Heat Definition and 1000 Threads

In thermodynamics, heat is energy in transfer to or from a thermodynamic system, by mechanisms other than thermodynamic work or transfer of matter. The various mechanisms of energy transfer that define heat are stated in the next section of this article.
Like thermodynamic work, heat transfer is a process involving more than one system, not a property of any one system. In thermodynamics, energy transferred as heat contributes to change in the system's cardinal energy variable of state, for example its internal energy, or for example its enthalpy. This is to be distinguished from the ordinary language conception of heat as a property of an isolated system.
The quantity of energy transferred as heat in a process is the amount of transferred energy excluding any thermodynamic work that was done and any energy contained in matter transferred. For the precise definition of heat, it is necessary that it occur by a path that does not include transfer of matter.Though not immediately by the definition, but in special kinds of process, quantity of energy transferred as heat can be measured by its effect on the states of interacting bodies. For example, respectively in special circumstances, heat transfer can be measured by the amount of ice melted, or by change in temperature of a body in the surroundings of the system. Such methods are called calorimetry.
The conventional symbol used to represent the amount of heat transferred in a thermodynamic process is Q. As an amount of energy (being transferred), the SI unit of heat is the joule (J).

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  1. Pouyan

    What is the change in entropy of the water in a reversible heat engine?

    My attempt: I though : ΔQ_w= 1*4200 * (-100) J=-420000J Q_ice=334000*m_ice = ΔQ_w But it was totaly wrong! The solution showed : Because the heat engine is reversible the efficiency η = 1- (T_cold / T) T_cold is always 273 K while the hot temperature changes from 373 K to 273 K during this...
  2. patric44

    A question about the specific heat and Debye temperature

    this is my attempt of a solution , but my only equation is should i convert Θ to Celsius , and if i did the specific heat of the other substance is greater , how is that if its inversely proportional with temperature ! . and the other Θ is 200 K so it should be less ?!
  3. H

    A Crank-Nicholson solution to the cylindrical heat equation

    Hi, I am solving the radially symmetric heat equation with an internal heat source(this is meant to model the heating of a cylindrical battery). It's meant to model heat in a cylinder with conduction to the environment, so my outer boundary condition is Newton's law of cooling. The $T$ in the...
  4. Leonardo Machado

    A Boundary conditions for the Heat Equation

    Hello guys. I am studying the heat equation in polar coordinates $$ u_t=k(u_{rr}+\frac{1}{r}u_r+\frac{1}{r^2}u_{\theta\theta}) $$ via separation of variables. $$u(r,\theta,t)=T(t)R(r)\Theta(\theta)$$ which gives the ODEs $$T''+k \lambda^2 T=0$$ $$r^2R''+rR+(\lambda^2 r^2-\mu^2)R=0$$...
  5. DuckAmuck

    How Do Inelastic Collisions Distribute Energy Between Heat and Sound?

    In collisions that are inelastic or partially elastic, how can we predict how much of the energy lost to the surroundings becomes heat, and how much becomes sound? What determines that fraction?
  6. S

    Heat loss from a hot water tank

    This is a past exam paper Q. I think i might be missing info though. For i) i use q=UAdT A= 35m2 dT= (80-20) They haven't given U but if i know answer should be 1,050W So if i set U to 0.5 (0.5)(35)(60) = 1050W Am i missing something or could I calculate U with the info given? Also ii) Use...
  7. Y

    Low-Temp Liquid Helium System: Evaporation Heat & Temperature

    We consider a system composed of liquid helium in equilibrium with its vapor at very low temperature T, each phase being considered extensive. We neglect the mass of the gas compared to that of the liquid, as well as the heat capacities of the gas and the walls compared to that of the liquid...
  8. B

    How Do I Calculate Heat Transfer by Conduction Using Thermal Conductivity?

    Can anyone please guide me in what I'm missing to answer part b)i? I can't seem to work out the thermal conductivity (k) in order to find the heat transfer by conduction? Or is the different approach that I must take to do this question? Thanks for any help!
  9. B

    Constant of thermal conductivity and heat flow in a given time

    Here is the Q below I want to see if my values for part b) is okay? This is what I have tried: Any help would be nice! Thanks
  10. 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...
  11. Sabra_a

    When to use the Lumped Capacitance method in Heat Transfer problems?

