at different pressures

K = QL/W

K = QL/(QH-QL)

Second Law
• Is an ice cube melting at room temperature a reversible process?
• Kelvin-Planck
• Cannot convert heat completely into work
• Clausius
• Cannot move heat from low to high temperature without work
Carnot
• What two processes make up a Carnot cycle? How many temperatures is heat transferred at?

h = 1 - TL/TH

• Most efficient cycle
• Efficiency depends only on the temperature
Second Law
• The second law of thermodynamics can be stated:
• Engine cannot turn heat completely into work
• Heat cannot move from low to high temperatures without work
• Efficiency cannot exceed Carnot efficiency
• Entropy always increases
Entropy
• Entropy change is zero for all reversible processes
• All real processes are irreversible
• Can compute entropy for an irreversible process by replacing it with a reversible process that achieves the same result
• Entropy change of system + entropy change of surroundings = entropy change of universe (which is > 0)
Determining Entropy
• Can integrate dS to find DS

dS = dQ/T

DS =  dQ/T (integrated from Ti to Tf)

• Examples:
• Heat reservoir (or isothermal process)

DS = Q/T

• Isobaric

DS = CP ln (Tf/Ti)

Pure Substances
• Can plot phases and phase boundaries on a PV, PT and PTV diagram
• Saturation
• condition where substance can change phase
• Critical point
• above which substance can only be gas
• where (dP/dV) =0 and (d2P/dV2) = 0
• Triple point
• where fusion, sublimation and vaporization curves intersect
Properties of Pure Substances

cP = (dQ/dT)P (per mole)

cV = (dQ/dT)T (per mole)

b = (1/V)(dV/dT)P

k = -(1/V)(dV/dP)T

• cP, cV and b are 0 at 0 K and rise sharply to the Debye temperature and then level off
• cP and cV end up near the Dulong and Petit value of 3R
• k is constant at a finite value at low T and then increases linearly
Legendre Transform:

df = udx +vdy

g= f-ux

dg = -xdu+vdy

Useful theorems:

(dx/dy)z(dy/dz)x(dz/dx)y=-1

(dx/dy)f(dy/dz)f(dz/dx)f=1

dU = -PdV +T dS

dH = VdP +TdS

dA = - SdT - PdV

dG = V dP - S dT

(dT/dV)S = - (dP/dS)V

(dT/dP)S = (dV/dS)P

(dS/dV)T = (dP/dT)V

(dS/dP)T = -(dV/dT)P

Characteristic Functions and Maxwell’s Relations
Key Equations
• Entropy

T dS = CV dT + T (dP/dT)V dV

T dS = CP dT - T(dV/dT)P dP

• Internal Energy

(dU/dV)T = T (dP/dT)V - P

(dU/dP)T = -T (dV/dT)P - P(dV/dP)T

• Heat Capacity

CP - CV = -T(dV/dT)P2 (dP/dV)T

cP - cV = Tvb2/k

Joule-Thomson Expansion
• Can plot on PT diagram
• Isenthalpic curves show possible final states for an initial state

m = (1/cP)[T(dv/dT)P - v] = slope

• Inversion curve separates heating and cooling region

m = 0

• Total enthalpy before and after throttling is the same
• For liquefaction:

hi = yhL + (1-y)hf

Clausius-Clapeyron Equation
• Any first order phase change obeys:

(dP/dT) = (sf -si)/(vf - vi)

= (hf - hi)/T (vf -vi)

• dP/dT is slope of phase boundary in PT diagram
• Can change dP/dT to DP/DT for small changes in P and T
Open Systems
• For a steady flow open systems mass and energy are conserved:

Smin = Smout

Sin[Q + W + mq] = Sout [Q + W + mq]

• Where q is energy per unit mass or:

q = h + ke +pe (per unit mass)

• Chemical potential = m = (dU/dn)

mi = mf

• For open systems in equilibrium: