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. D

    How does the rise in temperature of fuse wire depend upon its radius?

    I tried the following: We know, Q = mc (delta t) but by Joule's law of heating, we also know that H = I^2 R t Assuming that no heat is lost in the surroundings, we get Q = H => mc (delta t) = I^2 R t => (delta t) = (I^2 R t)/(mc) But m = V d = AL d and R = (rho L) / A => (delta t) =...
  2. G

    I How does BTU convert to heat in degrees in a greenhouse?

    Information online claims sun is 200 btu per sq ft in winter only when sun is perpendicular to plastic or glass of a green house. When sun hits glass or plastic at an angle there are losses. The steeper the angle the more losses will be. Is it possible to calculate how warm a green house will...
  3. MatinSAR

    Question about the thermodynamic temperature scale

    My first problem is to find the absored and rejected heat. Can I say that it is equal to the work done in an isothermal proccess (##dQ=Pdv##)? My reasoning : We have ##dQ=C_V d\theta + Pdv##. For constant temperature it becomes :$$dQ=Pdv$$
  4. MatinSAR

    I Work done in cycle of a refrigerator (in Heat and Thermodynamics by Zemansky)

    I guess the first one is wrong. Because in this cycle we have ##|Q_H| > |Q_L|## then ## |Q_L| - |Q_H| ## is negative and caanot be equal with ##|W|##. Am I right?
  5. J

    I Measurement of heat as an interval measurement or a ratio measurement

    Hello! I am new to this forum and I'm not even sure if this is the right place to ask but here goes: I am studying Research Methods. I got to the part where the teacher is teaching us about levels of measurement. They said that there are a few levels but the two that are significant to my...
  6. A

    B How much heat energy can give out put by ionization gas?

    I mean by accelerating electrons and ions for energy levels in atom
  7. L

    Why does this question make this simplifying assumption?

    The question says to "neglect the change of volume of the water". This confuses me. Why do we neglect the change of the water's volume? Let's say we didn't. A = Q/It, where A is area, Q is heat, I is intensity, t is time. Q = mc(deltatheta), mass is constant, specific heat is constant (as mass...
  8. bushmonk

    I Measurement and Thermal Effects Similar?

    A weak (one photon at a time) beam of laser light strikes a double slit. In case 1 the barrier forming the double slit is a mirror. In case 2 the barrier is flat black. An array of detectors surround the experiment and can detect the photon whether it goes through the slit or is reflected. In...
  9. R

    I Is low entropy found in something very hot?

    If we have a kg of something that is 100miljon Celsius degrees, and can controlably use this heat somehow, we can sustain life, grow crops, drive steam engines and with these we could build a whole city like New York, we can create a lot of mass with very low entropy, things that are very...
  10. E

    I Why is ##dQ = dH## still valid for chemical reaction (Callen)?

    Consider a system which is characterized by the extensive variables ##(U,V,N_1,...,N_m)##. For a quasistatic process which occurs in contact with some pressure reservoir and where the ##N_i## are constant, one has $$dU = TdS -PdV \implies TdS = dQ,$$ where the implication follows from the First...
  11. S

    Correct statement about thermodynamics process

    I know process B absorbs heat but I can't determine the heat of process A. In adiabatic process, Q = 0 but process A is not adiabatic. I only know both W and ΔU will be negative for process A but how to know Q? Thanks
  12. RobbyQ

    I Does heating weaken the strong nuclear/electromagnetic forces

    In this steel forging video, does the heat weaken the strong nuclear and electromagnetic forces allowing compression?
  13. jackkk_gatz

    Electric heat generation from parallel rods carrying current in an oil bath

    I know that ##\dot e_{gen} = \frac {R_{e}i^2} {\pi r^2 L}## the thing is I don't know the value of i. I didn't write it, but another thing that the problem asks is to determine the flow of electric current. The model I came up with using heat generation due to electrical current is $$ \frac...
  14. singularcell