    I attached below my attempt to solve the question.
  12. Parzeevahl

    Obtaining work from two bodies by a heat engine

    Here's my attempt for the first part: For the first body, the work obtained is ##W_1 = C_P (T_1 - T_f)## while for the second body, it is ##W_2 = C_P(T_2 - T_f).## So the net work obtained is the sum of these two: ##W = W_1 + W_2 = C_P (T_1 + T_2 - 2 T_f)## and that proves the first part...
  13. Graeme M

    Contribution of surface heat to atmospheric warming

    I'm not sure if this should be here or under Earth Sciences. GHGs warm the atmosphere, but they do that largely in response to LWR from the heated surface. The relative contributions of the major GHGs are known, but I can't seem to find anything about the contribution from surface heat. For...
  14. O

    B Highly conductive, reflective and heat resistant material

    Which material or combination of materials is the most highly conductive(super-conductor) and highly reflective and heat resistant at the same time? I am asking this because I am trying to block radiation. T
  15. godiswatching_

    Spring stiffness and Heat Capacity (Equipartition of energy)

    Here's a picture of what I tried. I was wondering if this is correct?
  16. gregorspv

    The efficiency of a heat engine

    The first picture was provided along the problem statement. The second has my annotations. I initially began by calculating the ratio of efficiencies, since the work done is obviously the same and cancels out, but after failing and having seen the form of the solution I saw that that cannot...
  17. B

    Working out the rate of heat loss from a radiator

    Here is the Q below This is what I have tried so far. I ended up getting a value of 1772.4 watts Is this the correct way of doing this problem?
  18. B

    Finding latent heat of vaporisation

    Can anyone please help me on how to tackle this question? I really do not have any clue to make a start on it? Thanks for any help!
  19. S

    What is the Formula for Calculating Enthalpic Capacity in Steam Heat Transfer?

    I) Area = 3.14 x L xD 3.14 x 60 x 0.11 = 20.72m2 Q=UADT = (1.0Wm2k-1)(20.72m2)(150-20) = 2693.6 W Changed flow rate from 0.1kgs-1 to 360kghr-1 II) Enthalpic Capacity in dry steam = steam flow rate x latent heat vaporisation Q=mDh = 360 x 2114 (formula looks for lat heat vap but we have latent...
  20. F

    How many reversible thermodynamic cycles are there between two heat reservoirs?

    Hi, I was revisiting my (high school level) understanding of thermodynamic cycles and I think I still have some doubts. Last year and more recently I posted a few questions which surely helped me, but I think I need more clarifications. In a nutshell, what I'd like to know is the following...
  21. A

    Relationship between Heat Capacity Cv for Different Models

    I think the C_V for van der waals gas will be larger than ideal gas since it‘s a more precise description. However, for the relationship I cannot come up with a specific equation.
  22. carter7gindenv

    Infrared Lamp vs Red Heat Lamp: What's the Difference?

    Hi I'm going to use a light based source of heating for a science experiment at my university and I'm a bit confused. Sometimes the names infrared (IR) lamp and heat lamp are used as two different thing and sometimes not but I can't find clear info about it. Is there a difference? thanks in...
  23. F

    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...
  24. maistral

    Boundary conditions for convective heat and mass transfer + wall Temperature

    I am operating via finite differences. Say for example, I have this pipe that contains a fluid. I have the boundary condition at x = x1: k is the effective thermal conductivity of the fluid, T is the temperature of the fluid at any point x, hw is the wall heat transfer coefficient, and Tw is...
  25. C

    Physics problem: body heat loss via radiation

    I do not know where I am doing wrong. I asked a tutor, he said I was doing it correctly. Below is my work. Please help! Net loss by radiation in 10.0 mins is 74.8 kJ
  26. Diracobama2181

    Heat Capacity of a Fermi Gas at Low Temperature

    I find that $$U=\int Z \epsilon D(\epsilon) e^{-\epsilon β}d\epsilon=\frac{gV}{(2\pi)^3}\int Z \frac{(\hbar)^2k^2}{2m}k^2 (4\pi)e^{-β\frac{(\hbar)^2k^2}{2m}}dk$$ where g=2s+1=2, $$Z=e^{βµ}$$ and $$D(\epsilon)=\frac{gV}{(2\pi)^3}k^2 4\pi$$ for the density of states From here, I can use $$c_v...
  27. O

    How can I evenly heat a series of metal plates to a controlled temp?

