Lecture 4 Hydrostatics and Time Scales
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Lecture 1 Overview Time Scales, Temperature-Density Scalings
I. Preliminaries The life of any star is a continual struggle between the force of gravity, seeking to reduce the star to a point, and pressure, which holds it up. Stars are gravitationally Lecture 1 confined thermonuclear reactors. So long as they remain non-degenerate and have not Overview encountered the pair instability, overheating leads to Time Scales, Temperature-density expansion and cooling. Cooling, on the other hand, leads to contraction and heating. Hence stars are generally stable. Scalings, Critical Masses The Virial Theorem works. But, since ideal gas pressure depends on temperature, stars must remain hot. By being hot, they are compelled to radiate. In order to replenish the energy lost to radiation, stars must either contract or obtain energy from nuclear reactions. Since nuclear reactions change their composition, stars must evolve. Central Conditions The Virial Theorm implies that if a star is neither too at death the degenerate nor too relativistic (radiation or pair dominated) iron cores of massive stars 2 GM are somewhat MN kT (for an ideal gas) R A degenerate GM T N AkR 1/3 ⎛ 3M ⎞ R for constant density ⎝⎜ 4πρ ⎠⎟ GM 2/3 So ρ1/3 T 3 N Ak ρ ∝ T That is as stars of ideal gas contract, they get hotter and since a given fuel (H, He, C etc) burns at about the same temperature, more massive stars will burn their fuels at lower density, i.e., higher entropy. Four important time scales for a (non-rotating) star to adjust its The free fall time scale is ~R/vesc so 1/2 structure can be noted. -
Hydrostatic Shapes
7 Hydrostatic shapes It is primarily the interplay between gravity and contact forces that shapes the macroscopic world around us. The seas, the air, planets and stars all owe their shape to gravity, and even our own bodies bear witness to the strength of gravity at the surface of our massive planet. What physics principles determine the shape of the surface of the sea? The sea is obviously horizontal at short distances, but bends below the horizon at larger distances following the planet’s curvature. The Earth as a whole is spherical and so is the sea, but that is only the first approximation. The Moon’s gravity tugs at the water in the seas and raises tides, and even the massive Earth itself is flattened by the centrifugal forces of its own rotation. Disregarding surface tension, the simple answer is that in hydrostatic equi- librium with gravity, an interface between two fluids of different densities, for example the sea and the atmosphere, must coincide with a surface of constant ¡A potential, an equipotential surface. Otherwise, if an interface crosses an equipo- ¡ A ¡ A tential surface, there will arise a tangential component of gravity which can only ¡ ©¼AAA be balanced by shear contact forces that a fluid at rest is unable to supply. An ¡ AAA ¡ g AUA iceberg rising out of the sea does not obey this principle because it is solid, not ¡ ? A fluid. But if you try to build a little local “waterberg”, it quickly subsides back into the sea again, conforming to an equipotential surface. Hydrostatic balance in a gravitational field also implies that surfaces of con- A triangular “waterberg” in the sea. -
Thermodynamics Notes
Thermodynamics Notes Steven K. Krueger Department of Atmospheric Sciences, University of Utah August 2020 Contents 1 Introduction 1 1.1 What is thermodynamics? . .1 1.2 The atmosphere . .1 2 The Equation of State 1 2.1 State variables . .1 2.2 Charles' Law and absolute temperature . .2 2.3 Boyle's Law . .3 2.4 Equation of state of an ideal gas . .3 2.5 Mixtures of gases . .4 2.6 Ideal gas law: molecular viewpoint . .6 3 Conservation of Energy 8 3.1 Conservation of energy in mechanics . .8 3.2 Conservation of energy: A system of point masses . .8 3.3 Kinetic energy exchange in molecular collisions . .9 3.4 Working and Heating . .9 4 The Principles of Thermodynamics 11 4.1 Conservation of energy and the first law of thermodynamics . 11 4.1.1 Conservation of energy . 11 4.1.2 The first law of thermodynamics . 11 4.1.3 Work . 12 4.1.4 Energy transferred by heating . 13 4.2 Quantity of energy transferred by heating . 14 4.3 The first law of thermodynamics for an ideal gas . 15 4.4 Applications of the first law . 16 4.4.1 Isothermal process . 16 4.4.2 Isobaric process . 17 4.4.3 Isosteric process . 18 4.5 Adiabatic processes . 18 5 The Thermodynamics of Water Vapor and Moist Air 21 5.1 Thermal properties of water substance . 21 5.2 Equation of state of moist air . 21 5.3 Mixing ratio . 22 5.4 Moisture variables . 22 5.5 Changes of phase and latent heats . -
Introduction to Hydrostatics
