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. W

    Why most engine wear occurs during cold start up?

    What is phyisical reason for 75%-90% wear engine occurs on cold start up? I can't find exact reason.. Is that beacuse ? 1) rich fuel mixture on choke, splash oil film from cylinder wall? 2)corrosive wear on cylinder and piston rings? 3)cold oil don't lubricated well?? (but oil when is cold...
  2. D

    Fluid temperature change over cold surface?

    For my research I am trying to study the temperature change of a warm fluid (air) flowing over a cold surface, but I have not studied fluid mechanics. Does anyone know of an equation that can be used to analyze this problem? To give some reference, the hot air is 49 Celsius and the cold air is...
  3. B

    Catching cold when wet in rain

    Whenever I got wet in the rain. I always get cold (running nose, etc.) I know I'm not catching the virus in the rain. Is the explanation because one's body thermo regulation (hypothalamus) gets affected which lowers the immune system that makes the existing virus in the body multiply? Right now...
  4. D

    I Exploring the Effects of Cold Welding in Space

    I've read that when two pieces of the same kind of metal touch in space, they will fuse because there is no way for the electrons to "know" that they are in different pieces of metal. What would happen if two different metals were to touch in space? Would electrons still flow freely between...
  5. S

    Anisotropy of cold rolled aluminium

    Sorry for my poor english. I did tensile test of aluminium(purity more than 99%. 1000series). The three specimens had tensile directions of 0, 45, and 90 degrees for each rolling direction. strength(both yield and UTS)is higher degree closer 0 degree. and %elongation is too. but modulus of...
  6. J

    Would this device(pic inside) work for boiling cold water?

    Imagine some giant glass(or other material) dome placed on top of some ocean or lake. Then pump some air out of this dome, until the water level rises a few meters under the dome. Place another dome below the first dome, and repeat the process, resulting in a little less pressure in the second...
  7. K

    Is EN353 cold forgeable as much as SCM420

    Hi, We are doing a gear box for a two wheeler (for the first time). Most of our benchmarks are of Suzuki and Honda bikes. All their gears are made of SCM420. Surprising part is these gear teeth are formed by cold forging. No machining at all! Now, I was comparing the chemical composition of...
  8. F

    Dispersion Relations in Cold Plasma waves

    Homework Statement Im stuck on a old exam in plasma physics. It is about how to determine dispersion relations for high frequency waves in cold plasma's. I'm not sure how they do in the solution manual. Homework Equations B = B_0z^ E = E_0exp(i(kx-wt))z^ The Attempt at a Solution The...
  9. Monoxdifly

    MHB How Hot is 10 Degrees Celsius Compared to 5 Degrees Celsius?

    What is 4 times hotter than -4 degrees Celcius? A. -16 degrees Celcius B. -1 degrees Celcius C. 12 degrees Celcius D. 16 degrees Celcius https://www.physicsforums.com/threads/how-much-warmer-is-10-degree-celsius-than-5-degrees-celsius.731713/
  10. J

    Can cold air become water in a vacuum cylinder?

    can cold air become water in a vacuum cylinder?
  11. wolram

    B Dark matter can be cold warm or hot

    According to this paper https://arxiv.org/pdf/1701.03128.pdf is this a fudge factor or can we have a mixture of energies for DM. Abstract. Various particle physics models suggest that, besides the (nearly) cold dark matter that accounts for current observations, additional but sub-dominant dark...
  12. Timmykuku

    Does Ambient Temperature Affect Cold-Blooded Animals' Size?

