What is Heat: Definition and 1000 Discussions

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. J

    Specific heat capacity and concentration

    Hello. I am doing a research paper on the relationship of specific heat capacity and refractive index of liquids. I am doing this by finding the relationship between specific heat capacity and concentration and refractive index and concentration. With this I can find a general trend and conclude...
  2. J

    Heat transfer: lumped system

    Hello all, I have a question regarding heat transfer. When the Biot number is small (<0.1) a mass can be regarded as lumped. Which means that the temperature is the same everywhere in the mass. Now if the Biot number is larger than 0.1 (say around 0.5-0.8) and I still do a lumped system...
  3. A

    Fin area and heat transfer direction

    So, i was studying some fin design in a heat transfer course , and then came the part where the efficiency is to be calculated, then i noticed that when he calculated the surface area and the sides of a rectangular fin weren't included, so i searched and i found out that it was neglected...
  4. R

    Heat transfer direction in fins

    Homework Statement I'm unsure of what exactly is changing the heat transfer direction in the triangular fin. Homework Equations $$q_{x} = -kA(x)\frac{dT(x)}{dx} (1)$$ $$q_{x+dx} = -kA(x)\frac{dT(x)}{dx} - k\frac{d}{dx}[A(x)\frac{dT(x)}{dx}] (2)$$ $$dq_{conv} = h(x)dS(x)P[T(x) - T_{∞}] (3)$$...
  5. H

    Exploring the Source of Heat in a Vehicle CD Player: Uncovering the Mystery

    While changing a CD in my vehicle CD player I noticed the CD is quiet a few degrees above ambient after it has been playing for awhile, not measured just felt very warm to touch. the question is what is the source of the heat? I considered the LED but that makes no sense. Is it the motor...
  6. Harkaran Singh

    Heat Transfer Calculations for Hot Pipe Coil Setup

    Hi guys, I need help with heat transfer calculations The scenario is : Installing a coil around a hot pipe and extract the heat from the hot pipe to heat the fluid flowing through the coil. The whole assembly is insulated. I need methods/formulas to calculate the amount of heat transferred so...
  7. MathWA

    Saucepan Handle: Heat Conduction Benefits

    Homework Statement Explain why a saucepan has a plastic or wooden handle. Homework Equations Maybe Ko = oC + 273o The Attempt at a Solution I tried looking at a book. I also know a saucepan has a plastic or wooden handle because of conducting heat. I think.
  8. P

    Can convergent nozzles convert heat into motion?

    It's a well known fact that convergent and/or convergent-divergent nozzles convert internal enthalpy into forward motion i.e. dynamic pressure. But, enthalpy means both internal heat and the pressure-volume. I want to know whether the internal heat of a fluid can also be converted into forward...
  9. C

    Does air conditioning remove more heat from humid air?

    Hello, assume I have an air condition unit which is not very strong and thus always runs at full power. If I let water evaporate in the room without adding heat (e.g. by hanging lots of wet towels), will this improve the heat removal by the aircon unit and thus lower the room temperature, or...
  10. S

    Problem with boiling water and raising piston

    Homework Statement Consider a cylindrical tank closed by a movable piston with mass ##m=3 kg##. The radius of the cyclinder is ##r=7.5 cm##. In the tank there is a mass ##m'=2 kg## o water at temperature just below ##100°C##. At the base of the cyclindrical tank there is an electrical heater...
  11. andrejen88

    Overall heat transfer coefficient for water cooling

    Hi all! I'm analyzing a fermenter and the water cooling system that cools it down. In analyzing this I've found kW and kWh required to cool down the fermenter using Q = dt*cp*m However when I try to find the overall heat transfer coefficient U using the data, I find that it varies with...
  12. R

    B Polytropic process with heat addition

    In polytropic process if we know p1,v1,v2 and q(heat added in process) then what will be the value of p2 and the coff. Of expansion (n) ?
  13. S

    I Does ice really melts if in conctact with reservoir at 0°C?

