What is Cold: Definition and 394 Discussions

Cold is the presence of low temperature, especially in the atmosphere. In common usage, cold is often a subjective perception. A lower bound to temperature is absolute zero, defined as 0.00 K on the Kelvin scale, an absolute thermodynamic temperature scale. This corresponds to −273.15 °C on the Celsius scale, −459.67 °F on the Fahrenheit scale, and 0.00 °R on the Rankine scale.
Since temperature relates to the thermal energy held by an object or a sample of matter, which is the kinetic energy of the random motion of the particle constituents of matter, an object will have less thermal energy when it is colder and more when it is hotter. If it were possible to cool a system to absolute zero, all motion of the particles in a sample of matter would cease and they would be at complete rest in this classical sense. The object would be described as having zero thermal energy. Microscopically in the description of quantum mechanics, however, matter still has zero-point energy even at absolute zero, because of the uncertainty principle.

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  1. K

    Heater power keeping a room constantly warm from low temperatures outside

    I've been trying for 45 minutes and doing research but I can't even find a start to how to approach this. :( [Mentor Note: New user reminded to always show their work on schoolwork problems. That work is shown in a subsequent post below]
  2. R

    I Estimating Hot & Cold Spots in CMB Sky Maps

    Is it possible to estimate the number of hot and cold spots of average diameter of 1° in a full-sky map of the CMB?
  3. B

    I Time for a cold volume of air to reach a higher air temperature

    I have a cube with a volume of 1000m3 at an initial temp of 290K. The bottom side (10m by 10m) is open to the ambient air. I put this cube into a huge fridge and cool the whole volume by 5K. I close the open side by placing a cover on it. This cube has now got a volume of air at a temperature of...
  4. S

    I Centrifugal separation into hot & cold air streams?

    I have been reading about the Ranke-Hilsch vortex tube. Details of the explanation tend to differ somewhat among different sources, but it got me thinking about the following thought experiment. Air enters a tube of about 0.5 x 4 cm cross section. It passes through a section that is channelized...
  5. archeryemily9085

    B Filling a bathtub (Hot water to cold water exchange)

    Hey there, I honestly don’t know if this is the right place for this, but I figured I would ask. lets say you are going to take a bath, but someone just took a shower and you know your hot water heater will be running low on the goods. Just for arguments sake, let’s say you have just enough...
  6. George Jones

    Is -41C Too Cold for a 20-Minute Walk to the Bus Stop?

    Went for a 20-minute walk to the bus stop. The temperature was
  7. G

    I Data Showing Dark Matter Is Not Cold Neutrinos?

    How do we know that cold neutrinos do not make up 100% or a large percentage of the dark matter content in the universe? In my mind, the only way to prove that dark matter is not simply cold neutrinos would be to measure the density of cold neutrinos in the universe and then calculate the...
  8. P

    I What are the possible states of cold iron balls?

    This thread is meant to discuss a topic that arose in https://www.physicsforums.com/threads/is-there-a-theoretical-size-limit-of-a-planet.1045983/ in a more precisely specified, idealized way. Further details: 1) Standard model + GR assumed, no other theories intended for this...
  9. H

    Refrigerator Too Cold: Wrapping Veggies in a Blanket?

    My refrigerator is on the minimal setting but still freezes the vegetables a little bit. It didn't do that a week ago. I'm renting and moving out in a few days so I'm not going to do anything drastic. How about wrapping my veggies in a blanket? I never understood the physics of those...
  10. B

    B Thermos Bottle: Stainless Steel Inside & Out - 24h Cold & 12h Hot

    Manufacturer information: Interior material: stainless steel Exterior materials: Stainless steel Keeps cold liquid for: 24 hours Keeps hot liquid for: 12 hI was looking for a thermos bottle and came across a situation that I'm having a hard time answering. Why is the cold keeping time different...
  11. BillTre

    Which Dinosaurs Were Warm Blooded or Cold Blooded?

    In another well directed chemical analysis of fossils, a research group has indications that some but not all dinosaurs were warm blooded (endothermic), although some (Stegosauri and Triceratops) had apparently reverted to being cold blooded (ectothermic). In addition, it seems that the flying...
  12. Erwinux

    I Calculating the thermal coefficient between the insulated interior of a telescope instrument package and the cold ambient air outside

    I've built an insulated chamber to protect a sensitive instrument at freeze temperatures in the winter. The instrument is mounted on a telescope, so the heat inside the chamber will slowly dissipate in the ambient. A digital PID thermostat is used to keep the temperature at a safeguard level...
  13. shivajikobardan

    Comp Sci Cold Start & Early Rater: Making Predictions with Limited Data

    cold start-: system requires huge amt of current user data to make accurate predictions early rater-: new user hasn't rated many items to make predictions. both same? isn't it?
  14. shivajikobardan

    MHB Cold Start & Early Rater: Same Problem?

    cold start-: system requires huge amt of current user data to make accurate predictions early rater-: new user hasn't rated many items to make predictions. both same? isn't it?
  15. S

    How can a human body create such effects on a freezing cold winter day?

