Thermodynamics Definition and 560 Discussions

Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, radiation, and physical properties of matter. The behavior of these quantities is governed by the four laws of thermodynamics which convey a quantitative description using measurable macroscopic physical quantities, but may be explained in terms of microscopic constituents by statistical mechanics. Thermodynamics applies to a wide variety of topics in science and engineering, especially physical chemistry, biochemistry, chemical engineering and mechanical engineering, but also in other complex fields such as meteorology.
Historically, thermodynamics developed out of a desire to increase the efficiency of early steam engines, particularly through the work of French physicist Nicolas Léonard Sadi Carnot (1824) who believed that engine efficiency was the key that could help France win the Napoleonic Wars. Scots-Irish physicist Lord Kelvin was the first to formulate a concise definition of thermodynamics in 1854 which stated, "Thermo-dynamics is the subject of the relation of heat to forces acting between contiguous parts of bodies, and the relation of heat to electrical agency."
The initial application of thermodynamics to mechanical heat engines was quickly extended to the study of chemical compounds and chemical reactions. Chemical thermodynamics studies the nature of the role of entropy in the process of chemical reactions and has provided the bulk of expansion and knowledge of the field. Other formulations of thermodynamics emerged. Statistical thermodynamics, or statistical mechanics, concerns itself with statistical predictions of the collective motion of particles from their microscopic behavior. In 1909, Constantin Carathéodory presented a purely mathematical approach in an axiomatic formulation, a description often referred to as geometrical thermodynamics.

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  1. AJSayad

    A Thermodynamics Derivative Reduction Problems

    Hi everyone, I'm in a graduate level mechanical engineering thermodynamics class. We're working on derivative reductions using the gibbs and maxwell relations. I was wondering if anyone has any good sources of practice problems that I could use. I've looked through my textbook and there are...
  2. S

    I Conservation of energy in quasar outflows?

    I found this article* about the behavior of quasar outflows in cosmology and how they can create a magnetic field. In section 2.1.4., the authors say that when a quasar produces a "wave" or an outflow, the material will be emitted with energy coming from both the quasar itself and the Hubble...
  3. S

    I Avoiding heat death in an accelerated expanding universe? Is the heat death of the universe completely unavoidable in an universe with an accelerated expansion dominated by dark energy like ours? Or can there be any way to avoid it according to current knowledge, observations and experiments...
  4. S

    I Methods for energy harvesting in expanding universe?

    There has been much discussion about how could we (theoretically) extract energy from the accelerated expansion of the universe. However, the only gedankenexperiment I can found is the "tethered galaxies" one (e.g. However, has somebody proposed an...
  5. S

    I Internal energy of a comoving volume increasing as space expands?

    I was reading an article by Edward Harrison, which tackles the problems of conservation of energy at cosmological scales. At some part (point 2.4) he cites several article, including one by Rees and Gott, which he says indicates that the internal energy of a comoving volume (e.g. a cosmic...
  6. S

    I Energy from quantum systems in an expanding universe?

    I found a paper ( which talks about quantum systems emitting energy due to spacetime expansion. Is this true or only a hypothesis?
  7. S

    I Can trains use permanent magnets to be propelled?

    Can a train (e.g. like a maglev train) use a set of permanent magnets (not electromagnets) that somehow can be propelled and maintain at least a constant speed with them? Is this an example of such system...
  8. S

    I Is energy conserved during the formation of local systems?

    I found an old article ( which talks about conservation of energy in an expanding space. Apparently, the author found that energy is conserved at local scales (like the motion of planets in our solar system) as one would expect, but...
  9. H

    I Difference between work done during quasi-static and non quasi-static expansion

    I'm wondering what's the difference between work done on quasi-static and non quasi-static expansion. In a quasi-static process, the gas inside the system must do a work to "extend". However, in a non quasi-static process, where the gas inside the system doesn't move fast enough to "push" the...
  10. C

    I Physics of paper absorbing Water -- Doesn't this decrease Entropy?

    Summary: doesn't this decrease entropy ? Cellulose is known for its hydrophilic quality, which can be explained from the polarity of its hydroxyl groups. We all know water can overcome the force of gravity through a piece of paper you put in the water. Correct me if I'm wrong but this is a...
  11. S

    Engineering Looking for calculations I could apply to a vacuum-forming machine

    So far I have determined the evacuation time, a basic heat transfer between the heating element and plastic sheet, and a hold down force of the forming bed.
  12. G

    B Does gravity defy the 2nd Law?