    Heat Engine cyclic process

    I didn't have much trouble with part a but I'm struggling with b,c, and d. I considered the efficiency formula for a heat engine e = work done by engine/ qh but i am unsure of how to approach it. for part c) not sure how i can get to Tc without knowing Th for d) my gut is telling me 5/2 but i...
  15. pinball1970

    Record Heat for the Oceans in 2022

    "Changes in ocean heat content (OHC), salinity, and stratification provide critical indicators for changes in Earth’s energy and water cycles. These cycles have been profoundly altered due to the emission of greenhouse gasses and other anthropogenic substances by human activities, driving...
  16. R

    Problem about a block of ice melting (specific latent heat)

    Energy lost by water = Energy gained by ice Energy lost by water = 0.16 x 4200 x (100-t) Energy gained by ice = 0.205 x L + 0.205 x (t) (where t is the temperature at thermal equilibrium). However, there does not appear to be enough info to continue. The solution, however, considered t to be...
  17. C

    Heat added to system increases mass

    According to this, The heat added to the system is proportional to the mass. Does someone please know how that highlighted statement is so? I think it is because the heat increases the internal energy of a system and internal energy is the sum of the translational, rotational, and vibrational...
  18. S

    Improving heat transfer in domestic hot water tank?

    A typical UK domestic hot water tank holds c. 200 litres of water. Hitherto the common method of heating it to say 60C has been by means of an internal coil of copper pipe through which is pumped hot water from a gas boiler at ~ 70C. Heat transfer on the outside of the coil is by natural...
  19. Scott Chapel

    Enclosure Panel Material and Heat Transfer

    Referring to the attached, the bottom right is an electronics compartment. The external panels are 20GA steel powder coated yellow. There is 1-inch of R-6 foam board insulation on the inside of all external walls. The skid is exposed to ambient heat and direct sunlight when on site. If we...
  20. C

    Deriving general specific heat capacity formula

    For this, Dose anybody please know of a better way to derive the formula without having ##c = \frac{\Delta Q}{m \Delta T}## then taking the limit of both sides at ##\Delta T## approaches zero? I thought ##\Delta Q## like ##\Delta W## was not physically meaningful since by definition ##Q## is...
  21. R

    B Heat transfer -- Several questions

    I am constantly research a wide range of areas and often run into questions I cant seem to find the answer to, no matter how much I ask uncle google. I am looking for open dialog and discussion, so I dont mind getting off topic a bit. I like the inspiration. Tho periodically I may need a...
  22. R

    Passive Heat Collection

    I ask this question making refference to the Evacuated Tube Solar Water Heaters that have alcohol in their core tube to transfer heat from the Evacuated Tube to the water. Is it possible to make a passive water heater that uses a step system of tubes filled with water in a gradually decreasing...
  23. C

    Finding how much heat released by evaporation of water

    For this, Dose anybody please know where they got the 120W from? Many thanks!
  24. C

    Heat exchanged in Expanding and cooling gas

    For part(b), My solution is, ##\Delta E_{int} = Q - W = \frac{3}{2}(P_fV_f - P_iV_i)## ##Q = W + \frac{3}{2}(P_fV_f - P_iV_i)## ##Q = 4000 + \frac{3}{2}((1 \times 10^6)(6 \times 10^{-3}) - (3 \times 10^6)(2 \times 10^{-3})## ##Q = 4000 J## However, according to the solution b. ##−4000 J## Can...
  25. D

    Rate of cooling of aluminum parts after a glycol heat process

    I work for an aerospace supplier across different departments. I was recently shown how the production workers straighten aluminum parts after a glycol heat solutionizing process. The aluminum parts undergo a heat treatment for a few hours and are quenched in glycol. I can't get too in-depth...
  26. M

    A Practical use of an overall heat transfer coefficient?