    (In opening, hi. I'm a lawyer, not a physicist, and I'm entirely out of my depth here.) I need to make roughly 8 wooden frames (19 inches x 1-1/16 inches x 9-1/8 inches). Into each, I'd like to place a thin metal plate with hexagonal cells (5.27 mm cell diameter) pressed into them. I need to...
  28. Gvozden

    Is Heat Capacity Derivable for Non-Ideal Liquid Mixtures?

    Can I derive heat capacity of one phase mixture of three liquids as a sum of their mass shares multiplied by heat capacities of solitary components at given temperature? All components are miscible, of course ... thank you in advance
  29. J

    Solar heat gain on a typical solar cell

    what will be the joules per meter using thermal gain and wattage gain from a solar cell if the cell is in an insulative vacuum not counting the thermal degredation of 25% for wattage loss due to heat gain
  30. M

    Engine wont start unless I heat up the engine first

    I took my car to the shop and they had it for 3 straight weeks even changed the spark plugs but they never could figure out what was wrong with it. Now that it's getting cooler weather it's taking bout 2 hours of having the hair dryer blowing on the back of the engine for it yo start up, does...
  31. maistral

    Heat Transfer Coefficient in Large Pipe w/Smaller Pipes

    Hello, I have a question with regard to equipment design. How do I calculate the overall heat transfer coefficient for a large pipe who has smaller pipes in it? Say I have water flowing on the large pipe, and inside it there are three pipes with hot acid in them. Assuming they flow concurrently...
  32. Philip Robotic

    I don't know how I got "mysterious energy" in a Carnot heat pump

    So first I transformed the equation no 2 like this: $$|Q_L|=K\cdot|W|$$ And then I transformed the first equation to find ##|Q_Z|## $$|Q_L|=|Q_H|-|W|$$ Plugging the result into the first equation $$|Q_H|=K\cdot |W|+|W|$$ $$|Q_H|=|W|\cdot (K+1)$$ We know that the efficiency coefficient K is...
  33. Thatrandomdudeacross

    Heat Transfer in Metal Tank: Time to Drop from θ2 to θ1

    I'm trying to find a heat transfer equation that includes time. Metal tank, partially filled with liquid. I need to find the time it would take for the temperature on the outside of the tank to drop from θ2 to θ1, given thickness, metal and liquid's properties, and tank dimensions
  34. A

    HVAC Semi-snorkel stove heat transfer

    Hi! Came across this forum while researching my project, and this seems like a good place to ask. Thanks in advance! I'm building a hot tub that will use a 55 gallon steel drum wood stove as a heat source. The drum is built into one of the sides so that the majority of the stove is surrounded...
  35. H

    Calculate the Heat Loss in a Hot Water Tank from a Shower

    Hi, If it is assumed the temperature of a shower is to be 43°C and the electric shower consumes 50 litres of water at that temperature, then the Energy required for an electric shower would be: QElectric Shower = mcΔT QElectric Shower = 50 * 4.181 * (43-10) QElectric Shower = 6.89945 kJ or...
  36. Rifat amin

    Below 20K, The specific heat capacity c of silver varies with temperature

    Not attempt
  37. harishree90

    Exit flow rate in a Plate Heat Exchanger

    Hello All, I am working on a project with plate heat exchangers connected in a cascade system to heat up water connections in a huge system The Problem Statement : A boiler produces and circulates hot water at 65 degC to a Heat Exchanger and come out at 50 degC, where as the other fluid is...
  38. yecko