Introduction to Hydrostatics Hydrostatics Equation The simplified Navier Stokes equation for hydrostatics is a vector equation, which can be split into three components. The convention will be adopted that gravity always acts in the negative z direction. Thus, and the three components of the hydrostatics equation reduce to Since pressure is now only a function of z, total derivatives can be used for the z-component instead of partial derivatives. In fact, this equation can be integrated directly from some point 1 to some point 2. Assuming both density and gravity remain nearly constant from 1 to 2 (a reasonable approximation unless there is a huge elevation difference between points 1 and 2), the z- component becomes Another form of this equation, which is much easier to remember is This is the only hydrostatics equation needed. It is easily remembered by thinking about scuba diving. As a diver goes down, the pressure on his ears increases. So, the pressure "below" is greater than the pressure "above." Some "rules" to remember about hydrostatics Recall, for hydrostatics, pressure can be found from the simple equation, There are several "rules" or comments which directly result from the above equation: If you can draw a continuous line through the same fluid from point 1 to point 2, then p1 = p2 if z1 = z2. For example, consider the oddly shaped container below: By this rule, p1 = p2 and p4 = p5 since these points are at the same elevation in the same fluid. However, p2 does not equal p3 even though they are at the same elevation, because one cannot draw a line connecting these points through the same fluid. -
14 Timescales in Stellar Interiors
14Timescales inStellarInteriors Having dealt with the stellar photosphere and the radiation transport so rel- evant to our observations of this region, we’re now ready to journey deeper into the inner layers of our stellar onion. Fundamentally, the aim we will de- velop in the coming chapters is to develop a connection betweenM,R,L, and T in stars (see Table 14 for some relevant scales). More specifically, our goal will be to develop equilibrium models that describe stellar structure:P (r),ρ (r), andT (r). We will have to model grav- ity, pressure balance, energy transport, and energy generation to get every- thing right. We will follow a fairly simple path, assuming spherical symmetric throughout and ignoring effects due to rotation, magneticfields, etc. Before laying out the equations, let’sfirst think about some key timescales. By quantifying these timescales and assuming stars are in at least short-term equilibrium, we will be better-equipped to understand the relevant processes and to identify just what stellar equilibrium means. 14.1 Photon collisions with matter This sets the timescale for radiation and matter to reach equilibrium. It de- pends on the mean free path of photons through the gas, 1 (227) �= nσ So by dimensional analysis, � (228)τ γ ≈ c If we use numbers roughly appropriate for the average Sun (assuming full Table3: Relevant stellar quantities. Quantity Value in Sun Range in other stars M2 1033 g 0.08 �( M/M )� 100 × � R7 1010 cm 0.08 �( R/R )� 1000 × 33 1 3 � 6 L4 10 erg s− 10− �( L/L )� 10 × � Teff 5777K 3000K �( Teff/mathrmK)� 50,000K 3 3 ρc 150 g cm− 10 �( ρc/g cm− )� 1000 T 1.5 107 K 106 ( T /K) 108 c × � c � 83 14.Timescales inStellarInteriors P dA dr ρ r P+dP g Mr Figure 28: The state of hydrostatic equilibrium in an object like a star occurs when the inward force of gravity is balanced by an outward pressure gradient. -
The Underlying Physical Meaning of the Νmax − Νc Relation
A&A 530, A142 (2011) Astronomy DOI: 10.1051/0004-6361/201116490 & c ESO 2011 Astrophysics The underlying physical meaning of the νmax − νc relation K. Belkacem1,2, M. J. Goupil3,M.A.Dupret2, R. Samadi3,F.Baudin1,A.Noels2, and B. Mosser3 1 Institut d’Astrophysique Spatiale, CNRS, Université Paris XI, 91405 Orsay Cedex, France e-mail: [email protected] 2 Institut d’Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août 17, 4000 Liège, Belgium 3 LESIA, UMR8109, Université Pierre et Marie Curie, Université Denis Diderot, Obs. de Paris, 92195 Meudon Cedex, France Received 10 January 2011 / Accepted 4 April 2011 ABSTRACT Asteroseismology of stars that exhibit solar-like oscillations are enjoying a growing interest with the wealth of observational results obtained with the CoRoT and Kepler missions. In this framework, scaling laws between asteroseismic quantities and stellar parameters are becoming essential tools to study a rich variety of stars. However, the physical underlying mechanisms of those scaling laws are still poorly known. Our objective is to provide a theoretical basis for the scaling between the frequency of the maximum in the power spectrum (νmax) of solar-like oscillations and the cut-off frequency (νc). Using the SoHO GOLF observations together with theoretical considerations, we first confirm that the maximum of the height in oscillation power spectrum is determined by the so-called plateau of the damping rates. The physical origin of the plateau can be traced to the destabilizing effect of the Lagrangian perturbation of entropy in the upper-most layers, which becomes important when the modal period and the local thermal relaxation time-scale are comparable. -