    Since cold-blooded animals, such as lizards, turtles, &c., take on the temperature of their environment, can we expect them to linearly and volumetrically expand/shrink in warmer/cooler environments? In addition, can we expect them to follow the simple equation: ΔV=βV0ΔT, for some volume...
  13. wolram

    B Cold or warm dark matter or a mixture

    This paper favors a mixture, but there are many papers that predict Cold dark matter, is there a particle that can be ruled out, or pushed to the back of favored particles. arXiv:1611.09362 [pdf, other] Hints against the cold and collisionless nature of dark matter from the galaxy velocity...
  14. wolram

    B Is dark matter self-interacting?

    https://www.sciencedaily.com/releases/2014/09/140908204603.htm According to this article dark matter is warm, and provides a mechanism for showing the lack of satellite around the milky way. Date: September 8, 2014 Source: Royal Astronomical Society (RAS) Summary: Scientists believe they have...
  15. A

    B Is There a Connection Between Thermostats and Shower Water Temperature?

    If I turn up thermostat to 70, the rooms get warm. I notice shower head gives warmer water at this temperature. If I turn thermostat lower, the rooms get cooler as it should be. Again I notice shower gives less warm water. What is the connection? Or is it coincidental? It happens too often that...
  16. Greg Bernhardt

    Troubleshooting Cold Radiator - Boiler Fall Start Up

    Turned the boiler for the first time this fall and an upstairs bathroom rad is stone cold. All other rads heat up. I fiddled with the valve, which is new, with no luck. What is strange is that even the pipe from the floor leading to the valve is cold. This is a water system. It must cycle. How...
  17. wolram

    B Cold spot still baffles scientists

    https://www.sciencedaily.com/releases/2016/10/161012095819.htm Date: October 12, 2016 Source: University of Portsmouth Summary: Astrophysicists have created the largest ever map of voids and superclusters in the Universe, which helps solve a long-standing cosmological mystery. So what is...
  18. wolram

    B Positions of atoms at super cold temp

    I thought this was an interesting paper, good enough to share https://www.sciencedaily.com/releases/2016/09/160915162930.htm September 15, 2016 Source: Massachusetts Institute of Technology Summary: If you bottle up a gas and try to image its atoms using today's most powerful microscopes, you...
  19. O

    I The cold interior of the Sun seen in a sun spot

    The sun we see has a measured surface temperature of 5800K. This is the temperature of the photosphere , a glowing layer of plasma radiating energy from the sun. But when there is a hole in this layer and we look deeper into the interior the temperature drops to 3800K. How can this possibly be...
  20. R

    Need advice on pinch-off tool for cold cold-weld sealing

    Hi, My name is Rodrigo and I need to achieve a permanent seal for a vacuum chamber in the high vacuum range (I’ll be working between 1x10-4 and 1x10-torr). We were thinking of using a cold-weld sealing of an annealed copper tube to separate the chamber from the vacuum pump once the vacuum level...
  21. C

    Just for interests sake, on a Saturday night

    Why is there an excess of foam when I add ice to rum and coke? When I add ice to vodka and sprite/lemonade there is a short burst of foam which quickly dissipates. What causes the reactions to differ? Sugar?
  22. S

    Airflow Across a Persistent Cold Air Pool in a Basin

    This question is regarding the general fluid dynamics involving warm air advection aloft a persistent cold air pool in a small basin that is recessed in a horizontal planar surface. For scale, the basin has a compound parabolic shape, with a 30° viewing angle, a .146m radius at the base, a .292m...
  23. T

    I left a soda in the car and it still feels cold.

    After a cookout on Saturday, I left two cans of Coca-Cola in my bag. Around Monday morning which is almost two days later, when I left for school, the cans were still cold with moisture on it. The next day, I put another can of Coca-Cola in the car and only left it overnight to see what the...
  24. P

    X died of pneumonia doing experiments in the cold myths?