    Summary: Trying to understand the relationship between gravity, thermodynamics and entropy, thank you. Gravity can take a diffuse cloud of gas filling a given volume of space at equilibrium density and temperature, and turn it into a burning star surrounded by empty space. Does this mean that...
  13. L

    Finding both temperature and the amount of gas added

    The volume of the cylinder is ##V=\pi r^2 h=\frac{7\pi}{250}\ m^3## the number of moles is ##n=\frac{15}{16}\ mol## so from ##PV=nRT## we get ##P=\frac{nRT}{V}=25975.5\ Pa##. Now, for the second question, it should be an isochoric process so ##V_2=V_1## and ##P_2=P_1+0.8P_1=\frac{9}{5}P_1## and...
  14. Dario56

    I How Can Internal Energy of the Canonical Ensemble Change (Fluctuate)?

    Canonical ensemble is the statistical ensemble which is applicable for the closed system in contact with the reservoir at constant temperature ##T##. Canonical ensemble is characterized by the three fixed variables; number of particles ##N##, volume ##V## and temperature ##T##. What is said is...
  15. mohamed_a

    I Problem regarding understanding entropy

    I was reading about thermodynamics postulates when i came over the differnetial fundamental equation: I understand that the second element is just pressure and last element is chemical energy, but he problem is i don't understand what is the use of entropy and how does it contribute to a...
  16. Hurcane

    Answer the question about time of cooling of a ball

    Hello, I'm not sure if this is the right place to post my question, however I still want to know. Recently, my teacher asked me the following question: "In which of the cases will the cooling time be the least: 1. the ball is suspended from the ceiling 2.the ball lies on a stand with a thermal...
  17. P

    Environmental Physics - Radiative forcing effect on greenhouse effect

    I think part (a) is simple enough. Here is what I have done. (a) ##G=\sigma T^{4}_{s} - OLR## $$ =\sigma (294)^{4} - 160 = 254 Wm^-{2} $$ Part (b) is where I am confused. I think I'm supposed to apply the second relevant equation, in order to get the change in average surface temperature due to...
  18. ContrapuntoBrowniano

    B Ideal gas formula not working?

    Hi! I wanted to do some basic calculations for temperature T on a water-filled pot. I noticed something strange on my calculations, and couldn’t figure out what was wrong... So here it is: The ideal gas formula: k=PV The actual formula Relates equally the product PV with the a constant...
  19. L

    Change of entropy in the Universe in a thermodynamic cycle

    (a) We first find that: ##T_A=\frac{P_A V_A}{nR}=\frac{1\cdot 10^5 \cdot 4}{40\cdot 8.314}K\approx 1202.7904 K##, ##\frac{T_B}{T_A}=\frac{\frac{P_B V_B}{nR}}{\frac{P_A V_A}{nR}}=\frac{P_B V_B}{P_A V_A}=\frac{P_A \frac{V_A}{2}}{P_A V_A}=\frac{1}{2}##, ##\frac{T_C}{T_B}=\frac{P_C...
  20. S

    I Partition function rotation

    Hey, I have a question about proving Saha's equation for ionizing hydrogen atoms. The formula is \frac{P_{p}}{P_{H}} = \frac{k_{B} T}{P_{e}} \left(\frac{2\pi m_{e} k_{B}T}{h^2} \right)^{\frac{3}{2}}e^{\frac{-I}{k_{B} T}} with P_{p} pressure proton's, P_{H} pressure hydrogen atoms, m_{e}...
  21. M

    Partition Function for system with 3 energy levels

    I determined the partition function of the particle A, B and C. C should be the same as B. I then considered the situation, where all particles are in the system at the same time, and drew a diagram of all possible arrangements: The grey boxes are the different partitions, given that we...
  22. L

    Finding equilibrium temperature when there are phase changes

    If there weren't phase changes occurring I know that the temperature equilibrium would be ##T_e=\frac{m_{ice}c_{ice}T_{ice}+m_{w}c_{w}T_{w}}{m_{ice}c_{ice}+m_{w}c_{w}}##. Now, by repeating the reasoning to get the above formula (##\sum \Delta Q=0##) and adding the phase changes of the water...
  23. Dario56

    Derivation of McCabe - Thiele Method

    McCabe - Thiele method is used to find minimum number of separation stages or theoretical plates for a given efficiency of separation in distillation for two - component mixture if components have similar enthalpy of vaporization. If components have similar enthalpy of vaporization ,than on...
  24. S

    I Thermodynamic Identity

    I have a question about the Thermodynamic Identity. The Thermodynamic Identity is given by dU = TdS - PdV + \mu dN . We assume that the volume V and that the number of particles N is constant. Thus the Thermodynamic Identity becomes dU = TdS . Assume that we add heat to the system (we see that...
  25. U