    I came up with a basic doubt on heat exchange. Consider this example situation. A cube of ice of mass ##m## and at temperature ##\theta <0°C## is put in contact with a resevoir exactly at the temperature ##T=0°C##. The question is: does the ice melts, i.e. does the ice pass to liquid state? Or...
  14. Harkaran Singh

    Calculating Heat Sink Area for 6ft Deep Heat Rejection

    Could anyone please help me with the area of heat sink required if I want to dump heat 6 feet below the surface? The heat to be rejected is 20000 kW Temperature of the fluid has to be dropped from 30 deg C to 19 deg C. I need rough estimates of the area required to lay down looped pipelines to...
  15. E

    Heat and Entropy Increase Confusion

    This may be a matter of me being confused by the definition of heat. However, I view heat as the energy passed between systems of different temperatures. My problem is the following: By the principle of minimum energy/max entropy, in an isolated system (and therefore fixed internal energy)...
  16. S

    Efficiency of engine when non-boundary work done is done?

    The thermal engine efficiency is defined as $$\eta = \frac{W_{\mathrm{produced}}}{Q_{\mathrm{absorbed}}}\tag{1}$$ A more general definition of efficiency (not only for thermal machines is) $$\eta = \frac{\mathrm{Work \, produced}}{\mathrm{Energy \, absorbed}}\tag{2}$$ But suppose that, in one...
  17. S

    Internal energy in irr. process with molar heat not constant

    I'm a bit confused about the following situation. In a irreversible thermodynamics process the molar heat of an ideal gas changes according to a function of the temperature, say ##c_v=f(T)## (which also leads to ##c_p=R+f(T)##) and I'm asked to determine the heat exchanged during that process...
  18. Uthpala Kaushalya

    Heat transaction of a non-ideal gas

    Here I want to know how to find the heat interactions of non-ideal gases in the following processes. 1. Isobaric process 2. Isochoric process I know that internal energy U depends on temperature and volume. And the enthalpy depends on temperature and pressure. How calculation of heat...
  19. R

    Heat Transfer Q&A: Sun, Metal Heat Sinks, & Conductivity

    Hi, I'm working on a project and would like some help regarding heat transfer and heat conductivity. Any help would be appreciated, I'm more or less a layman. If I wanted to use heat from the sun to heat water via a metal conductor, does the size of the heat sink have proportional heating...
  20. Chemchampa

    What is the mechanism of heat transfer and air flow in coffee roasters?

    Hello, I have here a complex question. My background is Chemistry and Optoelectronics. I wasn't the best when we were studying Thermodynamics at uni so I have some knowledge-gaps. Anyways, here is my problem. Can somebody explain me the whole process of the convection in the coffee roaster. So...
  21. A

    How is thermal energy stored in different types of materials?

    I'd like to get a general and clear picture of all the ways heat can be stored in a body. Suppose we have a kiln/furnace in which we put : particles a) electrons,... monoatomic molecules b) atoms of He... diatomic molecules c) atoms of H, O ... pieces of metal d) Fe, Cu... compounf...
  22. S

    Can the work in a isochoric process be non zero?

    I came up with a doubt regarding isochoric irreversible processes. Is it always true that, for any isochoric process, reversible or not, the work exchanged by the system is zero and the heat exchanged is ##Q=\Delta U##? I'm asking this because, in a exercise on thermodynamics trasformations of...
  23. S

    Reversible heat exchange between water and ice

    Homework Statement In an adiabatic container are placed , in rapid succession , a mass of ice , ##m_I= 2 kg## , at temperature ##T_I = -10 ◦C## and a mass of water , ##m_W = 1 kg## , at the temperature ##T_W = + 20 ◦C## . Determine : a) the total mass of water present in the container at...
  24. D

    Internal heat of ideal gass

    Homework Statement An ideal gass is at constant volume risen to a new pressure level of ##P_f##. Find te expression for the total heat brought to the system. Homework Equations 3. The Attempt at a Solution [/B] So ##PV=nRT## and ##E=Q## ##Q=C_v(T_f-T_i)## so i just have to find...
  25. C

    How Can I Determine the Maximum Flow Rate for a Heat Exchanger Cooling Oil?