    Can science text writers please stop repeating these sorts of unfounded, unscientifically grounded anecdotes in science texts as if they are fact. I was just reading in Universe 8th ed. astronomy text and found this passage: "Sadly, Goodricke paid for his discoveries with his life; he caught...
  25. I

    I Does insulation protect outdoor objects from cold

    I have an object ( a windsurfer board) which I want to protect it from the cold but I cannot store it inside. The outside temperature now is a few degrees above 0 Celsius during the day, but is expected to drop to -8 at night for the next few days.. Assuming that during the day the board...
  26. Elena14

    I Hot liquid mixed with a cold liquid

    My textbook says - When a hot liquid is mixed with a cold liquid, the temperature of the mixture - "is undefined for some time and then becomes nearly constant". Shouldn't the temperature first decrease and then become constant ? Why does it say "undefined" ?
  27. P

    How to find the temperature of the cold reservoir?

    Homework Statement Carnot engine A has an efficiency of 0.60, and Carnot engine B has an efficiency of 0.80. Both engines utilize the same hot reservoir, which has a temperature of 650 K and delivers 1200 J of heat to each engine. Find the magnitude of the work produced by each engine and the...
  28. A

    Effect of a distant cold object in focus of concave mirro.

    Pictets 1790 experiment shows the apparent reflection of cold from ice in the focus of a concave mirror onto an air thermometer held in the focus of a second concave mirror 12 feet away. As soon as the screen shielding the 'cold' concave mirror is moved aside the distant thermometer in the...
  29. B

    What does cold hydrogen gas emit at 1420 MHz?

    I just read Paul Davies book The Eerie Silence. In The Eerie Silence, Paul Davies asserts that the radio astronomers looking for radio signals from extraterrestrial civilizations search for radio signals at the frequency 1420 MHz because that is the emission frequency for cold hydrogen gas...
  30. S

    Why is Hot Air Lighter than Cold Air?

    If E=mc^2, why is hot air lighter than cold air? Hot air has more energy and should thus have greater mass and therefore be heavier as weight = mg
  31. P

    Magnetic waves/fields that precede cold weather fronts....

    Are magnetic waves that travel ahead of low pressure wx fronts, a certain frequency? Are they a sign wave? Is it possible to cancel a magnetic wx front wave on a local basis by generating an 180 degree out of phase signal at the same frequency like you would a voltage wave? Is the magnetic wave...
  32. W

    Air vs water as insulation for my cold frame

    Hi, new here, with bits & pieces of basic college physics rattling around in my brain from years ago, so explain things at that level, please. I am building a cold frame in order to try growing food through the winter in NJ, USA. It will get below freezing here at night but not every day. I...
  33. wolram

    My 5-Day Hospital Adventure - Surviving Cold Sprays and X-Rays!

    So i spent 5 day in hospital, apart from three occasions every thing went well i elected to go for an epidural, and o boy, they sprayed my back with something that was colder than the arctic, it was a real pain full cold:mad:. After the operation i laid in bed with no feeling from my waist down...
  34. wolram

    Cold Spot in CMB: Real or Systemic Error?

    Is this a systemic error? Quote from wiki The cold spot is mainly anomalous because it stands out compared to the relatively hot ring around it; it is not unusual if one only considers the size and coldness of the spot itself.[7] More technically, its detection and significance depends on using...
  35. I

    Why does cold water hurt my teeth but warm water doesn't?

    Whenever I brush my teeth and I squish the water around in my mouth, I get an intense sharp pain in some of my top back teeth. I've been to the dentist several times and they say there's nothing wrong with my teeth. However whenever I drink warm water or squish warm water around in my mouth it...
  36. HugeDragon

    Extreme Environment Robotics: Materials for a -1000°C Planet

    Hi guys, My science teacher gave me a project to design a robot for a specific planet. I must also include what materials the robot were to be made of. Unfortunately the planet I received has very hostile conditions 1. Temps of -1000 degrees C. Do not ask my why my planet is colder than 0K (3...
  37. K

    Need help in calculating amount of cold water required

    need to cool down 1000 ltrs of water from 121 Degree C to 36 Degree C.Jaket volume of fermentor is 200 ltrs. How much cold water at 25 Degree C required ?at what flow rate?how much time required.?
  38. PytrTchaikovsky

    How can the effect of air temperature on an object be quantified?