    Adiabatic expansion with temperature-dependent gamma

    The statement does not say whether the process is reversible or not, but I suppose the only way to solve the problem is by thinking it actually is. I tried using the formula for reversible adiabatic processes, i.e. PVγ = constant. First, I calculated the initial volume with the ideal gas law...
  26. G

    I Negative thermal Power

    I have a cylinder that is separated with an insulator. In the internal cylinder there is a thermal source, while outside the insulator we have a thermal sink. The power of the internal cylinder is positive, while of the external one in total is negative. How I should interpet the results. Like...
  27. M

    I Partition function of mixture of two gases

    I have a question about statistical physics. Suppose we have a closed container with two compartments, each with volume V , in thermal contact with a heat bath at temperature T, and we discuss the problem from the perspective of a canonic ensemble. At a certain moment the separating wall is...
  28. C

    Entropy of spin-1/2 Paramagnetic gas

    As we know, dipole can be only arranged either parallel or anti-parallel with respect to applied magnetic field ## \vec{H} ## if we are to use quantum mechanical description, then parallel magnetic dipoles will have energy ## \mu H ## and anti-parallel magnetic dipoles have energy ## -\mu H##...
  29. Istiak

    I Book suggestions for classical thermodynamics

    I was looking for book on classical thermodynamics. I found lot of related posts in PSE but couldn't find a book which type I was expecting. I was searching for book which covers the whole thermodynamics (not QM but it's ok if there's some knowledge of Relativity), and I want some problems in...
  30. W

    Coping mechanisms for thermodynamics?

    Wasn’t sure whether I should post this here since it’s a more qualitative question, or under the Thermodynamics thread because that’s a more specific topic. For all practical purposes, the laws of thermodynamics are inviolable, and statistical mechanics puts them on an even firmer theoretical...
  31. Sai Maurice

    Comp Sci Populating a 1001x101 2D array in MATLAB

    The method I employed was based on a nested loop. I ran into two issues with this approach 1. The code took way too long to run, easily going for over 7 minutes. 2. In the end, it didn't even completely work, due to the "index exceeding the array length". This confuses me For the relevant...
  32. B

    Net efficiency of a cascaded Carnot Engine and Fridge

    Here is a diagram of my interpretation of the problem: Where I'm thinking that the engine originally takes heat from ##T_h## to ##T_l##, in which case ## \frac { Q_{h} } { T_{h} } = \frac { Q_{l} } { T_{l} } ## and ## W_{out} = Q_{in} - Q_{out} = Q_h \left( 1 - \frac {T_l} {T_h} \right) ##...
  33. D

    I Steam flow rate in 2-chamber steam engine system

    Our system of interest has a duct on the left and a piston chamber on the right that make the shape of the letter T rotated 90º clockwise. The smaller tube on the left is abbreviated as P1 has an unspecified length while the piston chamber is P2. The air in P2 heats up and expands while the...
  34. burian

    I Entropy after removing partition separating gas into two compartments

    Summary:: Proving that entropy change in mixing of gas is positive definite > >An ideal gas is separated by a piston in such a way that the entropy of one prat is## S_1## and that of the other part is ##S_2##. Given that ##S_1>S_2##, if the piston is removed then the total entropy of the...
  35. Dario56

    Vapor - Liquid Equilibrium Equation (VLE)

    Most fundamental equation for VLE is $$ \mu_i^L = \mu_i^V $$ It states that for every component chemical potential must be equal in both liquid and vapor phase at equilibrium. However, in my thermo textbook, this equation is derived for isolated systems while usually when dealing with VLE...
  36. L

    Number of moles necessary to get piston back to initial position

    a) ##T_A=\frac{p_AV_A}{nR}=300.7K, P_A V_A=kL^2=nRT_A##, ##P_B S=k\frac{L}{2}\Rightarrow P_B V_B=k(\frac{L}{2})^2 \Rightarrow P_B=\frac{kL^2}{2V_A}=\frac{P_AV_A}{2V_A}=\frac{P_A}{2}##, ##W_{spring\to gas}=\int_{L}^{L/2}kxdx=-\frac{3}{8}kL^2=-\frac{3}{8}nRT_A####\Rightarrow Q=L+\Delta...
  37. L

    Finding the increase in entropy of the universe in gas expansion

    a) ##P_f=\frac{nRT_f}{V_f}=\frac{nR\frac{T_i}{2}}{2V_0}=\frac{1}{4}\frac{nRT_i}{V_0}=\frac{1}{4}P_i## b) ##Q=\Delta U=nC_V \Delta T=n\frac{5}{2}R(-\frac{T_i}{2})=-\frac{5}{4}nRT_i=-\frac{5}{4}P_i V_0## (##L=0## since the gas expands in a vacuum; Now, (a) and (b) are both correct but not (c)...
  38. warhammer