Astronomy 201 Review 2 Answers What Is Hydrostatic Equilibrium? How Does Hydrostatic Equilibrium Maintain the Su
Astronomy 201 Review 2 Answers What is hydrostatic equilibrium? How does hydrostatic equilibrium maintain the Sun©s stable size? Hydrostatic equilibrium, also known as gravitational equilibrium, describes a balance between gravity and pressure. Gravity works to contract while pressure works to expand. Hydrostatic equilibrium is the state where the force of gravity pulling inward is balanced by pressure pushing outward. In the core of the Sun, hydrogen is being fused into helium via nuclear fusion. This creates a large amount of energy flowing from the core which effectively creates an outward-pushing pressure. Gravity, on the other hand, is working to contract the Sun towards its center. The outward pressure of hot gas is balanced by the inward force of gravity, and not just in the core, but at every point within the Sun. What is the Sun composed of? Explain how the Sun formed from a cloud of gas. Why wasn©t the contracting cloud of gas in hydrostatic equilibrium until fusion began? The Sun is primarily composed of hydrogen (70%) and helium (28%) with the remaining mass in the form of heavier elements (2%). The Sun was formed from a collapsing cloud of interstellar gas. Gravity contracted the cloud of gas and in doing so the interior temperature of the cloud increased because the contraction converted gravitational potential energy into thermal energy (contraction leads to heating). The cloud of gas was not in hydrostatic equilibrium because although the contraction produced heat, it did not produce enough heat (pressure) to counter the gravitational collapse and the cloud continued to collapse. -
STARS in HYDROSTATIC EQUILIBRIUM Gravitational Energy
STARS IN HYDROSTATIC EQUILIBRIUM Gravitational energy and hydrostatic equilibrium We shall consider stars in a hydrostatic equilibrium, but not necessarily in a thermal equilibrium. Let us define some terms: U = kinetic, or in general internal energy density [ erg cm −3], (eql.1a) U u ≡ erg g −1 , (eql.1b) ρ R M 2 Eth ≡ U4πr dr = u dMr = thermal energy of a star, [erg], (eql.1c) Z Z 0 0 M GM dM Ω= − r r = gravitational energy of a star, [erg], (eql.1d) Z r 0 Etot = Eth +Ω = total energy of a star , [erg] . (eql.1e) We shall use the equation of hydrostatic equilibrium dP GM = − r ρ, (eql.2) dr r and the relation between the mass and radius dM r =4πr2ρ, (eql.3) dr to find a relations between thermal and gravitational energy of a star. As we shall be changing variables many times we shall adopt a convention of using ”c” as a symbol of a stellar center and the lower limit of an integral, and ”s” as a symbol of a stellar surface and the upper limit of an integral. We shall be transforming an integral formula (eql.1d) so, as to relate it to (eql.1c) : s s s GM dM GM GM ρ Ω= − r r = − r 4πr2ρdr = − r 4πr3dr = (eql.4) Z r Z r Z r2 c c c s s s dP s 4πr3dr = 4πr3dP =4πr3P − 12πr2P dr = Z dr Z c Z c c c s −3 P 4πr2dr =Ω. Z c Our final result: gravitational energy of a star in a hydrostatic equilibrium is equal to three times the integral of pressure within the star over its entire volume. -
1. A) the Sun Is in Hydrostatic Equilibrium. What Does
AST 301 Test #3 Friday Nov. 12 Name:___________________________________ 1. a) The Sun is in hydrostatic equilibrium. What does this mean? What is the definition of hydrostatic equilibrium as we apply it to the Sun? Pressure inside the star pushing it apart balances gravity pulling it together. So it doesn’t change its size. 1. a) The Sun is in thermal equilibrium. What does this mean? What is the definition of thermal equilibrium as we apply it to the Sun? Energy generation by nuclear fusion inside the star balances energy radiated from the surface of the star. So it doesn’t change its temperature. b) Give an example of an equilibrium (not necessarily an astronomical example) different from hydrostatic and thermal equilibrium. Water flowing into a bucket balances water flowing out through a hole. When supply and demand are equal the price is stable. The floor pushes me up to balance gravity pulling me down. 2. If I want to use the Doppler technique to search for a planet orbiting around a star, what measurements or observations do I have to make? I would measure the Doppler shift in the spectrum of the star caused by the pull of the planet’s gravity on the star. 2. If I want to use the transit (or eclipse) technique to search for a planet orbiting around a star, what measurements or observations do I have to make? I would observe the dimming of the star’s light when the planet passes in front of the star. If I find evidence of a planet this way, describe some information my measurements give me about the planet. -
Module 2: Hydrostatics