    I'm trying to figure a range of flows of coolant that will pull 200,000 Joules per minute from a heat exchanger cooling oil. The oil is passing through a head that adds 200,000 Joules per minute to 0.5 L / min. The 0.5 L is dumped into a sump with ~7 gallons of oil. The oil then passes...
  26. C

    Pressure Changes Along Flow Experiencing Heat Transfer

    For an airflow experiencing a large heat transfer, say a delta of ~600 degrees celsius, I assume compressible effects cannot be ignored. I am struggling however with some conceptual elements, such as how the pressure gradient changes along the flow when compared with compressible and inviscid...
  27. G

    Effects of heat on compressed silicates

    Hi, I'm hoping to make small batches of synthetic minerals by compressing & heating small liquid or semi-liquid samples of sodium silicate w/ variable amounts of other salts dissolved within. As I understand it, many of these will require high pressure & temperature to form. So my strategy is to...
  28. Daringpear

    Formula for determining the heat dissipation of a radiator

    Are there any equations out there which can calculate the heat dissipation of a water-cooled radiator with a fan on it? I would assume some of the variables would include: -Volume of the radiator (240 x 199 x 45mm)=2149200mm^3 -Specific heat of water (4.187 kJ/kgK) -Specific heat of the copper...
  29. Tulio Cesar

    How Do Carnot's Efficiency Formulas for Heat Engines Align?

    My question is: according to Carnot cycle, the maximum efficiency of a heat engine is given by 1 - T2/T1, where T2 is the temperature of the cold source and T1 the temperature of the hot source. So, accordingly, as higher T2 is for a same T1, lowest is the efficiency of the engine. But, the...
  30. timelesstrix0

    Specific heat capacity of a mixture

    so for one of my assignments i need to make a calculation which involves the mixture to be made of different types of substance... the heat capacity for each of the different unknown substances that were given: substance 1 = 1250J/kg °C substance 2 = 1130J/kg °C substance 3= 4100J/kg °C i need...
  31. F

    Heat is infrared radiation....

    Hello Forum, Thermal energy (heat) is associated to the infrared portion of the electromagnetic spectrum. Why? I know the human body emits radiation in the infrared (10 micron). Heat is associated to warming. Are infrared wavelength the resonant wavelengths at which molecules in most objects...
  32. A

    Which cheap metal would absorb and radiate heat well?

    I'm working on a solar heated pump project and require and suitable metal ..that'll absorb as well as radiate heat well. It should should also be cheap to use .
  33. Asafish

    How much heat required to raise the temperature of potassium

    Homework Statement At low temperatures, the specific heats of metals is described by the expression ## c=kT + AT^3 ## , where k and A are constants. Here the first term describes the contribution of free electrons and the second the lattice contribution. How much heat is required to raise the...
  34. trigger701

    What moves the piston in a Carnot heat engine?

    Hi everyone, I have been looking at Carnot heat engines in a bid to better understand entropy, and I can't figure out how it actually does work. Why does the piston move? In some diagrams I have seen weights being removed from the piston, reducing the pressure at constant temperature and...
  35. Planobilly

    Advice on heat gun temp for de-soldering

    This is more of a general question about what temps you set your hot air gun. I just got a new rework station and messed around with it for a bit on a old PCB removing large and small components. I find it easy to damage the PCB. In general I tend to solder pretty hot, anywhere from 300C to 400C...
  36. Parsifal1

    Calculate heat required to melt ingot

    Homework Statement Given that aluminium has a specific heat of 0.9KJ/KgK, a melting point of 660 degrees Celsius and a latent heat of fusion of 390KJ/Kg, calculate the heat energy required to melt a 10Kg ingot, starting from a room temperature of 20 degrees Celsius. Homework Equations dQ= m c...
  37. A

    Do things burn because of the flame or the heat?

    So let's say a cactus will catch on fire at 500 degrees Celsius. If you use a 500 degree flame it will light on fire. But if you took it into a 500 degree room will it also light on fire?
  38. M

    Some simple heat transfer formula derivations and questions

    Hi. I would like to ask a simple question. Here is the link of the file I study on. Immediately before the formula 4.9 for Biot number. Lc=V/As but I cannot understand it and I think it is not clear enough. How it appears, for what the word "characteristic" stands for, for example a pipe? For...
  39. T

    Predict whether a molecule will decay due to heat.