    I was in doubt if this is the right chapter to place this thread, but it seems there are many talents just here. "we were observing from a decent distance through optics. The weather was frosty, clear, calm. When хххххх appeared from under the arch of the house (he was хххххххх to walk to the...
  16. BWV

    Its cold, but the NY Post should have used Google to convert C to F

    Oymyakon is cold, but not that cold https://nypost.com/2021/12/03/fears-children-could-freeze-to-death-in-worlds-coldest-town/here is the Sun article they credited but failed to convert the temps https://www.the-sun.com/news/4196960/fears-children-freeze-temperatures-drop/The Siberian village...
  17. B

    Cooling tower -- hot and cold water tanks

    Hi In some projects, I saw that hot and cold water tanks have been used for open circuit cooling towers. What is the need to use these tanks?
  18. T

    COVID Antibody from Common Cold reacts to COVID

    THIS POST TITLE IS MISLEADING. I paraphrased/lifted from the jpost. com article but did not read the research article. @Ygggdrasil graciously points out the error in post #3 below. Thanks...
  19. T

    COVID Common Cold Virus Suppresses COVID-19

    I'll trade a cold for Covid-19 any time! https://www.medicalnewstoday.com/articles/a-common-cold-virus-may-help-fight-covid-19 Cheers, Tom
  20. M

    Can polyester resin eventually cure if it was too cold at first?

    I finally brought in a heater and kept it at about 30C for 3 hours. It still smells like uncured offgassing. Will it ever cure or did the freezing/prolonged cold ruin it and i have to remove it?
  21. T

    1420 MHz--- the emission frequency of cold hydrogen gas

    I recently finished reading Paul Davies book The Eerie Silence, which is a book about the SETI (Search for ExtraTerrestrial Intelligence) project. In The Eerie Silence, Davies says that scientists using radio telescopes to search for radio messages from space aliens set their radio telescopes...
  22. iVenky

    I Are quantum computers required to be cold to reduce Brownian motion?

    I understand that based on what I have read online quantum computers are required to be close to absolute zero because it introduces less error. Is it because brownian motion due to thermal agitation of molecules reduces with temperature?
  23. F

    B Cold vs hot on the atomic scale

    So I've been hunting google for an answer but i cannot find a definitive one. If 0 Kelvin is the coldest temperature and it is where atoms cease to move does this mean that a photon in a vacuum is the hottest particle because it is in turn moving at the fastest speed possible? Second question...
  24. iVenky

    B Why does my head feel heavy when I go out on a cold weather day?

    Is it because of increase in pressure on my blood vessels due to cold weather outside?
  25. kyphysics

    If My Water Heater Doesn't Work, Is Water Still Safe to Drink (if Cold)?

    My water heater has an issue. I still get cold water, but is that safe to drink if the water heater is not working? Thanks!
  26. T

    I A cold, massive, rotating disk galaxy 1.5 billion years after the BB

    Neeleman, M., Prochaska, J.X., Kanekar, N. et al. A cold, massive, rotating disk galaxy 1.5 billion years after the Big Bang. Nature 581, 269–272 (2020). https://doi.org/10.1038/s41586-020-2276-y Abstract Massive disk galaxies like the Milky Way are expected to form at late times in traditional...
  27. S

    I Wringability or cold welding

    What are standard quantitative measurements of wringability? What is the quantitative limit between wringing and cold welding? What are the basic reasons of weakness at wringing/cold welding fault?
  28. Harini Krishna

    Food temperature question -- eating cold meals

    Hi, Can i know how having cold meals daily effects out health in long term? Thank you
  29. Indula

    A Why does a layer-like structure occur in a cold plasma?

    I have observed a layer like structure (made of plasma) in a cold plasma and wonders what causes it. Following image shows the observation.
  30. 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...
  31. J

    (resolved) Seebeck effect: why can't electron+hole combine at cold end?

    Hi, In seebeck effect: Ref: Once the red region is hot, the electrons and holes move toward cooler region i.e. bottom. So there is a positive potential toward right side of cold side and negative region toward left side of cold side. So why can't the electrons and holes combine here only...
  32. S

    Municipal water lines in cold climates - history of technology?

    In the USA, the usual solution to keeping water lines from freezing is to bury them. In cold climates this implies digging deep trenches. (On another forum, a person from Minnesota said water lines are typically buried 7 ft deep.) In the days before modern excavating equipment, were...
  33. 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?
  34. gmax137

    No evidence for globally coherent warm and cold periods

    No evidence for globally coherent warm and cold periods over the preindustrial Common Era This is being reported in the popular news, here's a link to the Nature abstract. https://www.nature.com/articles/s41586-019-1401-2
  35. S

    B Does warm air rise or does cold air sink?