Module 2: Hydrostatics . Hydrostatic pressure and devices: 2 lectures . Forces on surfaces: 2.5 lectures . Buoyancy, Archimedes, stability: 1.5 lectures Mech 280: Frigaard Lectures 1-2: Hydrostatic pressure . Should be able to: . Use common pressure terminology . Derive the general form for the pressure distribution in static fluid . Calculate the pressure within a constant density fluids . Calculate forces in a hydraulic press . Analyze manometers and barometers . Calculate pressure distribution in varying density fluid . Calculate pressure in fluids in rigid body motion in non-inertial frames of reference Mech 280: Frigaard Pressure . Pressure is defined as a normal force exerted by a fluid per unit area . SI Unit of pressure is N/m2, called a pascal (Pa). Since the unit Pa is too small for many pressures encountered in engineering practice, kilopascal (1 kPa = 103 Pa) and mega-pascal (1 MPa = 106 Pa) are commonly used . Other units include bar, atm, kgf/cm2, lbf/in2=psi . 1 psi = 6.695 x 103 Pa . 1 atm = 101.325 kPa = 14.696 psi . 1 bar = 100 kPa (close to atmospheric pressure) Mech 280: Frigaard Absolute, gage, and vacuum pressures . Actual pressure at a give point is called the absolute pressure . Most pressure-measuring devices are calibrated to read zero in the atmosphere. Pressure above atmospheric is called gage pressure: Pgage=Pabs - Patm . Pressure below atmospheric pressure is called vacuum pressure: Pvac=Patm - Pabs. Mech 280: Frigaard Pressure at a Point . Pressure at any point in a fluid is the same in all directions . Pressure has a magnitude, but not a specific direction, and thus it is a scalar quantity . -
Hawking Temperature for Near-Equilibrium Black Holes
KUNS-2372 Hawking temperature for near-equilibrium black holes Shunichiro Kinoshita1, ∗ and Norihiro Tanahashi2, † 1 Department of Physics, Kyoto University, Kitashirakawa Oiwake-Cho, 606-8502 Kyoto, Japan 2 Department of Physics, University of California, Davis, California 95616, USA (Dated: June 18, 2018) We discuss the Hawking temperature of near-equilibrium black holes using a semiclassical analysis. We introduce a useful expansion method for slowly evolving spacetime, and evaluate the Bogoliubov coefficients using the saddle point approximation. For a spacetime whose evolution is sufficiently slow, such as a black hole with slowly changing mass, we find that the temperature is determined by the surface gravity of the past horizon. As an example of applications of these results, we study the Hawking temperature of black holes with null shell accretion in asymptotically flat space and the AdS–Vaidya spacetime. We discuss implications of our results in the context of the AdS/CFT correspondence. PACS numbers: 04.50.Gh, 04.62.+v, 04.70.Dy, 11.25.Tq I. INTRODUCTION The fact that black holes possess thermodynamic properties has been intriguing in gravitational and quantum theories, and is still attracting interest. Nowadays it is well-known that black holes will emit thermal radiation with Hawking temperature proportional to the surface gravity of the event horizon [1]. This result plays a significant role in black hole thermodynamics. Moreover, the AdS/CFT correspondence [2] opened up new insights about thermodynamic properties of black holes. In this context we expect that thermodynamic properties of conformal field theory (CFT) matter on the boundary would respect those of black holes in the bulk. -
Modeling the Temperature-Mediated Phenological Development of Alfalfa (Medicago Sativa L.)
AN ABSTRACT OF THE THESIS OF Mongi Ben-Younes for the degree of Doctor of Philosophy in Crop Science presented on January 15, 1992. Title: Modeling the Temperature Mediated Phenological Development of Alfalfa (Medicago sativa L.) Redacted for Privacy Abstract approved:__ David B. Hannaway This study was conducted to investigate the response of seedlings of nine alfalfa cultivars (belonging to three fall dormancy groups) to varying temperature regimes and relate their phenological development to accumulated growing degree days (GDD) or thermal time. Simulation algorithms were developed from controlled environment experiments and were tested in field conditions to validate the temperature- phenology relationships of alfalfa. The percent advancement to first bloom per day (% AFB day 1) method was used to relate alfalfa phenological devel- opment to temperature. The % AFB day-1 of alfalfa cultivars was best described by the equation: I AFBday-1= 2.617 loge Tm 1.746 (R2=0.94), where Tm is the mean daily temperature. The X intercept (when the I AFB day-1 is zero) indicated that 4.6 °C was an appropriate base temperature for alfalfa cultivars. Alfalfa development stage and temperature treatments had significant effects on dry matter yield, time to maturi- ty stages, accumulated GDR), GDD5, and loges GDD46. Growth and development of alfalfa cultivars was hastened by warmer temperature treatments. A transformation of the GDD method using the loges GDD46 resulted in less variability than GDD0 and GDD5 in predicting alfalfa development. The equation relating alfalfa stages of development (Y) to loges GDD4.6 was: Y = 12.734 loges (GDD46) - 33.114 (R2=0.94) .