    I'm trying to understand the chemistry of stars. At what point do molecules really end? I guess there would be no one temperature where every molecule of hydrogen breaks apart in unison, but there should be an equation that says: if a diatomic gas is held together by 100 kJ/mol, at a...
  40. M M Ali Rafique

    Heat Transfer Problem in Ansys

    Dear All, I am facing a problem in Ansys Ansys issue. I am solving a model (Transient Thermal Analysis (Heat Flow from Hydrothermal Vessel)) in Ansys 16.2 Workbench While reaching modeling stage, and upon running the simulation, program gives an below errorAn error occurred while starting...
  41. Evangeline101

    Drawing a heat curve of ice melting

    Homework Statement Homework Equations equation with each part. The Attempt at a Solution I did part a) b) and c) on my own: Part d: I am having trouble at this part. I understand how to draw the heat curve but I am confused on what numbers I should use for the x-axis of the graph. I...
  42. D

    Specific heat help -- cooling down a metal bar

    I'm a design draftsman, I have bar of 17-4 stainless steel it weighs 6.9 lbs. The Product Data Sheet shows a specific heat 0.11 (BTU/lb/F (32-212F) (www.aksteel.com) It's at room temperature or 70 degrees F. I want to cool the bar to 50 degrees F. The surface area of the bar is 245 sq. inches...
  43. D

    Fourier's heat conduction equation

    Homework Statement This is the question as it was given...no other data was given. Obtain Fourier's heat conduction equation in three dimensions in an infinite medium in steady state.What modifications will be required in case of a finite body? 2. The attempt at a solution Well I can derive 3D...
  44. amrmohammed

    Logarithmic mean temperature difference for heat transfer

    Homework Statement [Update: just realized that the LMTD is a temperature difference, so my question was not valid] :P Calculate the logarithmic mean temperature difference (LMTD) to heat water flowing through a tube from 21 C (Ti) to 40 C (Te) if the tube has a fixed temperature of 45 C (Ts)...
  45. M

    Heat and mass transfer analogy to find average Nusselt no.

    I am trying to find an expression for the average Nusselt number corresponding to heat transfer from an isothermal disk. Given: ShD≡hm(r)D/DAB=Sho[1 + a (r/ro)n] (1) Sho=hm(r=0)D/DAB=0.814ReD1/2Sc0.36 (2) Relevant equations: Average nusselt number is defined as Nuav=havD/k where k...
  46. jmarkwalker

    Evaporating water using heat and reducing vapour pressure

    I want to reduce the amount of water present in the digestate coming out of my Anaerobic Digester. The traditional method would be to boil it until I've removed as much off as I want. However, this is expensive from an energy consumption point of view. There's a great video on Youtube showing a...
  47. Dakota

    Is it possible to magnify light heat?

    Is it possible for me to make a flash light into a "laser" hot enough to burn through paper. Here is what I have: a light prism, 2 light splitters,(the can separate light so it goes 2 ways) 2 magnifying glasses, and a microscope.(I don't want to use it though) If there is any thing else you...
  48. Miguel Velasquez

    When you heat a sheet, does it expand or compress?

    If a sheet containing two very thin slits is heated (without damaging it), what happens to the angular location of the first-order interference minimum? a) It moves toward the centerline. b) It moves away from the centerline. c) It doesn't change.Condition for m-order interference minimum...
  49. E

    What other forms of energy does latent heat convert to during a phase change?

    The kinetic theory says that temperature is the measure of the average kinetic energy. That would mean then : The temperature rises with the rise in kinetic energy. We know that when matter changes state 'latent heat' helps in 'breaking the bonds' at the transition phase and that this 'extra...
  50. Willfrid Somogyi

    Heat Extracted and Delivered to Reservoirs in a Heat Engine

    NO TEMPLATE---MISPLACED HOMEWORK So it seems like a pretty simple question, and in all likelihood it is, but my lecturer somehow managed to miss this bit in his lecture notes. A heat engine operates between 500K and 300K with 20% of the efficiency of Carnot engine operating between the same...
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