Read about thermodynamics | 493 Discussions | Page 1

  1. M

    Interpreting a thermodynamics formula using a picture

    I notice that ##pv=t## looks like the ideal gas law but with ##T## in units of energy. I know that ##pV = \text{constant}## means that the pressure of a gas decreases as you expand it (Boyle-Mariotte's law), explaining for instance how we breathe. I guess I could put everything in words even...
  2. Hamiltonian299792458

    B Work done by irreversible and reversible processes

    I am not able to follow the derivation of work done in a reversible and irreversible process as I don't get why the work done should be different in the two processes. a reversible process is said to be a process that occurs infinitesimally slowly and an irreversible process goes from initial to...
  3. M

    Energy as a state function

    Here is the figure: The answer is $$Q_A<Q_B$$ which I can show by calculation using the above equations. What's confusing to me is I thought that the change in internal energy was a state function. Which would mean since the initial and final points are the same, $$\Delta E_A=\Delta E_B$$ or by...
  4. bsharvy

    B Energy Conservation and the Dragons (Game of Thrones)

    I thought I'd calculate how much heat was required to melt the Iron Throne, and then multiply that by the number of flame-gushes during the sack of Kings Landing, to get a total amount of energy expended. Then I'd convert that to calories and use the average number of calories per goat to...
  5. Rahulx084

    Thermodynamic Phases

    #Can somebody please explain what is the difference between single phase and homogeneous phase in context with thermodynamics? #Also in the fundamental relations in thermodynamics like dg=vdp-sdT , it says this is applicable to homogeneous phase of constant composition. Isn't this equation...
  6. TechieDork

    Need help with calculating a Cv in PHYWE's Heat Capacity of gases

    I've conducted this experiment yesterday. The main goal of this experiment is to find a gas constant R and compare it with its theoretical counterpart but I get stuck in calculating a Cv so I tried to find out what's wrong with my calculations by trying to calculate a Cv from the given data...
  7. P

    Heat engine that uses a rubber band (Thermodynamics)

    Nota that the path a to b is isothermal since J is proportional to L, then we can find the value of T_a and T_b using the equation of state and the figure. We have, \begin{equation} J_0=\alpha L_0T_b \end{equation} or \begin{equation} T_b=T_a=\frac{J_0}{\alpha L_0}=T_0 \end{equation} Also, by...
  8. Ron19932017

    I Euler's equation of thermodynamics in free expansion (Joule expansion)

    Hi everyone, I am confused when I apply Euler's equation on the free expansion of an ideal gas. Consider a free expansion (expansion of gas in vaccum) where the volume is doubled (V->2V) The classical free expansion of an ideal gas results in increase in entropy by an amount of nR ln(2), a...
  9. T

    Calculate the average number of oscillators of an Einstein solid, in the grand canonical ensemble, when q>>N

    $$Q_{(\alpha, \beta)} = \sum_{N=0}^{\infty} e^{\alpha N} Z_{N}(\alpha, \beta) \hspace{1cm} (3.127)$$ Where ##Q## is the grand partition function, ##Z_N## is the canonical partition function and: $$\beta = \frac{1}{kT} \hspace{1cm} \alpha = \frac{\mu}{kT} \hspace{1cm} (3.128)$$ In the case of an...
  10. Amaterasu21

    B At what size is kinetic energy no longer related to temperature?

    Hi all, I've read so many times that "temperature is a measure of the average kinetic energy of the molecules in a substance," or sometimes "particles" to encompass atoms and ions too. But how big can "molecules/particles" be before their kinetic energy is no longer relevant to temperature? If...
  11. Refrigerators and Carnot Cycle (Sequence of 5 steps) #14

    Refrigerators and Carnot Cycle (Sequence of 5 steps) #14

    Refrigerators run on Carnot cycle. It is like a heat pump that operates on the reversed Carnot cycle. It utilizes the evaporation of the refrigerant to absor...
  12. densephysicist

    Calculus problem regarding Thermodynamics HW (entropy for C2H5OH at 348K)

    Summary:: Seems simple but has me stumped... [Thread moved from a technical forum, so no Homework Template is shown] Hello! Im struggling to use an equation given to me. To provide some context, I am trying to work out the entropy for C2H5OH at 348K. Using provided tabulated data, the...
  13. Second Law of Thermodynamics and Heat Engines #11

    Second Law of Thermodynamics and Heat Engines #11

    The Second Law of Thermodynamics is not an easy topic. However, if you understand the concept of direction of thermodynamic processes and heat engines, you w...
  14. S

    Engine working between multiple temperature baths worse than Carnot

    Let the new engine, NE, extract heat from a certain subset of these baths, and let heat obtained from the ##i^{\rm th}## bath be denoted by ##Q_i##, and let the heat rejected to the ##j^{\rm th}## be denoted by ##Q_j##. Let the engine perform an amount of work ##W##. Now right beside this...
  15. B

    Heat distribution in a piece of glass receiving protons

    First of all, I didn't know whether to pick this subforum or the engineering/compsci one, I understand this might need to be moved to a more appropriate subforum. The general approach is fairly obvious, use implicit method to construct the tridiagonal matrix for Thomas method and solve. However...
  16. A