    Let's say I have a cold bottle of water in a warm room (temperature is constant). Is it possible to calculate how long it will take for the water to reach a certain temperature? Do you guys know any formula for this? To be more general: What is the effect (W, J/s) that air in a certain...
  39. Greg Bernhardt

    Why hot water freezes faster than cold water?

    Interesting new study out http://www.telegraph.co.uk/news/science/science-news/10420496/Have-scientists-worked-out-why-hot-water-freezes-faster-than-cold-water.html Here is the paper http://www.nature.com/srep/2013/131021/srep03005/full/srep03005.html
  40. H

    Cold Spot in Cosmic Background Radiation

    At one time, I read that the universe was, on a very large scale, essentially the same in all directions. Recently I have learned about the Cold Spot in the fairly uniform Cosmic Background Radiation map. Scientists have attributed the artifact to a very very large void in the ordinary matter...
  41. Rotzooi

    Heat transfer of a hot plastic thread in a cold water bath

    Hi, I'll describe my problem more specific. I have a continuous flow of this polymer thread (polystyrene) at 465 °K (190°C sorry I'm metric) that needs cooling down. It passes trough a water bath which is kept at 310°K (35°C). Now i try to calculate the heat exchange into the water, to...
  42. manogyana25

    Average kinetic energy of the molecules in a cold liquid less?

    Why is the average kinetic energy of the molecules a cold liquid less? As the temperature of a liquid decreases, the average kinetic energy of its molecules reduce. What is the reason behind this?
  43. E

    Is it possible to be infected by the same cold virus twice?

    I am just getting over a relatively minor headcold, and I was curious if it was possible to get infected by the same bug you just got over. My keyboard, computer desk, and other objects are covered in these germs as I have touched them and sneezed on them countless times in the past 5-6 days...
  44. C

    Three big questions about Gravity, Cold and Magnets

    Hey, So I am a sophomore in high school and have never taken a physics class before (it hasn't been offered) but I look forward too taking one. I got really thoughtful today and came up with three questions. 1. Gravity... is it energy? And is it infinite? I mean we harness gravities energy...
  45. 0

    Getting cold water into pressurized hot water container

    How can this be done? So there is a container with hot water, and it's highly pressurized. It also has an output potential with a pressure reducing valve. I'm trying to workout how to feed the pressurized container with colder water (to refill). My first thought was that maybe when pressure...
  46. P

    What is the most effective way to prevent pipes from freezing in a cold attic?

    In January 2014 we had a pipe break on the third floor adjacent to an attic space which housed water pipes to the baseboard heat. After over 400,00 in repairs and eight months of not being able to use the house, we are facing another prolonged cold spell. The pipes in the attic do have...
  47. K

    Is There a Scientific Explanation for the Controversial Concept of Cold Fusion?

    Could somebody explain me if there's a theory, axiom, law or something that can refuse the possibility of "Cold Fusion" in experimental basis? Thanks a lot. Karmo
  48. Z

    Pre big bang infinite cold inflation?

    I was listening to Brian Cox on Intelligence Squared and he somewhat casually mentioned the general acceptance among physicists of a possibly "infinitely long" period of cold inflation predating the big bang and of the "sudden" stop to this inflation as the source of energy for the big bang...
  49. S

    Baby it's cold outside -- revived

    Well, here in Toronto, ON, Canada, it is currently -9 deg Celsius with a windchill of -18 deg Celsius (for Americans, that is 16 deg Fahrenheit with a windchill of -0.4 deg Fahrenheit). Tomorrow Toronto is expecting a high of -11 deg Celsius (12 deg Fahrenheit) with a windchill of -22 deg...
  50. S

    Why do some people not get the Common Cold?

    Sorry if this is in the wrong place, not sure where else to put it. Something has been puzzling me for a few years, I developed Pericarditis which is now recurrent on and off about 4-5 years ago. Ever since then, I have not had a bout of the Common Cold. If someone close to me has the Common...
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