    Say I have a real counter flow heat exchanger using air and water. I run a test so I know every parameter; mass flow rate of both fluids, surface area, and in and out temps. This is great. Now I can calculate the heat transfer rate, my log mean temperature difference, and overall heat...
  27. Darmstadtium

    B Conversion of Potential Energy: Sound or Heat?

    When a coin is dropped from a certain height and collides with a glass surface, is the majority of the potential energy converted to sound or heat? And how would one determine this as I only hear the sound and cannot measure the significant change in temperature?
  28. D

    A Heat exchange in a thermal storage based on phase change materials

    Hello, I want to model a thermal battery based on phase change materials (PCM). It is a plate heat exchanger immersed in a PCM bath. The diagram is given in the attached file. I want to determine the temperature at each moment and from everywhere in the battery. The hypotheses are the...
  29. Tone L

    Heat Transfer & Cooling Capacity

    Hi, zero education on heat transfer here, need some help. I have an electronics module that generates 15 W of electrical power over a 70mm x 20mm x 15mm heat load volume (aluminum). The ambient temperature is 25° C, the maximum temperature the heat sink can be is 40° C (ΔT = 15° C). Does...
  30. M

    Misc. Looking for help calculating heat transfer for PEX ground loop project

    Looking to build a suitable closed loop water (probably glycol mix) circulation system to prevent livestock water troughs from freezing. I assume that I need to calculate how much heat loss is predicted from the water tank in ambient air (varies, but assume worst case), then what length of heat...
  31. S

    B Liquid He heat of evaporation

    The dependency of liquid He latent heat of evaporation on temperature shows a conspicuous maximum. https://www.nist.gov/system/files/documents/srd/jpcrd551.pdf for 4He. Section 16, pages 1264-1266 on pages, 48-50 on pdf. From fig. 16.1 and table 16.3, the maximum is around 3,1 K and 94 J/mol. At...
  32. E

    I PDE - Heat Equation - Cylindrical Coordinates.

    Would method of separation of variables lead to a solution to the following PDE? $$ \frac{1}{r} \frac{ \partial}{\partial r} \left( kr \frac{ \partial T}{ \partial r}\right) = \rho c_p \frac{\partial T }{ \partial t }$$ This would be for the transient conduction of a hollow cylinder, of wall...
  33. kartini99

    I 1D Heat Transfer Analytical Wall

    Hello everyone, I'm trying to solve the transient heat transfer problem within the ID wall. The material is steel, and it is isotropic. The properties are given below : L = 5 mm qin = 0 Tinf = 100 deg C Tini = 20 deg C rho = 7850 kg/m3 cp = 460 W/Kg.K k = 45.8 W/m.K h = 20 W/m^2.K alpha = k /...
  34. R

    Calculating heat released from excess enthalpy

    Hello, I am currently trying to determine the method to calculate the heat absorbed / released when a gas and liquid mix. I believe that this is known as the heat/enthalpy of mixing (or excess molar enthalpy). I also believe, that if this value is negative it means an exothermic reaction (heat...
  35. V

    Heating up of tyres on the road

    I have searched online for this and found that it's due to friction. But for a rolling tyre there is only static friction acting and static friction produces no heat. For heat to be produced the two surfaces in contact must slide over each other i.e. there needs to be kinetic friction. Perhaps...
  36. V

    B Heat exchange after thermal equilibrium

    In screenshot below, systems A and B are separated by an adiabatic wall initially while each of them exchanges energy with system C via a diathermic wall. Once A and B reach thermal equilibrium with C, then A,B are allowed energy exchange via a diathermic wall, and energy exchange between A and...
  37. V

    I Cannot understand formula for molar heat capacities of an ideal gas

    Homework Statement:: I am trying to understand a formula given in our book for determining molar heat capacity of an ideal gas under different thermodynamic processes using a single formula, but it is confusing. The exact formula for different processes is in the screenshots below. Can someone...
  38. Alexey_H

    Heat loss from the temperature difference

    Hello, My house has some heat loss. As an example I know the outside door temperature. How can I calculate the heat loss delta if I reduce the door temperature by 1 degree? I know that there is a formula to calculate a heat loss based on the U-value of a fabric. But I can't get those values...
  39. F

    I Heat transfer via air cooling -- which airflow direction is best?