    Entropy Change & Heat Transferred to a Gas

    By using the given relationship that S=a/T --(1) along with the equation ∫ (delta Q rev)/T=∫dS -- (2) I found out that my answer for the value of Q is mc*ln (T2/T1)*a upon equating (1) & (2). But the solution is instead given as Q=a*ln*(T1/T2). I would be grateful if someone would point out...
  39. warhammer

    To find total work done from multiple reversible processes

    The question is given in 3 parts. For first part, process is isochoric so Work done=0. We know here that at end of the process (a), T2=T1 while V remains constant (we can take it as V1) so P2=2P1. For second part, process is isothermal so T is constant. At end of process we reach P1 again from...
  40. Bolhuso

    I Galaxy redshift and thermodynamics

    I just finished rereading the great "A Brief History of Time". To me, what stands out the most in this book, is its ability to keep raising questions while you read it. This thought came up. It's been stuck in my mind for days, so I will humbly post it here to get some feedback. Please forgive...
  41. R

    Courses What do I do to catch up on Physics 1?

    I am a nanobiology student about to start her 2nd year. This year I only had 2 physics courses and I did pretty bad in both. As I start my second year I would like to be very prepared in physics since we will have way more of this subject. Are there some books or tips you have to catch up on...
  42. warhammer

    Question on First Law of Thermodynamics (Paramagnet)

    For the first part, I have expressed it in the following differential form- dU= delta (Q) + BdM Now for the second part I am having major confusion. I know that B corresponds to P and M corresponds to V as generalised force and generalised displacement respectively for a Paramagnetic substance...
  43. T

    Calculating Transient Heat Transfer in Pipes with Thick Walls

    Hi Everyone, I am looking to find how much heat can be stored in a concrete pipe of roughly 0.3-0.4m diameter, and an internal diameter of 0.05m. Air will travel through the internal diameter at 500°C and 17.5bar which will provide the heat for the pipes. This system will then be reversed so...
  44. Istiak

    Calculate the volume change with gas temperature for this piston in a cylinder

    Solution attempt : Option : I am sure that my work is wrong. But, I must add solution attempt in PF that's why I just added that. How can I solve the problem?
  45. Ebi Rogha

    B Is time a consequence of 2nd law of thermodynamics?

    I have heard from a knowledgeable physics proffessor, time exists independently and it is not a consequence of arrow of time. Could some body explain this?
  46. Dario56

    Why Does Electrical Work Cause Changes in the Internal Energy of a System?

    If we look at system at constant temperature and volume which is galvanic cell, first law of thermodynamics states: $$ dU = dQ + dW' $$ Where W' is electrical work done by galvanic cell and Q is heat exchanged with surroundings. As far as I know electrical work is work done by electric field...
  47. bardia sepehrnia

    Engineering Average heat transfer coefficient (forced convection)

    So firstly, I don't understand if the mass flow rate is for steam or for water. If it is for water, I know I can find the heat transfer rate using equation:Q=mcdeltaT. But then I don't know how to find h (the average heat transfer coefficient) because I don't know the surface area (As). I can...
  48. Dario56

    Entropy Changes in Electrolytic/Galvanic Cell?

    One of the most fundamental equations in chemical thermodynamics states: $$ \Delta_rH_m^⦵ = \Delta_rG_m^⦵ + T \Delta_rS_m^⦵ $$ If we look at this equation in context of net chemical reaction in electrolytic or galvanic cell, it is usually interpreted as follows: Enthalpy of reaction denotes...
  49. mcas

    Show that a partial molar property is an intensive property

    I started by taking a derivative: $$E = \sum_{i=1}^{\alpha} (E_i^{(p)} n_i) \ \ \ | \cdot \frac{\partial}{\partial n_i}$$ $$\frac{\partial E}{\partial n_i}=\sum_{i=1}^{\alpha} [\frac{\partial E_i^{(p)}}{\partial n_i}n_i + E_i^{(p)} \frac{\partial n_i}{\partial n_i}]$$ $$\frac{\partial...
  50. P

    Thermodynamics problem (ideal gas law, kinetic theory, processes, etc.)

    It is a long problem, but it is simple to understand. I am having trouble with part A. My attempt: Pressure outside > pressure inside container. pV = constant (isothermal). At equilibrium, all gases are at atmospheric pressure. Because it is quasi-static, the pressures of both compartments are...