    I have a friend and he complained that at school they are teaching that warm air rises. He said it's wrong. He said instead the cold air sinks. Is this true?
  36. QuasarBoy543298

    The second law of thermodynamics -- What does "from cold to hot" mean?

    in Clausius formulation, what does the phrase "from cold to hot" means? I can understand it intuitively but in the language of the zero and first laws, we have not defined a temperature scale, only equivalence classes of systems that will be in equilibrium with each other (systems with the same...
  37. T

    Change in entropy when mixing cold and hot water

    I tried following: $$ dS_{\text{total}} = |\frac{dQ}{T_c}| |\frac{dQ}{T_H}| $$ where ## T_h ## is temperature of hot water and ## T_c ## is temperature of cold water. Coefficient for water wasn't provided in the assignment so i used following value c = 4190 J/kgK. $$ dS_{\text{total}} =...
  38. R

    Is hot gas more massive than cold gas?

    Homework Statement In principle, does a hot gas have more mass than the same gas when it is cold? In practice, would this be a measurable effect? Homework Equations E = γmc2 The Attempt at a Solution Since there's more energy, there's more mass. But I don't think these effects would be...
  39. Atlas3

    B Was the initial condition extremely cold?

    If all matter was at maximum density, would not the temperature have been extremely cold of the sum of all matter? temperature affects density I think.
  40. C

    B Barnard's Star, 6 Light-Years Away, May Have Cold SuperEarth

    The planet: Distance from star: 0.4 AU (comparable to Mercury) Orbital period: 233 days (comparable to Venus) Mass: at least 3 Earth masses Temperature: -270 F (comparable to Saturn) Barnard's Star: Distance from us: 6 light-years; second closest star system to Sun after Alpha Centauri...
  41. G

    APDL Programming buckling FEA C Section Cold Formed Steel

    Hi, I am facing issues with my program. Listed below. This is form my masters project. Also in what way can I connect the filleted surfaces shown in fig by a weld or just load or stress transfer connection. fini /clear /title,C160T2B70-1L /prep7 et,1,shell181 r,1,2 mp,ex,1,1.85e5...
  42. R

    Cold plasma technology and barrier discharge systems

    Hi there, I am currently doing a project involving cold-plasma technology systems. I am struggling to find papers relating to surface-micro barrier discharge systems, and the difference between those and dielectric barrier discharge systems. Any help would be greatly appreciated, however basic.
  43. Wrichik Basu

    Gene for allergies and the common cold

    Some people are more susceptible to allergies and common cold than others. For example, my mother and I suffer from dust allergies and blocked nostrils throughout the year. In fact, last year or so, my mother had to be hospitalised as her lungs had become stiff due to sudden allergy. Almost...
  44. H

    B Feasibility of a L1 Gravity Swing Cold Launch?

    At an L1 LaGrangian point between two bodies, one could - materials science notwithstanding - pit two of Newton's Laws (LM3,UG) against each other to provide thruster-free stationkeeping. Would it be feasible to use that to launch free from the system ? either spit out like a watermelon seed...
  45. A

    B I want to know about cold finger cryostat?

    hi guys,I want to know more about cold finger cryostat? can anyone help me to know this instrument?
  46. A

    Is there an Entropy difference between a cold and hot body

    Hello, If two bodies, who say start with ##T_{cold}=T_c## and ##T_{hot}=T_h## and then they are brought in contact with one another and then after some time they both have the same temperature. What would be the entropy of the entire system? Also another quick question, I've looked at some...
  47. E

    Why does entropy increase when hot water is mixed with cold?

    Why is entropy lost by hot water less than the entropy gained by the cold water?From perspective of energy,why is it better to take water and heat it to a temperature than it is to mix hot water and cold water to get a particular temperature.
  48. jim mcnamara

    Brumation - alligators beating the cold in North Carolina

    http://www.charlotteobserver.com/news/local/article193602684.html See the video. Brumation is the term used for reptiles. They simply shut down and literally freeze to get past very cold weather. Generally it is for short periods, although garter snakes in parts of Southern Canada can endure...
  49. Fanny Viksten

    Why does cold air seem to optically zoom in far-away objects?

    I have noticed that cold air makes far away objects, such as mountains, appear closer than they do in warm clear weather. Does anyone have an explanation for this?
  50. Petr Matas

    I Can We Cool a Macroscopic Crystal to 1 nK?

    Is it theoretically possible to cool a macroscopic crystal (for example NaCl 1×1×1 cm) to an extremely low temperature, like 1 nK? Will it retain its microscopic structure? Why? Does it have anything to do with zero point energy (I mean something like the lowest energy level of a quantum...