    I'm using a stainless steel pan and a fan to cool my coffee down as quickly as possible. Usually, I have the fan blowing down towards the coffee, but this morning I flipped it over to suck the heat out of the coffee and I'm wondering which of the two is the most efficient, especially as, in the...
  40. lxhull

    Specific Heat Capacity of a metal bar placed into water

    Previously solved thermal energy gained by water as Eth= 0.15(4180)(17.6) = 11035.2 J Not sure if its relevant
  41. DarkEnergy890

    The Mystery of Heat Loss: Examining Ice Melting Without Heat Loss

    During the procedure, 30% of heat is lost. So that means that 70% of water+container is contributing to melting the ice, right? And the other 30% contributing melting the ice is down to, well, the "heat being lost to the surroundings" (not sure what this really means). We compute the sum and...
  42. Salmone

    I try to solve a thermodynamics problem on heat transfer

    I have some doubts about a thermodynamics exercise I did-I'll write down the text and show how I tried to solve it to see if I reasoned it out right. In a container of thermal capacity ##C## containing ##1.0 kg## of ice at a temperature of ##-4.0 °C##, a mass of ##3.0 kg## of water is poured...
  43. V

    Heat Radiation question (skin's reaction to UV versus IR radiation)

    I am not sure of the answer. But I am guessing that infra-red rays carry more heat radiation than ultra-violet rays, and the heat is instantly felt on our skin when exposed to both these rays. So what we feel is the heat sensation from infra-red rays and that sensation is missing when it's...
  44. E

    Fourier's Law of Heat Transfer: Conceptual Explanation

    Hello, I'm looking over some old school notes and re-learning some basic heat transfer. I have known Fourier's Law (1D: ##q = k\frac{dT}{dx}##) for a long time, but when I look at it now I find it strange that heat flux is inversely proportional to the length of the conductor. I would think...
  45. V

    How could the discrepancy be reduced between the two readings?

    I am able to solve part (a) using the relationship ##\frac {P_1} {T_1} = \frac {P_2} {T_1}##, where ##T_1 = 273.16## since its the triple point of water and ##T_2 =T_s## ##(T_s = ## melting point of sulphur). I use the two readings for thermometer A to get ##P_1## and ##P_2## as mentioned in the...
  46. H

    pV diagram, ranking heat transfer between 4 processes

    Apologies, made a mistake when posting. Please see below post.
  47. T

    I Temperature change due to mixing liquids, heating and heat losses

    Hello, I am stuck how to proceed with the equations below. The problem: A tank containing ##m_0## mass, with a temperature of ##T_0##, is to be filled with a total mass of ##m_1##, with the constant mass flow rate ##G## and temperature ##T_1## during ##t_p## seconds. ##T_0>T_1##. Other than the...
  48. C

    Work greater than heat given to colder reservoir -- impossible?

    photo below... Is it possible to make a machine that would take more heat from the hot reservoir to do work than what would hot reservoir give to the cold reservoir(heat)? Apparently, it's impossible because it violates 1st law of thermodynamics. the thickness of the arrows symbolizes the...
  49. S

    Python Animating 2D Heat Map with Varying Time Intervals

    Consider the following heat map: from scipy import special import numpy as np import matplotlib.pyplot as plt u0=200 r0x=25 r0y=25 rmax=2.5 alpha=2 t=0.575 y, x = np.meshgrid(np.linspace(0, 50, 100), np.linspace(0, 50, 100)) r=np.sqrt((x-r0x)**2+(y-r0y)**2)...