    Gas - cylinder - piston problem

    Summary:: Seeking explanation to classical gas - cylinder - piston problem, not the solution. Problem 1.15 from 7th edition of Introduction to Chemical Engineering Thermodynamics by Smith, Van nes and Abbot) Classical problem, given: - gas in a confined cylinder - piston with weight is placed...
  17. Y

    Why use c_p and not c_v as specific heat - Thermodynamics

    Hey all, I am working on a problem that goes like this: The cargo space of a refrigerated truck whose inner dimensions are 12 m 3 2.3 m 3 3.5 m is to be precooled from 25°C to an average temperature of 5°C. The construc- tion of the truck is such that a transmission heat gain occurs at a rate...
  18. I

    Question about thermal physics -- Ice cubes melting in water

    First, I calculated the heat required for the ice to melt: Q=mLf Q=0.150×330 Q=49.5 J Then, I calculated the final temperature of the water by forming the following equation: Q=mcΔT −49.5=(0.15+0.35)×4200×(Tf −80) Tf=80.0 degrees Celcius But the answer says 32 degrees Celsius.
  19. B

    A Thermodynamics: calculate thermodynamic derivative from data?

    I don't understand how to use output from an NPT molecular dynamics simulation to compute a thermodynamic derivative. I need to compute this (where "d" is a partial derivative, "T" is a subscript that means, "at constant temperature," and "E" is internal energy): -(dE/dV)T I have a simulation...
  20. Y

    Engineering Find out the percent exergy loss

    Problem, with state values, and pie chart (Fig 4.20) showing answers: ^ This shows the system in question (Kapitza Liquefaction System). Methane gas enters into the compressor (c), then goes through the first heat exchanger (HX1). Some of it (z) gets routed to the expander (exp). Afterwards...
  21. obstinatus

    Isothermal pressure change in a U-shaped tube

    Hi, just reviewing some thermodynamics from the textbook by Sears and Salinger, having a hard time conceptualizing this one. It's an isothermal change in pressure, so the volumes of the mercury and the air both change to reach equilibrium, but if it's a "good vacuum pump", then won't the right...
  22. Anon_Miner

    Where do I put this Server Room's air exhaust?

    Hey guys. I have a room which i want to use as a server room. the devices need to work 24/7 and they get pretty hot. so it is imperative to keep the room cool otherwise the devices will be damaged. I have an air vent to bring cold air into the room for the devices so their fans can suck in cold...
  23. Z

    Thermodynamics: find the change of internal energy, the work and Q

    According to the first principle of thermodynamics: ΔU = W + Q Also noting that: W = -P⋅ΔV (Question: This P is the initial pressure or the final?) To find V2: (P1⋅V1) / T1 = (P2⋅V2) / T2 → Therefore, (P⋅V1) / T1 = [(P/5)⋅V2] / T2 → (P⋅V1) / T1 = (P⋅V2) / (5⋅T2) → V2 = (5⋅T2⋅V1) / T1...
  24. Rahulx084

    Thermodynamics -- Temperature of a Heat Source?

    In heat engine we define a heat source from where heat is transferred to the system, we say that heat source has a temperature ##T_h## , When we define a Carnot heat engine, the first process we have is an isothermal expansion and we say heat has to come in system through this process and here...
  25. C

    B Irreversible expansion of gas against gas

    Let's say I have a liter of gas at pressure of 4 atm and T=900K. I use it to move perfect massless frictionless and insulative piston to compress a liter of 1 atm, T=300K gas. When the pressure on both sides is equal and the piston stops moving, will the temperature on both sides of the piston...
  26. T

    B Adiabatic approximation in the derivation of the speed of sound

    The speed of sound in a gas at temperature T is given to be ## v=\sqrt{\frac{\gamma RT}{M}}##, where ##\gamma## is the adiabatic exponent, R is the gas constant and M is the molar mass of the gas. In deriving this expression, we assumed that the compression and expansion processes were so fast...
  27. F

    B Temperatures of the hot source and cold sink in a heat engine

    Hi, I was just wondering about the efficiency of a cycle that is not Carnot cycle. In that case one should use \eta = 1-\left|\frac{Q_{\rm out}}{Q_{\rm in}}\right|, where Q_{\rm in} and Q_{\rm out} are the amounts of heat absorbed and released during the cycle. For instance, I guess that in...
  28. WhiteWolf98

    Finding Isentropic Enthalpy, knowing Isentropic Entropy

    A short background: My question focuses solely on the part of the refrigeration cycle to do with the compressor, where the cycle begins. The first state is before the refrigerant enters the compressor, and the second state is after the refrigerant leaves the compressor. My goal is to obtain...
  29. C

    Is Specific impulse indicative of performance as a gun propellant?

    Black powder has specific impulse of around 80s, while rocket candy has up to 130s of specific impulse. Does that mean I could replace the propellant in a BP cartridge with 80/130 of the weight in rocket candy and obtain the same performance in an idealized gun? (as in without considering...
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