    Engineering Troubleshooting Heat Exchanger Error Msg

    How should I do to eliminate the error message? In what direction should I approach the question? (I have tried difference combination and sequences of the heat exchanger below pinch, all seems not working) Thanks
  39. ValeForce46

    Two objects exchange heat through a cyclical thermal machine

    This is how I solved part a) : ##Q_1=C\cdot (T_1-T_i)## This quantity is negative because object 1 loses heat. (positive for the machine) ##Q_2=C\cdot (T_2-T_i)## This one is positive because the object 2 absorbs heat.(negative for the machine) Then the exchanged heat FOR THE MACHINE is...
  40. dRic2

    I Lattice specific heat, help understanding a passage in Ziman's book

    Hi, I don't understand why the author in calculating the expression for the specific heat, divides by ##1/V## (the total volume). Also, in calculating the fraction of modes with frequency in the range ##v##, ##v+dv## he divides by ##V## (eq. 2.65, 2.67) Thanks, Ric
  41. TonyKonya

    Specific Heat at High Temperatures

    I am working on some simulations to determine pressures for extremely high-pressure combustion events. The temperatures in these events can range anywhere from 3000 K to 6000 K. In order to do this I need to find valuers for specific heats of gases at these temp ranges as I know they vary to a...
  42. PGaccount

    Why does heat flow from hot to cold?

    What is the origin of the fact that heat flows from hot to cold? Does it have something to do with fluctuations?
  43. Nikhil Rajagopalan

    Heat Absorbed by Gas: An Expert Explanation

    Dear Experts, While going through the explanation why heat absorbed by a gas is path dependent, there were two examples of comparison. 1. A gaseous volume of 2 liters expanding slowly to 4 liters , supplying heat from a heater keeping the temperature of the gas constant at 300K. 2. A gas allowed...
  44. mgkii

    Entropic Force - Heat Transfer Mechanism

    Is anyone able to explain something to do with entropic force at a level I might be able to understand please! Ok... you need to know what level I'm at? Formal Maths & Physics at high school (I'm 50, so distant past :-) Informal Maths & Physics - lifelong interest. Consume a lot of youtube...
  45. P

    A Can decoherence be associated with heat generation?

    This question is inspired by a comment that @thephystudent made where he said that "The dephasing between the Bragg pulses is not unitary, I believe it can be explicitly written in Lindblad form and generates heat. I believe this Point of view is the same as (among others) the papers of...
  46. A

    Heat Flow Through a Truncated Cone

    This problem seems best treated in cylindrical coordinates. There is azimuthal symmetry, and there is no heat loss or generation within the cone, so our thermal conductivity equation reads: $$\vec{q} = -k(\frac{\partial T}{\partial \rho} \hat{\rho} + \frac{\partial T}{\partial z} \hat{z})$$ We...
  47. rikku95

    Heat Transfer of a Dewar Nitrogen Container

    Hello guys, :smile: So like the Titels says I am Trying to get an Approximation of the Heat Transfer which ocours in a Deware container. Unfortunately all my calculations so far seem to suggest way to low Boil off rates for my Container its just to efficient, here is a summary of my Attempts...
  48. J

    What is the molar heat of self-decomposition of nitric acid?

    I am curious as to what is the value of the molar heat of self-decomposition of nitric acid. I've searched online and cannot seem to find the above value. Essentially, I am looking for the value for anhydrous nitric acid although I am aware that the acid is generally not provided in pure...
  49. J

    What is the heat of combustion of 1 kg carbon dioxide w/ magnesium?

    I am trying to find out the heat of combustion of one kilogram of carbon dioxide with magnesium. I am looking for results for stochiometric conditions of these two reactants. I have found the following equation online 2 Mg(s) + CO2 ---> 2 MgO(s) + C(s) yields a standard enthalpy of --810.1...
  50. F

    Pulling a vacuum on a heat pump with a recovery machine

    I am a student attending an HVAC program at a trade school. Today I recovered refrigerant out of a heat pump in order to debraze the compressor out of the heat pump. My instructor has taught me how to use a recovery machine to recover refrigerant out of an air-conditioner or heat pump before...
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